Concave Welding Symbol Explained: Meaning, Drawing Rules, Throat Math & Inspection

A fabricator once shipped 400 bracket assemblies that all failed incoming inspection — the welds looked smooth and pretty, but every fillet face was dished like a spoon, and the effective throat was a hair under the engineer’s minimum. The welder had read the drawing correctly except for one small curved arc sitting above the fillet triangle, an arc that quietly demanded a concave finished profile and a throat that had to be measured, not eyeballed. That tiny arc is the concave welding symbol, and misunderstanding it is one of the most common — and most expensive — drawing-reading mistakes in the trade.

Why One Small Curved Arc Decides Whether Your Weld Passes or Fails

That failed bracket shipment started with a single curved mark on a drawing. The welder set his machine correctly, ran clean beads, and produced welds that honestly looked better than most shop work. But “looks good” is not the same as “meets the print,” and when the inspector laid a fillet gauge across those dished faces, every assembly came up short on throat. The fix for the next 400 pieces was not new equipment or retraining. It was learning to read one tiny arc — the concave welding symbol — and understanding exactly what it demanded.

If you have landed here, you probably want three concrete things: to know what that inward-curving arc means, to know how to draw it correctly so your prints are unambiguous, and to understand how it affects weld strength so you can defend the call or catch the error. This guide delivers all three, in that order, with worked examples at the end.

What the Concave Welding Symbol Is, in Plain Language

A welding symbol is a compact instruction set on a fabrication drawing. It tells the welder what weld to make, where to make it, how big it should be, how long it should run, and what shape the finished face should have. That last part — the finished face shape — is handled by something called a contour symbol.

The concave contour symbol is simply a curved-inward arc placed directly above the basic weld symbol. If the basic symbol is a fillet triangle, the arc sits over the triangle. If the basic symbol is a single-V groove, the arc sits over the V. The arc mirrors the shape the finished weld face is supposed to have: dished inward, curving from one toe of the weld to the other like a shallow spoon. There is nothing decorative about it. It is an instruction that says, “when you finish this weld, grind or control the profile until the face is concave.”

The American Welding Society’s official weld symbol chart shows contour symbols sitting above the basic weld symbol for exactly this reason, and once you see the layout, you will never unsee it.

Why This Is Not Just a Drawing Convention

Welding symbols carry contractual weight. In the United States, the governing document is AWS A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination, now in its eighth edition (A2.4:2020). A print is a legal-technical language: the fabricator is obligated to weld to the symbol as drawn, and any departure — including a decision to leave a weld flat or convex when the print called for concave — requires an approved deviation through engineering. Welding inspectors are trained to reject work that does not match the symbol, which is precisely how our fabricator in the opening story lost a full production run.

That is why shops serious about drawing literacy keep a physical symbol chart posted at every welding station and every layout bench, and why welders themselves are expected to read symbols regardless of process. Whether you run stick, MIG, or flux core, or you are building out a home shop with a MIG TIG combo welder, the symbol language is the same. The American Welding Society maintains the standard, and shops worldwide reference it alongside the European approach.

How This Guide Is Built

Readers searching for “concave welding symbol” are usually in one of three situations: reading a print for the first time and trying to decode the arc above the triangle; drafting or engineering a concave finish callout without creating ambiguity; or standing on the shop floor in a throat-measurement dispute trying to work out who is right. We address all three.

The next section breaks down the full anatomy of a welding symbol — reference line, arrow, basic symbol, tail, size and length fields, all-around circle, field weld flag — and pinpoints exactly where the concave arc lives and how arrow-side versus other-side placement works. From there we compare concave against convex and flat contours, work through the throat math with real numbers, extend the idea to groove welds and reinforcement limits, and walk through three fully worked drawing examples.

You will also see how the undercut and notch concerns inspectors watch for at concave toes, how AWS A2.4:2020 and ISO 2553 differ in their conventions, and where shops get burned when they mix the two systems on one print. Finally, we cover when specifying concave is the smart engineering choice and when it quietly costs you throat you cannot afford to lose.

Why the Detail Matters More Than It Seems

It is tempting to treat a contour arc as a cosmetic note — a nice-to-have for appearance. That assumption is where the money gets lost. On a fatigue-sensitive joint, a smooth concave transition is genuinely the right call, because it reduces the stress concentration at the weld toe where cracks love to start. On a throat-limited structural fillet, the same concave profile can drop the effective throat below the value the engineer designed for, and the joint fails on paper before it ever fails in service. The arc does not care which outcome you intended. It only enforces the profile it draws.

The rest of this article gives you the vocabulary, the geometry, and the inspection logic to read that arc correctly every single time — and to catch it on someone else’s print before it costs you a production run.

