How is fillet weld strength calculated?

Fillet weld capacity = throat area × allowable shear stress = a × L × (fu / (√3 × βw × γM2)), where a is throat thickness, L is effective length, fu is ultimate tensile strength, βw is correlation factor, and γM2 is safety factor (typically 1.25).

What is the throat thickness of a fillet weld?

Throat thickness (a) is the shortest distance from the root to the face of the weld. For an equal-leg fillet with leg size s: a = s × cos(45°) = 0.707 × s. For example, a 6mm fillet weld has throat thickness ≈ 4.24 mm.

What is the difference between fillet and butt welds?

Fillet welds join two surfaces at right angles (T-joints, lap joints) and carry load in shear through the throat. Butt welds join two plates end-to-end with full or partial penetration and carry load in direct tension/compression through the weld thickness.

What is the correlation factor βw?

The correlation factor βw accounts for the material grade. Per EN 1993-1-8: S235 → βw = 0.8, S275 → βw = 0.85, S355 → βw = 0.9, S420 → βw = 1.0, S460 → βw = 1.0.

How do I calculate weld stress from an applied force?

Weld stress σ = F / (a × L_eff), where F is the applied force, a is throat thickness, and L_eff is the effective weld length (total length minus 2×a for end craters). Compare this stress to the allowable stress to check adequacy.

Free Engineering Tool

Weld Strength Calculator

Calculate fillet and butt weld capacity per EN 1993-1-8. Enter weld type, throat size, length, applied force, and material grade.

EN 1993-1-8Fillet & ButtStress Check

Results

Weld Capacity

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Applied Stress

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Allowable Stress

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Utilization Ratio

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Throat Thickness (a)

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Fillet Weld (Simplified Method)

Butt Weld

Full penetration butt welds are treated as parent metal — allowable stress equals the plate allowable.

⚠️ This is a simplified analysis. For combined stresses, use the directional method per EN 1993-1-8 clause 4.5.3.

Weld Strength Calculator | Fillet & Butt Weld | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Fillet Weld (Simplified Method)

Butt Weld