Fillet weld leg: meaning and how to check it
A fillet weld leg is the distance from the root of the joint to the toe of the fillet, measured along one member's surface. Every fillet has two legs, one on each member. When both legs are the same length you have an equal-leg fillet; when they differ, the drawing must say which leg goes on which member.
Use this diagram
How to measure a fillet leg
- Find the root and the toe. The leg starts at the inside corner where the members meet and ends at the weld toe on that member.
- Pick the gauge end. A fillet weld gauge blade has a leg end and a throat end. For a flat or convex fillet, use the leg end: set it against both members and check whether the blade meets the toe for the size you're testing.
- Check both legs. Measure the leg on each member separately. Equal-leg fillets that come out unequal point to arc placement or travel angle.
- Concave fillets are different. A dished face makes the legs longer than the fillet's real size, so check a concave fillet with the throat end of the gauge instead.
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Definitions and Terminology
A fatigue crack in a cyclically loaded fillet-welded attachment started along the line where the surface of the weld blends into the flange plate. At which weld feature did the crack start?
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Common mix-ups
- Leg versus size. For a flat or convex fillet, the size equals the shorter leg. For a concave fillet the size is smaller than the legs, because size comes from the largest right triangle that fits inside the weld. The theoretical throat page draws that triangle.
- Leg versus throat. The leg runs along a plate surface; the throat runs from the root to the face. The number on a fillet symbol is the leg size.
- Upright leg on the symbol. The vertical line of the fillet triangle on a welding symbol is always on the left. It doesn't tell you which plate gets the longer leg.
- Unequal on purpose. Some details call for unequal legs, for example to spread the weld along a thinner member. Read the detail before calling it a defect.
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Fillet legs on the CWI exam
Leg, size and throat are terms in Definitions and Terminology, at least 12% of the closed-book Part A. Welding Related Calculations, at least 6% of Part A, uses the leg to find the throat and the cross-section area.
On Part B, fillet legs are measured on the T-joint replica with the fillet gauges in the kit, and the reading is judged against the Book of Specifications. Welding Inspection and Discontinuities is at least 34% of the practical.
On a D1.1 Part C, minimum fillet sizes and fillet profiles come up in Clauses 5 and 7. Look them up in your own copy; no sizes are given here.
Practice it
Enter equal or unequal legs into the fillet weld calculator and it returns the throat, the cross-section area and the weld metal per foot. Then compare leg and throat side by side in the welding terms glossary.
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Sources
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- AWS bookstore, terminology and symbols listing, American Welding Society. Retrieved 20 Oct 2026.
- CWI Part A Fundamentals Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
- AWS B5.1:2025, Specification for the Qualification of Welding Inspectors, American Welding Society, Tables 5.1 and 8.1. Retrieved 20 Oct 2026.
Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.
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