Anatomy of a Welding Symbol: Where the Concave Arc Lives

Before that little curved arc means anything, you have to know exactly where to find it. A welding symbol is not a sketch of a weld; it is a compact code whose parts each have a fixed, legal position defined by AWS A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination. Shift one part a hair to the wrong side of the line and the drawing says something entirely different, which is how a shop ships 400 bracket assemblies that all fail incoming inspection.

So let’s take the symbol apart and drop the concave arc into its proper home.

The Eight Parts of a Welding Symbol

• Reference line. The horizontal backbone. Every other element hangs off it, and its two sides carry different meaning, the most important idea in this article.
• Arrow. The leader line pointing to the joint being welded. It may be straight, bent, or double-headed, and it must actually touch the joint; pointing vaguely at a corner is not good enough.
• Basic weld symbol. The shape that identifies the weld type: a triangle for a fillet, a V or bevel for a groove, a circle or rectangle for a spot or plug weld, and so on.
• Tail. The forked flag at the end of the reference line, used to name a process, a specification, or a welding procedure. A tail is optional, drawn only when something needs to be referenced there. If you want the shop to run a specific procedure, this is where you point them, whether that is a full pipe welding procedure or a local shop standard.
• Dimension fields. Size (leg length or groove depth), length, and pitch all sit in a specific zone relative to the reference line, never scattered around it.
• Weld-all-around symbol. A small circle at the intersection where the reference line meets the arrow. The Pearson/Goodheart-Georgian educational chapter on advanced welding symbols documents this exact placement: the circle sits at the reference line–arrow junction, not floating beside it. A circle there means “weld the entire perimeter of this member.”
• Field weld flag. A small pennant at that same junction, flagging that the weld is made in the field rather than in the shop.
• Contour and finish symbols. A shallow arc drawn on the basic weld symbol showing the required finished face shape, sometimes followed by a letter naming the finishing method. This is where our concave arc lives.

Arrow Side vs. Other Side: The Golden Rule

AWS A2.4 uses location significance. The weld on the side of the joint the arrow points to is called the arrow side, and it is specified by placing the weld symbol below the reference line. The weld on the opposite side is the other side, and it is specified above the reference line. Memorize that pairing and you can read roughly 90% of real-world drawings correctly. Flip it and the welder puts the weld on the wrong side of the joint.

Here is the layout in plain text:

   OTHER SIDE  →  symbol drawn ABOVE the line

        1/2  ⌒▽  1-1/2 @ 3 PITCH
   ───────────────────────────────  ← reference line
              △ 1/2
               ╲   ○ ← all-around circle
                ╲  ⚑ ← field weld flag
                 ▼ ← arrow points at the joint

   ARROW SIDE  →  symbol drawn BELOW the line

Where the Concave Arc and Finish Letter Sit

The contour symbol rides on top of the basic weld symbol, on both the arrow side and the other side. If the drawing also requires a specific finishing method, a letter goes right next to the arc: C for chipping, M for machining, G for grinding, and R for rolling. A concave fillet on a visible, cosmetic joint often carries a G, the cleanest way to produce a smooth, even curve without gouging the toe. Before you demand a finish, know what your process naturally gives you: a well-controlled TIG bead on thin material often comes out close to flush or slightly concave with no finishing at all, as covered in the guide to TIG welding sheet metal.

One more thing about the arc: it is a shape instruction, not a size instruction. It tells the welder what the finished face must look like, nothing about how big the weld is, and it does not relieve anyone of the duty to hit the required throat. That is a separate problem, handled in the throat math section.

Placement Mistakes Shops Actually Make

• Putting the arc on the wrong side of the triangle, so it reads as decoration instead of a contour requirement.
• Mirroring the symbol incorrectly, which puts an arrow-side weld on the other side of the joint.
• Drawing the all-around circle away from the arrow junction, where it means nothing.
• Adding a finish letter with no controlling note, leaving the shop guessing which standard governs.
• Reading an ISO drawing with AWS rules, a mix-up we untangle in the standards comparison section.

Concave vs Convex vs Flat: Comparing the Three Weld Contours

Once you know that a small arc above the basic weld symbol means “concave,” the natural next question is: what do the other marks mean, and how do I pick the right one? There are only three contour symbols in the welding symbol language, and they describe the shape of the finished weld face — nothing more. The arc curves inward for a concave face, bulges outward for a convex face, and a straight line sits flat across the top of the basic symbol for a flat (or mitre) face. Everything else about the weld — size, length, pitch, process — lives in other parts of the symbol. The contour mark is purely a instruction about final geometry.

The official AWS welding symbol chart shows all three side by side, and it is worth memorizing their shapes as a set rather than learning them one at a time.

