Fillet Weld Undersize: AWS D1.1:2025 Acceptance Criteria

Fillet weld size deficiency — the weld is smaller than what the drawing calls for — is one of the most common visual inspection findings in structural fabrication. It is also one of the most frequently handled incorrectly. CWIs who default to "mark it and repair it" on any weld that gauges short are creating unnecessary rework; CWIs who accept any slight shortfall without checking the tolerance rules are missing real defects. AWS D1.1:2025 is specific about where the line is.

The Specified Size vs. the Minimum Required Size

Before applying the underrun tolerance, a CWI needs to know two things:

  1. What is the specified fillet weld size? This comes from the structural drawing — usually a weld symbol with a size callout, or a general note. It reflects the design engineer's shear flow demand calculation.

  2. What is the minimum allowable fillet weld size per AWS D1.1:2025? This is a code floor based on the thicker part being joined. It has nothing to do with strength calculations — it exists to ensure adequate heat input for fusion.

A fillet weld can be oversized relative to the minimum required size but undersized relative to the specified size. The acceptance check must address both simultaneously.

Rule library based on AWS D1.1:2025; verify against your governing edition.

The Tolerance: What AWS D1.1:2025 Actually Allows

AWS D1.1:2025 permits fillet weld size underrun under specific conditions:

  • For fillet welds with a specified size ≥ 5/16 in. (8 mm): a maximum underrun of 1/16 in. (1.6 mm) from the specified size is acceptable, provided the undersize condition does not extend over more than 10% of the weld length.
  • For fillet welds with a specified size < 5/16 in.: the full specified size must be maintained along the entire length — no underrun tolerance applies to these smaller welds.

The 10% length limitation is important and often overlooked. A 10-foot fillet weld with 1/16 in. undersize at one 6-inch segment passes. The same weld with 1/16 in. undersize over 18 inches of its length fails the length criterion and requires repair.

These are visual acceptance criteria. They apply after the weld has cooled and slag has been cleaned from SMAW or FCAW passes. Measurement must be taken at representative points along the weld length, not just at the most accessible location.

How to Measure: Fillet Weld Gauges

Three gauge types are commonly used by CWIs for fillet weld size measurement:

Bridge cam gauge (most common for structural work): Measures leg length directly. The gauge bridges the toe of the weld and reads leg size on a calibrated scale. Available in sets covering 1/8 in. through 1-1/2 in. range. Best for equal-leg fillet welds.

Fillet weld gauge set (go/no-go): Individual gauges for each standard size (3/16, 1/4, 5/16, 3/8, etc.). The gauge for the specified size fits flush against a conforming weld. Faster than the bridge cam for pass/fail at a known size.

Combination weld gauge: Multi-function gauge that reads fillet leg size, reinforcement height, undercut depth, and weld angle. Useful for CWIs who need one tool for multiple measurements.

All gauges must be calibrated and in serviceable condition. A bent or corroded fillet gauge reads inaccurately. Many QA programs require gauge calibration records to be available during inspection.

For unequal-leg fillet welds, leg size alone does not confirm throat dimension. The effective throat for design purposes is the perpendicular distance from the root to the face, calculated as the shortest distance through the weld cross-section. If the weld legs are 3/8 in. and 5/8 in. due to unequal fusion on the two base metal faces, the throat is not 0.707 × average leg length. The CWI must note unequal-leg welds and the EOR should confirm the throat meets the structural requirement.

Underrun vs. Undercut: Different Defects, Different Rules

New CWIs sometimes confuse fillet weld undersize (the overall cross-sectional dimensions are smaller than specified) with undercut (a groove melted into the base metal along the weld toe that reduces the base metal section). These are two separate visual acceptance items with separate acceptance criteria in AWS D1.1:2025.

Underrun (undersize): The weld itself is geometrically smaller — the leg length is shorter than the specified dimension. The weld metal is where it should be but insufficient in volume. Correction is by additional weld metal deposition.

