Weld size and weld metal
Fillet weld leg length from throat thickness
- where
- equal leg length (mm)
- design throat thickness (mm)
Explain this equation
Converts a fillet weld throat thickness into the equal leg length the welder needs to lay down.
In an equal-leg fillet the throat runs at 45° from the root to the face. The leg is the hypotenuse of a right triangle whose two short sides are both equal to the throat, so it is √2, about 1.414, times the throat.
Simple 45° geometry, the reverse of the throat calculation for equal legs.
Sense check: an a5 fillet needs about 7.1 mm legs; an a4 about 5.7 mm.
Watch out: European drawings use "a" for throat and "z" for leg. Mixing them up gives a weld about 30% too small or 40% too big.
Converts a throat size called up on a drawing (a) to the equal leg length (z) the welder must lay down.
Enter your values to see the result.
Good to know
Round up to a leg you can gauge. Common pairs: a3 ≈ z4.2, a4 ≈ z5.7, a5 ≈ z7.1, a6 ≈ z8.5.
Common mistake
Rounding the leg down. A leg slightly short means the throat is below the design value.
Limits
Equal-leg fillet in a 90° joint with a flat face.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the welding procedure specification (WPS), BS EN ISO 15614/9606, BS EN 1090, BS EN 1993 and manufacturers’ data. Structural welds need qualified welders and engineered designs.