The Three Contours at a Glance

Read that throat column again, because it captures the whole trade-off in one line. A concave fillet face reduces the actual throat below the theoretical throat, while a convex face adds throat but creates a sharper weld-toe stress raiser. In other words, concavity costs you cross-section and convexity costs you fatigue life. There is no “free” contour — you are always choosing which compromise to accept.

Why Fatigue Sensitive Designs Lean Concave

The weld toe is where cracks start. When a weld stands proud of the plate, the load path has to turn sharply at the toe, and that sharp geometric notch concentrates stress. A smooth concave transition does the opposite: it blends the weld toe into the base metal so the load flows through the joint gradually instead of slamming into a corner. That is why good welding symbol references consistently point out that contour selection is a fatigue decision as much as an appearance decision.

The same reasoning shows up in process choice. Achieving a clean concave profile on thin material is far easier with a well-tuned TIG setup than with a hot stick bead — which is exactly the kind of control discussed in The Complete Guide to TIG Welding Sheet Metal. On heavier fillets, heat input and travel speed do most of the work, as covered in 3/32 E7018 Amperage: Settings, Polarity & How to Run It.

What Changed in AWS A2.4:2020

The flat contour symbol has a slightly messy history. Before 2020, drawings used the straight-line mark somewhat loosely across different weld types, and shops interpreted it inconsistently — some read it as “grind flush,” others as “finish by welding.” AWS A2.4:2020 tightened the rule by limiting the flat contour symbol to fillet welds and by allowing the contour to be achieved by any method — mechanical (grinding, machining) or by welding technique alone. That is a genuine simplification for detailers: you no longer need a separate note to say whether the flat face comes from a grinder or from careful pass control.

There is also good news for shops that worry about rejecting welds for looking imperfect. AWS D1.1 permits slightly convex, flat, and slightly concave fillet faces, provided the weld-size criteria are met. “Slightly” is the operative word — the profile has to stay within the code’s limits, and the required throat still has to be there.

Practical Selection Guidance

Use this decision order when you are detailing a joint:

1. Is the joint fatigue-critical or hygienic? Specify concave. It gives the smooth toe transition and the crevice-free face that sanitary and cyclic-loaded work demands.
2. Is the joint throat-limited and heavily loaded? Specify flat, or leave the contour unspecified and design around the required throat. Concavity here means a larger leg to recover the throat you lost.
3. Is appearance irrelevant and you want margin? A slightly convex face is acceptable and adds a little throat, but watch the toe angle.
4. Is the weld visible to the customer? Concave or flat almost always looks better and hides fewer ripples.

Whichever you choose, remember that the contour symbol is only half the instruction. It sits on top of the basic symbol, and if you place it on the wrong side of the reference line, you have specified the wrong joint entirely — a mistake we will walk through step by step in the drawing section ahead.

The Math Behind a Concave Fillet: Leg Size, Throat, and Why Concavity Costs Strength

Size can be deceiving on a fillet weld. Plenty of welders have laid down what looked like a full-sized bead only to have it rejected for an undersized throat, and a concave face is the usual suspect. A quick glance won’t catch the problem, but the cross-section math will. That dish in the face isn’t cosmetic; it removes metal from exactly the part of the weld that carries the load, which is why the concave welding symbol matters more than most people realize.

Fillet Weld Geometry: Leg Length vs Theoretical Throat

Two measurements drive everything for an equal-leg fillet weld. The first is the leg length, noted as “z”: the distance from the root to the toe, measured along each of the two base metals being joined. The second, and the one that governs design strength, is the throat, the shortest distance from the root of the weld to the face of the weld.

With a perfectly flat face, that cross-section forms a right isosceles triangle, so the theoretical throat (a) comes from a constant multiplier. As defined by AWS A2.4 and AWS D1.1, the theoretical throat for a flat-faced equal-leg fillet is:

A 10 mm leg therefore gives about 7.07 mm of theoretical throat. In imperial, a 3/8 in. (0.375 in.) leg equates to roughly 0.265 in. Engineers use these figures to calculate the load capacity of a joint, but the calculation only holds for a flat face. A concave one breaks it.

How Concavity Shrinks the Actual Throat

A concave face curves inward toward the root, forming a dish-like profile. Where a flat fillet gives you a straight hypotenuse, that inward arc cuts into the weld’s cross-section, pulling the actual measured throat below 0.707 × z even when your leg lengths still hit their specified size.

Technique therefore matters as much as math. Clean puddle control from MIG welding or from choosing the correct electrodes helps keep the face profile where you want it. But no process excuses the requirement: if the symbol calls for a given throat, the concave profile has to deliver it. As AWS standards make clear, the effective throat is what determines acceptability, not how “pretty” the bead looks.

Worked Example #1: Metric Breakdown

A drawing calls for a 10 mm fillet weld with a minimum required throat of 7.0 mm. A flat 10 mm leg delivers 7.07 mm, so it passes easily. Deposit that same weld with a moderate concave face and the measured throat drops to 6.85 mm. The legs still read 10 mm on an inscribed circle gauge, yet the throat sits under the required 7.0 mm, making the weld undersized and a rejection at inspection.

