How much can a fillet weld be undersized?

Not at all — the leg must not be smaller than specified. It may be up to 1/16 in (2 mm) oversize on a square edge, but any undersize reduces the joint's capacity in direct proportion. Length may be up to 1-1/2 in short per weld.

Is a bigger weld stronger?

Only up to the specified size. Capacity comes from the throat, which is 0.707 times the leg, so making the weld larger than specified does add capacity — but the design already accounts for the specified size, and excess convexity that does not increase the throat adds nothing while creating a stress concentration at the toe.

What is the maximum undercut allowed?

1/32 in (1 mm) deep on primary members regardless of length, and 1/16 in (2 mm) on secondary members with an additional limit on cumulative length. Undercut deeper than 1/32 in on a primary member must be repaired.

Why is excess weld reinforcement a defect?

Because it does not increase the throat — the dimension that carries the load — while creating a sharper transition at the weld toe. That sharper angle raises the stress concentration exactly where fatigue cracks initiate, so excess reinforcement makes a cyclically loaded joint worse, not better.

Are arc strikes acceptable?

No. An accidental arc strike leaves a locally hardened, crack-prone region in the base metal and is treated as a discontinuity. It must be ground out to sound metal, and the area inspected afterwards — the repair is often larger than the strike itself.

Weld Size and Tolerance

Twelve acceptance criteria for weld size, profile and discontinuities, with why each limit exists rather than only what it is.

Data verified 2026-09-29 · based on AWS D1.1/D1.1M:2020

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Weld Size and Profile Acceptance Criteria (AWS D1.1)

Criterion^[1]	Limit^[1]	Consequence^[1]	Why the limit exists^[1]
Fillet weld leg size — minimum #	Not less than the specified leg	Undersize reduces capacity directly	A fillet weld smaller than specified is a strength shortfall, not a cosmetic defect. It is the most common weld sizing nonconformance.
Fillet weld leg size — maximum #	Not more than 1/16 in (2 mm) oversize on a square edge	Oversize is permitted up to a limit	The limit exists because the leg cannot overhang the edge of the thinner part by more than that.
Fillet weld length — minimum #	At least four times the nominal leg size	Short welds may not achieve capacity	The ends of a fillet weld are less effective than the middle because of the stress concentration at the termination.
Fillet weld length — undersize #	Not more than 1-1/2 in (38 mm) shorter than specified per weld	A limited length shortfall is permitted	Based on a length allowance rather than a percentage, which is why long welds have a proportionally tighter requirement.
Fillet weld profile — convexity #	Not more than 1/8 in (3 mm) convex	Excess convexity is a defect, not extra strength	A convex weld is not stronger than a flat one — the capacity comes from the throat, and excess metal above it adds weight and stress concentration without adding strength.
Fillet weld profile — concavity #	Not more than 1/16 in (2 mm) concave, and never below the throat	Concavity reduces throat and is limited	The critical measure is the actual throat. A concave weld measuring below the required throat fails regardless of the leg size.
Groove weld reinforcement #	Not more than 1/8 in (3 mm) above the base metal surface	Caps the reinforcement height	Excess reinforcement adds a stress concentration at the weld toe without increasing capacity.
Undercut — primary members #	Not more than 1/32 in (1 mm) deep, any length	Tighter limit on fatigue-critical members	Undercut is a stress raiser at the toe, which is where fatigue cracks start. Primary members carrying cyclic load have the tightest limits.
Undercut — secondary members #	Not more than 1/16 in (2 mm) deep, and not more than 1/8 in in any 12 in	Looser limit off the primary load path	Undercut exceeding 1/32 in deep is required to be repaired on primary members regardless of length.
Porosity #	Visual inspection: no visible porosity; not more than 3/8 in in any linear inch	Size and frequency limits	Acceptance criteria for porosity differ between visual, ultrasonic and radiographic methods, and depend on the weld's criticality.
Crater cracks #	Not permitted	Craters must be filled to full cross-section	A crater crack is a crack, and cracks are never acceptable in a structural weld.
Arc strikes #	Not permitted on the base metal	Arc strikes are discontinuities and stress raisers	An accidental arc strike leaves a hardened, crack-prone spot. It must be removed by grinding to sound metal.

Weld capacity comes from the throat, not from the visible size. A fillet weld's throat is 0.707 × leg, so the leg is 41% larger than the dimension that actually carries the load. That is why an undersized leg is serious — a 10% shortfall in leg is a 10% shortfall in capacity.

It also explains why excess convexity is a defect rather than a bonus. Adding metal above the throat does not increase the throat, so it adds nothing to strength — and it creates a sharper transition to the base metal at the toe, which is where fatigue cracks initiate. A neatly flat or slightly concave weld is better on both counts.

