Weld Length Calculator Formula

Understand the math behind the weld length calculator. Each variable explained with a worked example.

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Formulas Used

Minimum Weld Length

min_length = throat > 0 && allowable_shear > 0 ? applied_load / (throat * allowable_shear) : 0

Effective Throat

throat_size = throat

Capacity per Inch

capacity_per_inch = throat * allowable_shear

Variables

VariableDescriptionDefault
applied_loadApplied Load(lbs)10000
weld_sizeWeld Leg Size(in)0.25
allowable_shearAllowable Weld Shear Stress(psi)21000
throatDerived value= weld_size * 0.707calculated

How It Works

Fillet Weld Length

L = P / (t x Fv)

Where P is the applied load, t is the effective throat (0.707 x leg size for a 45-degree fillet), and Fv is the allowable shear stress on the weld metal. For E70 electrodes, Fv = 0.30 x 70,000 = 21,000 psi.

Worked Example

10,000 lb load, 1/4-inch fillet weld, E70 electrode.

applied_load = 10000weld_size = 0.25allowable_shear = 21000
  1. 01Throat = 0.25 x 0.707 = 0.177 in
  2. 02Capacity = 0.177 x 21,000 = 3,713 lbs/in
  3. 03Min length = 10,000 / 3,713 = 2.69 in
  4. 04Use at least 3 inches of weld.

Frequently Asked Questions

What does 0.707 represent?

For a 45-degree fillet weld, the shortest distance through the weld (the throat) is the leg size multiplied by sin(45) = 0.707. The throat is the weakest cross-section.

What is the minimum weld length?

The AISC requires minimum weld length of 4 times the weld size or 1.5 inches, whichever is greater. Very short welds may not develop full strength due to start/stop effects.

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