Worked Example #2: Imperial Reality Check

Now the scenario that haunts many shops: a 3/8 in. fillet specified to achieve a required throat of 0.265 in. Theoretically, a flat 3/8 in. leg produces exactly that. If that weld is severely concave, inspectors can measure the actual throat at 0.250 in. or less. A 0.015 in. drop may not be visible to the naked eye, but it lands well under the required value.

On critical applications such as pipe welding procedures, losses that small carry real consequences. This is why inspectors don’t trust leg measurements alone on a concave profile; they measure the throat, or verify with the inscribed circle method, in accordance with the code.

Why the Required Throat, Not the Leg, Is What Matters

A welded connection is designed around stress transfer, and the throat is the effective cross-sectional area resisting shear and tensile forces. Drawings therefore almost always specify the required effective throat, or a weld size that correlates to it. Leg size is a fabrication target; the required throat is the non-negotiable acceptance criterion.

The symbol itself hints at this. Under AWS conventions, a number placed before the fillet symbol denotes leg size, while the same number in parentheses denotes effective throat. If your drawing shows parentheses, the throat, not the leg, is the number you have to hit.

Sometimes that means deliberately running larger legs. If the joint design calls for a concave contour, or your technique naturally leaves a slight dish, adding leg size is common practice so the effective throat still meets or exceeds the code minimum after the inward curve takes its bite. Watching for welding defects undercut and profile matters just as much as holding a proper travel angle welding technique.

Concave Groove Welds: Reinforcement Limits and Underfill

The concave arc does not belong only to fillet welds. Above a groove symbol it still describes shape, but it starts driving a different set of numbers. On a butt, V-groove, or corner joint, a concave contour symbol tells everyone on the job that the finished face must curve inward, sitting flush with or slightly below the surrounding plate surface instead of crowning above it. That one instruction changes how the joint is run, whether it gets ground, and exactly where the inspector puts the gauge.

What the Arc Means on a Butt or Groove Weld

Reinforcement is simply the weld metal standing proud of the base metal surface, and on a structural butt weld it is not a defect on its own. Groove welds may be welded flat, ground flush, or left with a slight crown, and visual acceptance criteria for structural work generally allow groove weld faces to be convex, flat, or concave as long as the reinforcement and underfill limits in the governing code are respected and the weld still meets its size requirement. The NRC’s reviewed visual weld acceptance criteria restate the same idea for both fillet and groove faces.

A concave contour symbol tightens that freedom. It says the face is not merely permitted to be dished, it is specified to be dished. Where no contour symbol appears, the welder runs the joint and leaves the face roughly as it falls, and the inspector checks it against generic profile limits. Where the arc appears, the finished profile becomes a contractual requirement checked against that symbol, not against the welder’s judgment.

Reinforcement Limits and Why They Exist

Capping reinforcement is not about looks. A tall crown at the weld toe creates a sharp change in geometry, and that notch is where fatigue cracks like to start. AWS’s welding symbol chart places the concave arc directly above the basic groove symbol to signal a reduced or eliminated crown. In practice, AWS D1.1 has historically limited groove weld reinforcement to roughly 1/16 in. (1.6 mm) for most steels, about the thickness of a dime, while underfill is held to small fractions of the plate thickness. Pipe and pressure-part work follows similar logic through the National Board’s visual acceptance guidance. On thin sheet, where appearance and heat control both matter, a specified concave or zero-reinforcement butt weld is common enough to be covered in detail in The Complete Guide to TIG Welding Sheet Metal.

Underfill: Where Concavity Becomes a Reject

Here is the distinction that trips people up. Intentional concavity is a specified profile. Underfill, also called an incompletely filled groove, happens when weld metal sits below the base metal surface beyond the permitted limit, so the groove is simply not full. One is a shape the engineer asked for; the other is a missing-volume defect that thins the joint and leaves a notch at each toe. Underfill is a rejection condition on essentially every structural code, and it is the number-one reason a groove weld with a concave callout comes back from inspection.

Underfill usually traces back to too few passes, too little filler for the groove volume, or a travel speed chosen to beat heat input. Filler-heavy processes and careful pass sequencing keep the groove full on the root side, which is why pass planning shows up so prominently in pipe welding procedures. Sharp concavity at the toe is a second reject path, because it behaves like undercut for fatigue purposes, the same notch mechanism described in Welding Defects Undercut.

Single-V, Double-V, and Corner Joints

The contour symbol applies only to the side of the symbol where it is drawn. On a single-V butt weld, one arc above the V glyph controls the face on that side of the joint. On a double-V groove, the joint has two finished faces, so each side needs its own symbol and its own contour mark; if only one side carries the arc, only that face is specified as concave. Corner joints behave the same way, with the arc telling the welder which visible face must end up dished or flush.