Undercut: A notch cut into the base metal along the weld toe by the arc. The base metal cross-section is reduced, creating a stress concentration. AWS D1.1:2025 has separate undercut depth acceptance limits (generally 1/32 in. maximum for transverse welds in tension zones). Undercut acceptance criteria are separate from the fillet size tolerance.

Both may be present simultaneously. A fillet weld can be undersized AND have undercut along the same toe. Each finding is evaluated independently against its own criterion.

Documentation: Recording Underrun Findings

When fillet weld undersize is found during inspection, the CWI's inspection report should record:

  • The joint identification (weld map reference, piece mark, weld number)
  • The specified fillet weld size
  • The measured size(s) at specific gauge points along the weld
  • The length of the undersize condition
  • The acceptance or rejection decision under AWS D1.1:2025

If the underrun is within tolerance (≤ 1/16 in. over ≤ 10% of length) and does not violate the minimum code size, mark it as "accepted within tolerance" with the measured values. This creates a documented record that the weld was evaluated, not just skipped.

If the underrun requires repair, the weld nonconformance report should describe the repair method (weld augmentation — additional passes deposited to the specified size), the WPS to be used for the repair, and the re-inspection result after repair.

The Repair: Weld Augmentation

An undersized fillet weld is repaired by augmentation: depositing additional weld metal on top of the existing passes until the specified size is met. Unlike a weld crack repair, augmentation does not require excavating the existing weld.

The augmentation passes must be made using a qualified WPS for the process and filler metal. Typically, the same WPS used for the original weld is used for the repair. If the original process is SMAW E7018 and the repair is FCAW-G, that is a process change — the FCAW-G WPS must be qualified for the same base metal and joint configuration.

The repaired area is re-gauged after cleaning. Both the augmented zone and a minimum of 2 inches on each side of the repair area should be re-inspected to confirm the size is now conforming.

Minimum Fillet Weld Size — the Floor Below the Tolerance

The underrun tolerance never excuses a weld that falls below the AWS D1.1:2025 minimum fillet weld size for the base metal thickness. Minimum fillet weld size is a heat input requirement, not a strength requirement — welding small fillets on thick plate with insufficient energy can produce an underheated HAZ susceptible to cracking.

If the specified fillet size on the drawing equals the code minimum for the base metal thickness, there is effectively zero underrun tolerance. The 1/16 in. allowance applies against the specified size, but the result must still meet the minimum — and at the minimum specified size, the 1/16 in. underrun would put you below the minimum.

This scenario is common on plate-to-plate connections where the EOR specified only the minimum required fillet. CWIs should be alert to joints where the drawing size matches the minimum table entry — the practical tolerance is zero on these joints.

Connecting Fillet Size to the WPS

The specified fillet weld size also needs to fall within the WPS's qualified range. A WPS qualified with a 5/16 in. single-pass fillet covers production single-pass fillets of 5/16 in. It does not automatically cover 1/2 in. multi-pass fillets on the same joint. If the shop is running larger fillet sizes than the WPS covers, that is a WPS essential variable issue separate from the visual acceptance question.

The CWI's pre-weld WPS review should confirm that the specified fillet size is within the WPS's qualified range before welding begins. Finding a size discrepancy after the fact creates both a WPS compliance issue and a potential rework situation. Addressing it pre-weld takes five minutes; addressing it post-weld takes much longer.

For shops managing fillet weld procedures across multiple base metal thicknesses and fillet sizes, keeping the WPS's qualified range explicitly documented — not implied or assumed — prevents the situation where the welder asks "is this covered?" and the answer isn't immediately clear.

Fillet weld undersize is a manageable inspection finding when the tolerance rules are applied correctly. The 1/16 in. with 10% length limitation is a practical code allowance for the normal variation in production welding. Applying it consistently, documenting it accurately, and requiring timely repair when it fails keeps the structural welding program in control.