What is the throat thickness of a fillet weld?

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.

What leg length should I specify for a fillet weld?

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.

What is the design strength of a fillet weld?

For Grade S275 steel (formerly Grade 43), the design weld strength pw = 220 N/mm² per BS 5950. For S355 steel (Grade 50), pw = 250 N/mm². Under Eurocode 3, the approach uses the ultimate tensile strength of the electrode (fu = 430 N/mm² for E42 electrodes) with correlation factor βw. The simplified method gives similar results.

How do I calculate the effective length of a fillet weld?

The effective length of a fillet weld is the overall length minus twice the leg length (to allow for the start and stop craters). So a 200mm weld with 6mm legs has an effective length of 188mm. The minimum effective length must be at least 40mm or 6 × leg length, whichever is greater. In practice, specify at least 50mm.

Does weld position affect capacity?

Yes, welding position affects quality and therefore capacity. Flat (downhand) welding gives the best quality and full design strength. Horizontal-vertical welding is slightly reduced. Vertical welding reduces capacity to about 90%, and overhead welding to about 85%. These reductions reflect the difficulty of achieving consistent penetration and profile in those positions.

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Weld Capacity Calculator — Fillet Weld Strength Check

Calculate fillet weld capacity per BS EN 1993-1-8. Checks throat thickness, weld strength, and total capacity for structural steel connections.

Applied Load

Total factored shear or tension on the weld group

Fillet Weld Leg Length

6mm is most common for standard connections

Weld Length

Total specified weld length per weld run

Number of Welds

e.g. 2 for both sides of a fin plate

How We Calculate This

This calculator checks the capacity of fillet welds in structural steel connections per BS EN 1993-1-8 (Eurocode 3) simplified method.

Calculation method

Throat thickness: a = 0.7 × leg length (equal leg fillet)
Weld strength: pw = 220 N/mm² (S275) or 250 N/mm² (S355)
Capacity per mm: a × pw (kN per mm length)
Total capacity: Capacity per mm × effective length × number of welds

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.