The most common drafting mistake is assuming the arc modifies the whole joint automatically, then getting an unexpected profile on the far side during fit-up inspection. The second is adding the arc without writing a note when a specific throat or face condition matters. Since ISO 2553 handles contour marks with its own side conventions, mixing an ISO-drawn groove detail into an AWS-controlled package is another reliable way to end up with the wrong face finished concave.

How to Draw and Specify the Concave Symbol Correctly (With Worked Drawing Examples)

Specifying a concave face correctly can be the difference between an approved part and a rejected one. While reading the symbol is important, drafting it incorrectly can create confusion that leads to scrap and rework. Once you learn the rules, however, it becomes a simple, repeatable process.

The Golden Rule: The Arc Always Sits Above the Basic Symbol

The most important rule is also the most frequent mistake: the concave contour arc is always drawn above the basic weld symbol, never below. This holds true for both arrow-side and other-side welds, as the arc is a contour instruction and not tied to which side the weld is placed on.

This placement is outlined by the AWS welding symbol chart and falls under the requirements of AWS A2.4:2020, Standard Symbols for Welding, Brazing, and Nondestructive Examination.

Layering the Full Callout

To avoid missing modifiers, build the symbol in this order:

1. Reference line and arrow: Point the arrow directly to the joint to be welded.
2. Basic weld symbol: Place it below the line for arrow-side welds, or above the line for other-side welds.
3. Contour arc: Add the concave arc above the basic weld symbol. Its position never flips with the weld.
4. Finish letter: If needed, add a letter such as G for grinding or C for chipping beside the arc.
5. Size, length, and pitch: Place these to the left of the symbol (leg size first, then segment length and pitch for intermittent welds).
6. Additional modifiers: Add a weld-all-around circle at the arrow and reference line junction, or a field weld flag at the open end of the reference line when required.

These additions don’t change the concave requirement. For a broader breakdown of the language, the Miller guide to reading weld symbols is a solid companion reference.

Worked Example 1: Concave All-Around Fillet on a Bracket

For a gusset that must be welded around its entire perimeter with a dished face for fatigue resistance, the callout would be a 1/4-inch leg fillet with a concave arc above it and an all-around circle.

A welder reading this knows exactly what to produce. If grinding is required, a G can be placed beside the arc. Before running the job, it’s worth dialing in your parameters using this MIG welder setup guide to get as close to the profile as possible with minimal material removal.

Worked Example 2: Concave Groove Butt Weld

For a single-V butt weld that needs a concave face with little to no reinforcement, the V-groove symbol goes below the reference line on the arrow side, with the concave arc stacked above it. Groove preparation dimensions (such as root opening and groove angle) can be added in the tail or as a note, and a G beside the arc will specify grinding.

This is especially common for sanitary tubing and pressure vessels. For process-specific execution, this pipe welding procedures guide covers WPS, positions, and passes in depth.

Worked Example 3: Other-Side Concave Fillet on a Lap Joint

In a lap joint, if the fillet goes on the far side, the fillet symbol moves above the reference line. However, the concave arc still sits above the fillet symbol, not below the line. This is where most beginners mirror it by mistake.

To help lock in your settings from the start, check out this MIG tack weld guide. Small adjustments at the tacking stage go a long way towards hitting a clean, concave profile.

Common Drafting Mistakes to Avoid

• Arc below the reference line: This is the most common error, rendering the contour callout ambiguous or reading as convex.
• Mirroring the arc with the weld symbol: The weld symbol flips sides, but the arc does not.
• Confusing concave with flush: A straight line denotes a flush contour, not the inward curve of a concave profile.
• Skipping the finish letter: If the profile is critical, call out how it’s produced to prevent guesswork.
• Ignoring required throat: When strength depends on it, add a fabrication note to ensure it’s measured.
• Mixing ISO and AWS conventions: ISO uses a dashed identification line and different symbol placement. Dropping an AWS-style arc onto an ISO layout can contradict the standard. The ANSI breakdown of the A2.4:2020 changes is worth reading before working with shared drawings.

Get the arc in the right place, pair it with the correct size and finish callouts, and your symbol will do its job: clearly specifying a concave face, produced by a stated method and measured against a stated throat.

AWS A2.4 vs ISO 2553: How the Concave Contour Differs Between Standards

The concave contour arc looks nearly identical in both systems: a small inward-curving arc sitting on top of the basic weld symbol, telling the welder to finish the face dished rather than flat or bulged. What changes is everything around it. AWS A2.4, the American Welding Society standard used across US drawings, and ISO 2553, used through most of Europe and the rest of the world, describe the same finished weld but arrange the symbol differently. And in this trade, arrangement is meaning. Get the scaffolding wrong and you will weld a perfectly concave bead on the wrong face of the joint.