The limits shown are typical AWS D1.1 criteria for statically loaded structures. Cyclically loaded structures have tighter limits, particularly on undercut and profile, and the applicable code section governs. Where a weld is critical, the acceptance criteria should be taken from the code edition the contract specifies rather than from a summary table.

The Weld Defects That Actually Matter

Weld inspection finds many discontinuities, and only some of them affect the joint's ability to do its job. Ranking them by consequence rather than by how easy they are to see is the useful discipline.

Cracks are never acceptable. They propagate under load and a cracked weld fails without warning. Crater cracks, toe cracks and hydrogen cracks all fall here, and all require repair.

Undersize is the most common real nonconformance and the most direct — the joint simply has less capacity than the drawing specifies. It is also the easiest to miss visually, because a slightly undersized weld looks like a weld.

Undercut matters where the joint will be cycled. Undercut is a stress raiser at the toe, and fatigue cracks start exactly there. On a statically loaded structure it is largely cosmetic; on a cyclically loaded one it governs fatigue life.

Porosity and slag inclusions reduce the effective throat but are distributed rather than sharp, so they are less severe than their size suggests. Acceptance limits are correspondingly more generous than for cracks.

Excess convexity and reinforcement are cosmetic in the sense that they do not reduce capacity — but they indicate poor technique, and they create a stress concentration at the toe. A welder who produces consistently convex welds is likely to produce other problems too.

Frequently Asked Questions

How much can a fillet weld be undersized?: Not at all — the leg must not be smaller than specified. It may be up to 1/16 in (2 mm) oversize on a square edge, but any undersize reduces the joint's capacity in direct proportion. Length may be up to 1-1/2 in short per weld.
Is a bigger weld stronger?: Only up to the specified size. Capacity comes from the throat, which is 0.707 times the leg, so making the weld larger than specified does add capacity — but the design already accounts for the specified size, and excess convexity that does not increase the throat adds nothing while creating a stress concentration at the toe.
What is the maximum undercut allowed?: 1/32 in (1 mm) deep on primary members regardless of length, and 1/16 in (2 mm) on secondary members with an additional limit on cumulative length. Undercut deeper than 1/32 in on a primary member must be repaired.
Why is excess weld reinforcement a defect?: Because it does not increase the throat — the dimension that carries the load — while creating a sharper transition at the weld toe. That sharper angle raises the stress concentration exactly where fatigue cracks initiate, so excess reinforcement makes a cyclically loaded joint worse, not better.
Are arc strikes acceptable?: No. An accidental arc strike leaves a locally hardened, crack-prone region in the base metal and is treated as a discontinuity. It must be ground out to sound metal, and the area inspected afterwards — the repair is often larger than the strike itself.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	AWS D1.1/D1.1M — Structural Welding Code, Steel	standard	AWS D1.1/D1.1M:2020
[2]	ISO 5817 — Quality levels for imperfections in fusion-welded joints	standard	ISO 5817:2023

Data Sources

Standard	Revision	What it covers on this page
AWS D1.1/D1.1M — Structural Welding Code, Steel	AWS D1.1/D1.1M:2020	the acceptance criteria for size, profile and discontinuities
ISO 5817 — Quality levels for imperfections in fusion-welded joints	ISO 5817:2023	the ISO quality-level system referenced in the comparison
AWS A3.0 — Standard Welding Terms and Definitions	AWS A3.0M/A3.0:2020	the terminology used in the criteria

Cross-checked against:

Every limit checked against the AWS D1.1 acceptance criteria for statically loaded structures
The throat relationship verified — throat = 0.707 × leg, matching the fillet weld size calculator on this site
Repair requirements cross-checked against the code's rejection criteria rather than inferred

Limits are typical AWS D1.1 criteria for statically loaded structures. Cyclically loaded structures have tighter limits, particularly on undercut and profile, and the contract-specified code edition governs. Where a weld is critical, take the acceptance criteria from that code rather than from this summary.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Weld Size Tolerance Chart." https://refmanual.com/charts/weld-size-tolerance/ (accessed 2026-09-29). Data from AWS D1.1/D1.1M — Structural Welding Code, Steel, AWS D1.1/D1.1M:2020.

BibTeX

@misc{weld-size-tolerance,
  title        = {Weld Size Tolerance Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/weld-size-tolerance/}},
  note         = {Accessed: 2026-09-29},
  source       = {AWS D1.1/D1.1M — Structural Welding Code, Steel, AWS D1.1/D1.1M:2020}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Weld Size Tolerance Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/weld-size-tolerance/
PY  - 2026
DA  - 2026-09-29
N1  - Source: AWS D1.1/D1.1M — Structural Welding Code, Steel, AWS D1.1/D1.1M:2020
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.