Actual throat: meaning and how to check it
The actual throat of a fillet weld is the shortest distance from the weld root to the weld face, measured across the real cross-section as it was welded. It includes any convexity on the face. That makes it the largest of the three fillet throats on a convex weld, and the one least useful for strength.
Use this diagram
How to find the actual throat
- Start at the weld root. That's the inside corner of the joint, or the deepest point of fusion if the weld penetrated past it.
- Go to the real face. Find the shortest straight line from the root to the weld face as it actually is, bulge and all.
- Compare with a flat face. Draw (or picture) a straight line from toe to toe. The distance from that line out to the crown is the convexity. Take it away and you're left with the throat the design counts.
- Know where you'll see it. The actual throat is found on a cut and etched cross-section, not from the surface. In the field you check fillet size with a gauge and rely on the drawing for the throat.
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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
- Actual versus effective. Both start at the weld root. The effective throat stops at a straight toe-to-toe face and leaves convexity out; the actual throat runs on to the real, crowned face.
- Actual versus theoretical. The theoretical throat comes from geometry alone: the largest right triangle that fits inside the weld, with no credit for fusion or crown.
- Bigger isn't stronger. Extra convexity raises the actual throat but adds no design strength, and a steep crown makes the toe sharper. That's why convexity is judged as a profile problem.
- Concave fillets. On a concave fillet the face dips in, so the actual throat and the theoretical throat come out about the same, and both are shorter than the legs suggest.
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The actual throat on the CWI exam
The three throats are a classic set in Definitions and Terminology, at least 12% of the closed-book Part A. Expect to pick the right one from a cross-section sketch, so learn what each includes and leaves out.
On Part B, the weld replicas are measured from the outside, so you check fillet size and convexity with the gauges rather than cutting a section. Welding Inspection and Discontinuities is at least 34% of the practical.
On a D1.1 Part C, fillet profiles and convexity are covered in Clause 7, and effective throats for design in Clause 4. Use your own copy for the requirements.
Practice it
The weld gauge reading trainer has convexity cases where you read the height of the face above the toe-to-toe line, the same distance that separates the actual throat from the effective one. The welding terms glossary keeps all three throats in one table.
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Sources
Show the list
- 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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