Measuring Fillet Weld Size

What it is

Fillet weld size is specified on drawings by leg size: the distance from the root of the joint to the toe of the weld. Strength depends on the throat: the shortest distance from the root to the weld face. For an equal-leg fillet on a 90° joint, the theoretical throat is the leg multiplied by 0.707. AWS D1.1 defines these terms for structural work and sets acceptance criteria. AWS B1.10 is the guide to visual examination of welds.

How to measure

1. Clean slag and spatter from the weld.
2. Convex weld: use the leg side of the gauge. Place it against one member and slide until the tip touches the weld toe. Check both legs; the smaller governs.
3. Concave weld: use the throat side of the gauge. Rest it on both members and read whether the face touches the gauge point. A concave weld is sized by its throat.
4. Check along the length, including starts and stops.
5. Compare to the drawing size and the code's undersize allowances.

Leg to theoretical throat

Throat = 0.707 times leg, for equal legs on a 90° joint.

Gauge types

Key points

• Weld size is the largest isosceles right triangle that fits inside the weld cross section.
• Extra convexity does not add to size and can be rejectable.
• Unequal legs must be measured and reported separately.
• Skewed (non-90°) joints change the throat; the code has its own rules.

Common mistakes

• Measuring a concave weld by its legs, which overstates its size.
• Checking one spot and assuming the whole length.
• Confusing leg with throat on drawings.
• Measuring before removing slag.

Governing standards: AWS D1.1; AWS B1.10; AWS A3.0 (terms).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.