Weld Capacity Calculator — Fillet Weld Strength Check

The throat thickness is the shortest distance from the root of the weld to the face. For a standard equal-leg fillet weld, the throat = leg length × 0.7 (i.e., leg × cos 45°). So a 6mm fillet weld has a throat of 4.2mm, an 8mm has 5.6mm, and a 10mm has 7.0mm. The throat thickness is used in all strength calculations.

The minimum fillet weld leg length is typically the thickness of the thinner plate being joined. Common sizes are: 6mm for plates up to 12mm, 8mm for plates 12-20mm, and 10mm for heavier plates. The maximum leg length should not exceed the thickness of the thinner plate minus 2mm. 6mm is the most common size for standard steelwork connections.

This calculator uses BS 5950-1:2000 Table 37. For Grade S275 steel (formerly Grade 43) with a matching E42/E35 electrode, the design weld strength pw = 220 N/mm². For S355 steel (Grade 50), pw = 250 N/mm². The capacity per mm of weld is then throat thickness × pw — for a 6mm fillet in S275 that is 4.2 × 220 = 924 N/mm (0.924 kN/mm), which matches the SCI Blue Book. Eurocode 3 (BS EN 1993-1-8 cl 4.5.3.3) takes a different route, using the electrode tensile strength fu with a correlation factor βw, and gives a slightly higher figure (about 0.98 kN/mm for the same weld); this tool follows the BS 5950 method.

BS 5950-1:2000 cl 6.8.2 defines the effective length as the length over which the fillet is full size, and states that — in the absence of better information — it may be taken as the overall length less one leg length s for each end that does not continue around a corner. For a normal run with two free ends that means deducting twice the leg length. So a 200mm run with 6mm legs has an effective length of 200 − 2 × 6 = 188mm. The same clause states that a fillet weld with an effective length less than 4s (four times the leg) or less than 40mm should not be used to carry load, so the minimum effective length is the greater of 40mm and 4 × leg (40mm for legs up to 10mm, 48mm for a 12mm leg). BS EN 1993-1-8 cl 4.5.1(1) takes a slightly less onerous route, deducting only twice the effective throat thickness (2 × 0.7 × leg = 1.4 × leg) and treating 30mm or 6 × throat as the minimum; this tool uses the BS 5950 leg-based deduction throughout.

Not in the design code. Neither BS 5950-1:2000 cl 6.8 nor BS EN 1993-1-8 cl 4.5.3 reduces the design capacity of a fillet weld for welding position — capacity is throat thickness × design strength only. Welding position is managed through the welding procedure specification (WPS) and the acceptance quality level (BS EN ISO 5817), not through a capacity reduction. The optional position setting in this calculator applies a conservative practical allowance (about 95% horizontal-vertical, 90% vertical, 85% overhead) to reflect the greater difficulty of achieving a consistent profile out of position; it is a workmanship judgement, not a code requirement. Leave it on Flat for a pure BS 5950 capacity check.