Measuring Fillet Weld Size and Throat: AWS D1.1 CWI Field Guide

The fillet weld is the most common weld type in structural steel fabrication, and its measurement looks simple — run a gauge across the toe, read the leg size, compare to the WPS callout. But getting the measurement right requires understanding what each gauge type actually measures, where to measure on a non-flat weld face, and what the acceptance criteria in AWS D1.1:2025 actually permit.

This is the field measurement reference for CWIs performing visual inspection of fillet welds on structural steel under AWS D1.1:2025.

Leg Size vs. Throat: Which One the Code Cares About

The WPS specifies fillet weld size as a leg size — a "3/8 in. fillet weld" means 3/8 in. measured along each plate face from the theoretical weld root to the weld toe. This is what the shop welds to and what the CWI measures in the field.

The structural engineer sizes the connection based on the effective throat — the perpendicular distance from the weld root to the weld face, which determines the shear area carrying load. For a flat-face equal-leg fillet weld:

Theoretical throat = 0.707 × leg size

For a 3/8 in. fillet weld, theoretical throat = 0.707 × 3/8 in. = 0.265 in. ≈ 17/64 in.

AWS D1.1 credits only the theoretical throat in design. A convex weld face does not increase the credited throat — excess convexity adds weight and stress concentration but no structural benefit. A concave weld face reduces the actual throat below the theoretical throat — a 3/8 in. leg specification with a severely concave face may deliver less than the expected 0.265 in. throat, making the joint structurally deficient even if the leg size gauges correctly.

This means measuring only leg size is not always sufficient. Understanding both measurements — and when the actual throat matters — is the difference between a cursory inspection and an accurate one.

Gauge Types and What Each Measures

Notched Fillet Weld Gauge (Skewed-T)

The most common gauge on a structural CWI's belt. A flat metal plate with notches cut at standard fillet weld sizes — typically 3/16, 1/4, 5/16, 3/8, 7/16, 1/2 in. and sometimes larger. The gauge is placed flat against the plate surface on one side of the fillet, with the notch extended to the weld toe on the opposite plate.

If the weld toe falls within the notch profile, the leg meets or exceeds the specified size. If the toe runs past the notch edge, the weld is undersize at that location.

Best for: Quick go/no-go check on standard equal-leg fillets with flat or slightly convex profiles. Fastest measurement for production inspection.

Limitation: Measures leg size only. Does not evaluate throat directly. On convex welds, the leg-size reading is accurate, but the CWI still needs to evaluate whether convexity exceeds the code limit. On concave welds, the gauge may read the full specified leg while the throat is actually deficient.

Bridge Cam Gauge

A sliding-rod combination gauge that reads actual throat, undercut depth, and weld reinforcement height. The bridge contacts the weld face at two points (the toes) and the sliding rod extends to the deepest or highest point of the weld face. This measures the actual perpendicular distance from the face to the plane of the plate surfaces — the actual throat.

Best for: Measuring actual throat on convex or concave fillet welds. Verifying that convexity has not created a stress concentration at the weld toe, or that concavity has not reduced the actual throat below acceptable limits. Also used for undercut depth measurement.

Limitation: Takes more time to position correctly. The bridge must contact the weld toes cleanly without riding on spatter or edge irregularities. For highly distorted plates or wide toe spread, the measurement can be ambiguous.

AWS Standard Fillet Weld Gauge

A multi-purpose gauge with cutout profiles at standard sizes, used to check both leg size and general weld profile in one placement. Position the appropriate profile against the fillet cross-section and read the fit against the profile template.

Best for: Profile conformance check — leg size plus overall shape — when the weld profile is straightforward. Common on inspection hold-point checks where a quick multi-attribute reading is needed.

Measurement Technique

Before Measuring: Clean the Weld Area

Spatter, flux, and slag residue on the weld toes create apparent leg size readings larger than the true leg. Wire-brush the measurement location before gauging. Do not accept a weld size reading taken on a surface with visible spatter that could be lifting the gauge off the plate face.

Identifying the Weld Toe

The weld toe — the line where the weld metal meets the base metal surface — is the reference for leg size measurement. On welds with gradual blending or underfill at the toe, the true toe can be ambiguous. Use a magnifier and adequate lighting to identify the actual fusion line before placing the gauge. When in doubt, the measurement defaults to the most conservative (shortest leg) interpretation.

Where to Measure

AWS D1.1 acceptance criteria for weld size apply along the full length of the weld. Do not measure only at the most favorable-looking spot and report that as the weld size.

Minimum measurement frequency:

  • Beginning, middle, and end of each fillet weld
  • At every visible restart location or bead transition
  • Every 10 ft for continuous fillet welds on long structural members
  • Both legs of horizontal (2F) and overhead (4F) position fillets, where the upper and lower legs often differ

Record each measurement location and reading. Do not report only the minimum or average — report the actual measured values by location so the inspection record accurately reflects where the weld is at minimum size.