Reference Lines: One Solid Line in AWS, a Dashed Identification Line in ISO

AWS A2.4 uses a single solid reference line. Weld symbols placed below the reference line are arrow-side welds; symbols placed above it are other-side welds. There is no dashed line and no ambiguity, which is why the official AWS weld symbol chart reads cleanly once you know the one rule: below means the side the arrow points to.

ISO 2553 solves the same problem a different way. When a single reference line is used, ISO adds an identification line, a dashed line drawn parallel to it, to resolve which side of the joint the symbols apply to. The 2019 revision documents this arrangement in Figure A.1 and the related annexes, which you can see in the ISO 2553:2019 preview. ISO also permits an older convention of two stacked reference lines instead of the dashed identification line. In either variant, the concave arc sits exactly where it always did, directly on top of the basic weld symbol, so the contour itself is never in doubt. Only the side is.

Why Mixing the Two Standards Flips the Weld

Because ISO and AWS assign opposite meanings to the same physical space on the drawing, a reader trained in one system can invert the other. A fillet symbol above the reference line means “other side” in AWS. Read through ISO habits, with an identification line present, that same line can resolve to the arrow side instead. The result is not subtle: the concave fillet lands on the wrong face of the lap joint, the throat is measured in the wrong place, and the assembly fails fit-up or inspection.

This shows up most on international projects, where structural steel, pressure equipment, and piping packages travel across borders and pipe welding procedures carry their drawings with them. A comparative guide by Gregory and Armstrong walks through weld symbol application under BS EN ISO 2553 and ANSI/AWS A2.4 side by side, confirming how easily the two conventions cross wires.

Practical Advice for Shops Working to Both Standards

• Declare the standard on every drawing. A note reading “Weld symbols per AWS A2.4” or “Weld symbols per ISO 2553” removes the guesswork before the first arc is struck.
• Keep both charts posted at the workstation. The AWS A2.4:2020 preview and the ISO 2553 annex figures are worth pinning side by side, next to references your operators already use, such as the settings in how to set up a MIG welder.
• Resolve side-of-joint questions at kickoff, not at fit-up. Walk the drawing with the fitter and point physically at the arrow. If a concave fillet is called out, confirm which face gets the dished profile before tacking.
• Train the reader, not just the drafter. The Miller guide to reading weld symbols and the ITW/Miller weld symbol chart make good refresher material for welders who learned one system only.
• Flag the drawing when it arrives. A print showing a dashed line but carrying an AWS title block is a query, not a shop-floor guess. Correcting a question mark on paper costs a phone call; correcting it in metal costs a plasma cut and a re-weld.
• Verify with a known reference joint. Before production, weld one sample to the drawing and have the inspector confirm side, contour, and throat. It is the cheapest insurance against a standards mix-up.

The bottom line: the concave arc means the same thing in both languages, a finished face that curves inward, but the frame around it decides which joint member it belongs to. Read the frame correctly and the symbol is unambiguous. Ignore it and you will make the most expensive mistake in welding: a flawless concave weld in the wrong place.

Inspecting a Concave Weld: Measurement Methods, Acceptance Criteria, and Common Mistakes

After the arc is struck and the bead laid, the final test for any concave weld is inspection, and nowhere is that more important than in structural, pressure, or cyclically loaded work. A concave profile can look smooth, blended, and perfectly finished while sitting well below the minimum throat the engineer specified. Verification is not optional.

The Goal of Inspecting Concave Welds

The objective is straightforward: prove the weld still has the strength the design assumes. Because concavity reduces the effective throat of a fillet or groove weld, the inspector must confirm that throat still meets the minimum required size. Acceptance criteria in AWS A2.4 and AWS D1.1 permit a concave fillet face so long as the minimum weld size is met; otherwise the weld is undersize and rejectable.

Safety frames every decision. Welders in the field rely on proper PPE to protect themselves while welding, but inspection protects the end user. Whether the work is critical piping systems or a specialized process such as railroad thermite welding, one undersized throat can be the difference between a sound joint and a catastrophic failure.

Visual Inspection: The First Line of Defense

Before any gauge comes out, a trained eye starts with visual inspection, the cheapest and most effective screen for immediate red flags: cracks, porosity, overlap, undercut. On concave welds the priority is profile and toe blending. A sharp, abrupt transition from weld face to base metal creates a stress riser that accelerates fatigue cracking, which is why codes limit toe geometry in cyclically loaded structures. On groove welds, underfill beyond code limits is a rejectable condition in its own right, distinct from a specified concave profile, a distinction the National Board of Boiler and Pressure Vessel Inspectors reinforces when referencing groove weld acceptance.

