Free Tool
Fillet Weld Size Calculator
Convert a fillet weld's leg size to its theoretical throat and back — for equal and unequal legs. Know the difference the CWI exam tests: size is the leg, throat is the effective dimension.
Try it now — no account, no download
Practice 5 Welding / CWI exam questions
Real questions — each with an instant AI explanation.
Welding / CWI · Question 1 of 5
Which welding process uses a non-consumable tungsten electrode?
Pick an answer to see the explanation + an instant AI breakdown.
Leg, Throat, and Why It Matters
A fillet weld is specified by its leg size — the distance from the joint root out to the weld toe along each member. The theoretical throat, the shortest line from the root to a flat weld face, is what actually carries load, and for an equal-leg fillet it is simply the leg times 0.7071. That 0.7071 factor is the sine of 45°, because an equal-leg fillet forms a 45° right triangle.
Inspectors and exam questions lean on this relationship constantly: a fillet called out as 1/4 in has a theoretical throat of about 0.177 in, and a convex or concave face changes the actual throat even when the legs measure correctly. For unequal legs the throat is the perpendicular from the root to the hypotenuse, (a × b) ÷ √(a² + b²). Use the calculator to move between leg and throat, then confirm the governing throat definition in your WPS or code.
Frequently Asked Questions
How do you calculate the throat of a fillet weld?
For an equal-leg fillet the theoretical throat equals the leg size times 0.7071 (the sine of 45°). For example, a 1/4 in (0.25) leg gives a theoretical throat of about 0.177. To go the other way, multiply the throat by √2 (about 1.414) to recover the leg.
What is the difference between the leg and the throat?
The leg is the distance from the joint root to the toe of the fillet along the base metal, and it is how a fillet weld's SIZE is specified. The throat is the shortest distance from the root to the face of the weld and represents the effective load-carrying dimension. Don't specify size by throat — fillet size is the leg.
How is the throat of an unequal-leg fillet found?
For unequal legs a and b, the theoretical throat is (a × b) ÷ √(a² + b²). For example, legs of 3 and 4 give a throat of (3 × 4) ÷ 5 = 2.4. This calculator handles both equal and unequal legs.
Is this the theoretical or actual throat?
This tool gives the THEORETICAL throat, based on the leg dimensions and a flat weld face. The actual throat can differ with a convex or concave profile, and the effective throat may include root penetration where the code allows it. Always follow the WPS and code for the throat definition that applies.
Studying for the CWI exam?
Practice real Certified Welding Inspector questions with instant explanations — symbols, codes, and inspection. No download required.