Reading Leg Size with a Notched Gauge

Place the flat reference face of the notched gauge on the plate surface on one side of the fillet. Extend the gauge to the weld toe on the other plate. Select the notch size matching the specified leg. If the weld toe falls at or inside the notch, the leg meets spec. If the toe extends past the notch edge, the leg is undersize.

For horizontal position fillet welds where the web leg (vertical plate) is typically longer than the flange leg (horizontal plate): measure both legs separately. A fillet weld that gauges correctly on the flange but is short on the web is a deficient weld despite appearing full-sized visually.

Checking Convexity

Place a straight edge across the fillet face from toe to toe. Measure the gap between the straight edge and the highest point of the weld crown using a taper gauge or the sliding rod of a bridge cam gauge. This is the convexity.

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

AWS D1.1 Table 7.1 sets maximum convexity based on weld leg size:

  • Weld leg 5/16 in. and under: maximum convexity = 1/16 in.
  • Weld leg over 5/16 in. through 1 in.: maximum convexity = 1/8 in.
  • Weld leg over 1 in.: maximum convexity = 3/16 in.

Excessive convexity must be corrected by grinding to the allowable profile. Do not add filler passes over a convex weld to reshape it — you would be burying the convexity under new weld metal rather than removing the excess. Excessive convexity concentrates stress at the weld toes under cyclic load and is a fatigue initiation site on dynamically loaded connections.

Checking Concavity

A concave fillet weld — where the weld face is depressed below the line between the two toes — reduces the actual throat below the theoretical throat. AWS D1.1 does not permit excessive concavity; the actual throat must equal at least the theoretical throat for the specified leg size.

To evaluate: use a bridge cam gauge to measure the actual throat from the root to the deepest point of the concave face. If this measured throat is less than 0.707 × the specified leg, the weld is deficient and must be built up with additional passes to restore the full throat dimension.

Concavity is most common in:

  • Overhead position (4F) fillet welds, where gravity pulls the molten pool away from the face
  • High travel speed, which produces a thin fast-cooling bead
  • Incorrect electrode angle in the horizontal (2F) position

Undersize Tolerance

AWS D1.1 Table 7.1 allows a fillet weld to be undersize by up to 1/16 in. for a cumulative length not exceeding 10% of the total weld length.

For a continuous fillet weld 30 in. long, 10% = 3 in. Up to 3 in. of total weld length may measure 1/16 in. under the specified leg size without rejection. Any single measurement more than 1/16 in. undersize requires immediate repair — additional weld passes to build up to the specified leg at the deficient location.

When multiple undersize locations are found along a weld, add up all the undersize lengths. If the sum exceeds 10% of the total weld length, the weld fails the cumulative acceptance criterion even if each individual undersize spot is only 1/16 in. short.

Additional Profile Checks During Measurement

While gauging leg size and throat, simultaneously check:

Undercut: The AWS D1.1 Table 7.1 limit for undercut at fillet weld toes depends on the connection orientation relative to the primary stress direction. Maximum allowed depth is 1/32 in. for welds transverse to primary tensile stress, and 1/16 in. for welds parallel to primary stress on connections not subject to fatigue. Use a bridge cam gauge or machinist's depth gauge — estimating 1/32 in. by eye is unreliable. Do not accept undercut measurements from visual estimation alone when the primary stress direction is tensile.

Overlap (cold lap): Weld metal that rolls over the weld toe without fusing into the base metal must be rejected regardless of the apparent leg size. The gauge may read full leg size while the toe has an unfused lip underneath. Check the toe interface carefully on vertical (3F) and overhead (4F) position welds, where cold laps are most common.

Surface porosity: AWS D1.1 Table 7.1 limits porosity on weld surfaces. Single pores greater than 3/32 in. in diameter and clusters exceeding the table limits require rejection and repair. Record porosity locations and sizes rather than estimating — a camera photo of the weld surface referenced to the inspection report is useful documentation.

For the complete visual acceptance criteria table and weld profile requirements, see Visual Acceptance Criteria Under AWS D1.1 and Fillet Weld Size Requirements in WPS Documentation.

Documentation Requirements

Each measurement in the inspection record should include:

  • Weld identification: piece mark, weld map reference, or connection ID
  • Measurement location: station point or distance from a reference edge
  • Measured leg size(s): upper leg and lower leg for 2F position welds
  • Convexity or concavity reading: when outside the flat range
  • Undercut depth: when present
  • Disposition: accept or reject, with the specific criterion referenced

For structuring inspection records that satisfy AWS D1.1 documentation requirements and hold up under third-party audit review, see CWI Inspection Report Documentation for AWS D1.1.

If you want a system for tracking fillet weld inspection records by weld map location across multiple active projects — with WPS parameter traceability built in — see our WPS and quality management tools.

Rule library based on AWS D1.1:2025. Verify against your governing edition — the AHJ or contract may specify 2020 or an earlier edition.