Theoretical throat: meaning and how to check it

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://filletgauge.com/charts/filletgauge/theoretical-throat.svg" alt="A large concave fillet in a T-joint corner with a dashed right triangle inside it touching the dished face, and a heavy line from the root at right angles to the triangle's long side, the theoretical throat" width="100%"><p><a href="https://filletgauge.com/guides/theoretical-throat?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=theoretical-throat.svg">The throat comes from the triangle, not the legs. On a concave face the triangle is smaller than the legs. by Fillet Gauge</a></p>

How to work out the theoretical throat

1. Draw the triangle. Inside the weld, fit the largest right triangle whose two short sides lie along the two members. For an equal-leg fillet it's an isosceles right triangle.
2. Measure to its long side. From the corner of the joint, go at right angles to the triangle's long side. That distance is the theoretical throat.
3. Use the shortcut. For an equal-leg fillet with a flat face, the throat is the leg times 0.707 (the sine of 45 degrees). A 0.25 leg gives 0.707 x 0.25 = 0.177, and a 0.375 leg gives 0.707 x 0.375 = 0.265, in whatever units the drawing uses.
4. Unequal legs. The 0.707 shortcut no longer works. Use the right-triangle height instead: the product of the two legs divided by the long side.

Common mix-ups

• Concave faces shrink it. The dished face limits how big a triangle fits, so the fillet's size and theoretical throat are smaller than its legs suggest. Size it from the triangle, not from the fillet weld leg.
• Convex faces don't grow it. On a convex fillet the triangle is set by the shorter leg, and the crown sits outside it.
• Theoretical versus effective. The effective throat can count fusion past the joint root. The theoretical throat never does. With no extra fusion and a flat face, the two match.
• 0.707 or 0.7. Some references round the factor to 0.7. The difference only shows in the third decimal.

The theoretical throat on the CWI exam

Throat terms sit in Definitions and Terminology, at least 12% of Part A. The 0.707 relation is a staple of Welding Related Calculations, at least 6% of Part A, and Part A is closed book, so know where the factor comes from rather than memorizing a table.

On Part B, the fillet gauge's throat end reads size on a concave fillet by touching the face at its middle, which is the theoretical triangle idea turned into a tool. Welding Inspection and Discontinuities is at least 34% of the practical.

On a D1.1 Part C, fillet weld design and effective areas are a Clause 4 subject. Look them up in your own copy.

Practice it

The fillet weld calculator does the throat for equal and unequal legs and shows the area and weld metal weight alongside it. The other two throats are compared in the welding terms glossary.

Know your throats? Check the rest of Part A

Take the free 10-question check. It's weighted to the Part A outline, scored by domain and needs no signup, so you see which domain costs you the most points. The Part A course then gives you a lesson on every exam subject, two full-length timed practice exams, and a worked explanation with every answer.

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.