Weld Gauges: The Most Common Field Tool

For most shops and job sites, the AWS fillet weld gauge set is the first tool off the belt. These compact gauges compare the leg of a fillet weld to the size called out on the drawing, and combination blades also check reinforcement and throat.

Concave faces need extra care. Many gauges use the inscribed circle method, which reads the leg along the hypotenuse formed by the two legs. Because the face curves inward, the gauge must sit flush on the actual legs rather than bridging over a dished center, or the reading will be optimistic. Clean, consistent readings also matter when dialing in parameters for processes like MIG or TIG welding sheet metal.

Throat Measurement: The True Test of Strength

Leg size is convenient, but the effective throat is what the design actually assumes. For an equal-leg fillet with a flat face, the theoretical throat equals 0.707 times the leg length; concavity pulls the actual throat below that value, sometimes drastically. Inspectors use a throat gauge or a dedicated fillet gauge blade to read it directly, and for deeper or irregular dishes, a radius gauge paired with profile templates pinpoints the minimum throat. That precision matters on heavily loaded joints made with specific filler electrodes, such as the settings covered for 3/32 E7018 amperage and 5/32 E7018 amperage.

Advanced Methods: Profilometers, Templates, and Critical Joints

For the most critical welds, gauges alone may not be enough. A profilometer traces the exact contour of the face, quantifying its radius and minimum throat, while physical templates cut to the allowable profile expose excessive concavity or a sharp toe transition. You will see this level of scrutiny under ISO 2553 structural work and anywhere failure is expensive, from precision sheet metal TIG to heavy pipe welding procedures. Scrutiny always scales with the consequence of failure.

Common Field Mistakes to Avoid

Even experienced inspectors slip up. The most common errors involving a concave welding symbol are:

• Confusing concave with flush: a flush face keeps its theoretical throat, while a dished face actively reduces it.
• Ignoring the required throat: never default to leg measurements; check the print and verify the throat when it is called out.
• Skipping the finish method: if the symbol calls for grinding or machining to achieve the contour, the final profile must match that requirement, not an arbitrary dish.
• Reading ISO drawings as AWS: as ANSI’s overview of AWS A2.4:2020 and Miller Electric’s guide to weld symbols show, notation and reference-line conventions differ sharply between the two standards, inviting misreads.
• Overlooking the toe: accepting a sharp concave toe transition is a fatigue death sentence for cyclically loaded parts; look for a smooth blend.

Final Takeaways

Inspection proves performance, not appearance. Start with a diligent visual check, verify leg size with a proper gauge, and always measure the effective throat when concavity is present, escalating to profilometers or templates for critical joints. The golden rule: the concave arc is a strength specification, not a cosmetic request. Uphold that and you prevent rework, stay code compliant, and keep the finished structure safe for years.

When to Specify a Concave Weld (and When Not To): Practical Tips and Takeaways

By now you know exactly what that little arc above the fillet triangle is asking for. The remaining question is whether you should ask for it at all. The concave welding symbol is a specialized tool: it buys you a smooth weld-toe transition and a surface with nowhere for crud to hide, and it costs you throat. Good detailing is simply knowing which side of that trade you are on.

Where Concave Is the Right Call

Specify a concave contour when the joint’s real problem is stress concentration or cleanliness rather than raw load capacity:

• Fatigue-loaded joints. A dished face blends the weld into the base metal instead of leaving a sharp shoulder at the toe. That gentle transition lowers the stress concentration where fatigue cracks almost always start, which is why concave profiles are preferred in cyclically loaded fabrications, vehicle frames, and mobile equipment.
• Sanitary and hygienic welds. Food, beverage, pharmaceutical, and vacuum service cannot afford crevices. A concave or flush profile eliminates pockets where product collects and bacteria grow, and it cleans easily. This same smoothness matters inside process piping, as covered in Pipe Welding Procedures: The Complete Guide to WPS, Positions, Passes & Inspection.
• Cosmetic, visible welds. Stainless panels, architectural trim, and consumer-facing enclosures are judged by eye. A controlled concave bead on thin material is the look clients expect; the settings and technique behind it are covered in The Complete Guide to TIG Welding Sheet Metal.
• Tubular and pipe joints. Smooth internal profiles keep flow characteristics clean and prevent turbulence and erosion at the joint, and they are far easier to pass on internal visual inspection.
• Low-load and thin-sheet fillets where the required throat is small and a slightly dished face still leaves plenty of metal in the design.

When Concave Is the Wrong Answer

Avoid it whenever the design is throat-limited. On heavy structural work, high-load fillets, and dynamically loaded corner joints, the engineer sized the weld around a required effective throat — and a concave face makes the actual throat smaller than the theoretical value. The fix is more passes and a bigger leg, not a prettier profile.

