Weld size and weld metal
Fillet weld throat thickness from leg lengths (BS EN ISO 2553)
- where
- design throat thickness (mm)
- leg length 1 (mm)
- leg length 2 (mm)
Explain this equation
Works out the throat thickness of a fillet weld from its two leg lengths. Use it when a drawing gives leg length (z) but the design is based on throat (a), or the other way round.
The weld is treated as a right-angled triangle with legs z₁ and z₂. The throat is the shortest distance from the root to the straight face, which is the height of that triangle measured to its hypotenuse. That height equals the product of the legs divided by the hypotenuse.
Area of the triangle can be written two ways, ½ z₁ z₂ or ½ × hypotenuse × throat; setting them equal gives this formula.
Sense check: equal 8 mm legs give a throat of about 5.66 mm, roughly 0.7 of the leg.
Watch out: any convexity on the face does not count towards the design throat. A load-carrying fillet needs at least a 3 mm throat.
Converts leg length (z on a drawing) to design throat thickness (a). For equal legs a = 0.707 × z.
Enter your values to see the result.
Good to know
BS EN ISO 2553 uses “a” for throat and “z” for leg on weld symbols; drawings to Eurocode design usually give a. Check which letter is on the drawing before you set the gauge. For a 90° joint with equal legs the throat is z/√2 ≈ 0.707z.
Common mistake
Reading an a-dimension as the leg. An “a5” weld needs about 7 mm legs, not 5 mm.
Limits
Assumes a 90° joint, a flat (mitre) weld face and no deep penetration. Convexity is not counted in the throat.
Sources
- BS EN ISO 2553:2019 Welding and allied processes – Symbolic representation on drawings – Welded joints (a = throat thickness, z = leg length)
- BS EN 1993-1-8:2005 Eurocode 3: Design of steel structures – Part 1-8: Design of joints (4.5.3.2 directional method, 4.5.3.3 simplified method, Table 4.1 correlation factor βw) and UK National Annex (γM2 = 1.25)
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.