Fillet weld calculator
A 6 mm fillet has a 4.24 mm throat, and in category SP with 490 MPa weld metal it carries 0.998 kN per millimetre: φ·0.6·fuw·tt·kr. Enter the legs, the weld category, the weld metal and the two plates for the throat, the design capacity per millimetre and for the run, the weld metal it takes, and the smallest fillet AS 4100 allows on the joint.
The joint is drawn in section with its legs, its throat and the shear plane through the throat, the weld growing as you type.
- Design capacity φvw
- Capacity of 200 mm of weld
- Design throat tt
- Capacity factor φ
- 0.8
- Weld metal fuw
- 490MPa
- Lap reduction kr
- 1.000
- Weld metal area
- Weld metal per metre
- Minimum fillet size
- 5mm
How it’s calculated
φvw = φ·0.6·fuw·tt·kr (AS 4100:2020 cl. 9.6.3.10; cl. 9.7.3.10 in AS 4100-1998). tt = tw1·tw2 / √(tw1² + tw2²), tw/√2 for equal legs. φ = 0.8 (SP) or 0.6 (GP). kr = 1 except on a welded lap joint over 1.7 m long: 1.10 − 0.06·lw (lw in m) to 8 m, 0.62 beyond.
Why is a fillet weld's strength set by its throat, not its leg?
A fillet fails in shear on the plane through its throat, the shortest path through the weld from the root to the face. For equal legs the throat is the leg over √2, so a 6 mm fillet works on about 4.2 mm of metal. The capacity per millimetre is that throat times the weld metal’s strength in shear (0.6 of its tensile strength), times the capacity factor for the weld category: 0.8 for SP, 0.6 for GP, which is for welds that are essentially statically loaded and lightly stressed.
On a long welded lap joint the ends of the weld load up before the middle, so past 1.7 m the capacity per millimetre is reduced by kr, down to 0.62 beyond 8 m.