Also reject concave profiles with a sharp, undercut-looking root at the toe; that notch behaves exactly like a crack initiator. Sharp toe conditions and their acceptance limits are treated in depth in Welding Defects Undercut: Causes, Acceptance Limits, and How to Fix It. And remember that position work makes profiles harder to control — running overhead is a different skill set, as Overhead Stick Welding: The Complete Guide to 4F & 4G Technique explains.

How to Call Out the Required Throat So Nobody Has to Guess

The single best habit a designer can build is pairing the concave contour symbol with a stated required effective throat, either beside the size field or in a drawing note. Something like: “ALL FILLET WELDS AS NOTED; REQUIRED EFFECTIVE THROAT 5 mm UNLESS OTHERWISE SHOWN; FINISHED PROFILE CONCAVE.” With that note in hand, the fabricator works backward from the required throat to a leg large enough that the dished face still leaves enough metal. For a flat-faced fillet the theoretical throat is roughly 0.707 times the leg, so a concave face means the leg must be noticeably larger than throat ÷ 0.707. Without the note, the welder guesses, the shop runs the smallest leg that “looks right,” and you end up with the 400 rejected brackets from our opening story.

Checklist: Reading and Drawing the Concave Symbol Correctly

1. Confirm which standard the drawing uses — AWS A2.4 or ISO 2553 — before interpreting anything.
2. Find the arc above the basic weld symbol; arc curving inward toward the symbol means concave.
3. Check which side of the reference line the symbol sits on: arrow side or other side.
4. Read size, length, and pitch fields, then add the all-around circle or field-weld flag if present.
5. Look for a required-throat note and size the leg to satisfy it after concavity.
6. Check for a finish-method note (grind, machine, hammer-peen) if the contour must be produced by material removal.
7. Verify on the first article with a fillet gauge before running production quantity.
8. Never substitute flush or convex because it is easier to run; the symbol, not the welder’s preference, controls the finished profile — a point both the National Board and AWS training materials make explicitly.

Summary and Next Steps

The concave contour symbol is a small mark with a real engineering consequence: a smoother toe, a cleaner surface, and a thinner throat. Use it deliberately for fatigue, hygiene, and appearance; stay away from it where the design counts on metal you would be removing. The cheapest insurance against contour-related rework is three steps: put a current AWS weld symbol chart on every drawing board and machine, own a copy of the governing AWS A2.4:2020 standard, and train the shop against them until every hand on the floor can explain what that arc demands — and measure it with a gauge instead of an opinion.

Frequently Asked Questions

What does a concave welding symbol mean?

It is a curved-inward arc drawn above the basic weld symbol that instructs the welder to finish the weld face with a concave (dished) profile. It specifies the finished shape of the weld face, not the process or the filler metal.

Where is the concave arc placed on a welding symbol?

The arc sits directly on top of the basic weld symbol, above the reference line region for that weld. It stays above the basic symbol whether the weld is on the arrow side (symbol below the line) or the other side (symbol above the line).

Does a concave fillet weld make the joint weaker?

It can, because a dished face reduces the actual throat below the theoretical throat (0.707 x leg for an equal-leg fillet). The joint is acceptable only if the required effective throat is still achieved, so fabricators often increase the leg size to compensate.

What is the difference between concave, convex, and flat weld contours?

Concave is an inward curve, convex is an outward bulge, and flat (or mitre) is a straight face. Each is specified with its own contour symbol above the basic weld symbol; AWS D1.1 permits all three for fillet faces provided weld size requirements are met.

Can a concavity on a weld be rejected even if it looks smooth?

Yes. If the concavity reduces the weld below its required size or causes underfill in a groove weld, it fails inspection regardless of appearance. Smooth-looking concave welds are rejected as undersize when the measured throat falls short.

How is a concave weld measured during inspection?

Inspectors use weld gauges and the inscribed-circle method to measure leg length, and they measure the throat directly or derive it from the profile. The measurement is compared against the required size shown on the drawing, not against a visual impression.

How does ISO 2553 handle the concave contour differently from AWS A2.4?

Both systems place the contour mark with the basic weld symbol, but ISO uses an identification (dashed) reference line to clarify which side of the joint the weld applies to. Reading an ISO drawing with AWS conventions can flip the weld to the wrong face of the joint.

What changed for contour symbols in AWS A2.4:2020?

The 2020 edition allows the specified contour to be produced by any method, whether welding or mechanical finishing, and it limits the flat contour symbol to fillet welds. Always confirm which edition your drawing set cites.

When should a designer call for a concave weld?

Use it for fatigue-critical joints, sanitary or hygienic welds, tubular connections, and visible cosmetic welds where a smooth toe transition matters. Avoid it on throat-limited heavy structural fillets unless the required throat is explicitly noted.