How do I size a fillet weld?

The effective throat is a = 0.707 × leg. Direct shear stress is τ = P/(a·L). Size the leg for your allowable shear: leg = P/(0.707·L·τ_allow).

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Fillet Weld Size

Equal-leg fillet: effective throat a = 0.707·leg; direct shear τ = P/(a·L). Units: leg and length in mm, load in N → stress in MPa.

Calculator

Leg size (mm)

Weld length (mm)

Applied load P (N)

Allowable shear (MPa) — for required-size

Throat (mm)

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Shear stress (MPa)

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Required leg (mm)

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Eccentric weld group (weld as a line)

Rectangular box A_w = 2(b+d), J_u = (b+d)³/6; circular pattern A_w = πD, J_u = πD³/4. Direct and torsional shear combine vectorially at the critical point.

Pattern

Width b (mm)

Depth d (mm)

Load P (N)

Eccentricity e (mm)

Allowable shear (MPa)

Direct shear

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Resultant

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Required leg

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Elastic (weld-as-a-line) method. A helper, not a substitute for engineering judgment.