Weld Strength Calculator Formula
Understand the math behind the weld strength calculator. Each variable explained with a worked example.
Formulas Used
Effective Throat
throat_mm = throatWeld Load Capacity
capacity = throat * weld_length * allowable_shear / 1000Strength per mm Length
per_mm = throat * allowable_shear / 1000Variables
| Variable | Description | Default |
|---|---|---|
weld_size | Fillet Weld Leg Size (a)(mm) | 6 |
weld_length | Weld Length (L)(mm) | 200 |
allowable_shear | Allowable Weld Shear Stress(MPa) | 124 |
throat | Derived value= weld_size * 0.707 | calculated |
How It Works
Fillet Weld Strength
Fillet welds fail through the throat, which is the shortest path through the weld cross-section.
Key Relationships
Effective throat = 0.707 x leg size (for equal-leg fillets at 45 degrees)
Capacity = throat x length x allowable shear stress
The 0.707 factor is cos(45 degrees) = sin(45 degrees), representing the minimum cross-section through the triangular fillet.
Worked Example
A 6 mm fillet weld, 200 mm long, with 124 MPa allowable shear (E70 electrode).
- 01Throat = 0.707 x 6 = 4.24 mm
- 02Capacity = 4.24 x 200 x 124 = 105,152 N = 105.2 kN
- 03Strength per mm = 4.24 x 124 / 1000 = 0.526 kN/mm
Frequently Asked Questions
What is the minimum fillet weld size?
Minimum weld size depends on the thicker plate being joined. For plates up to 6 mm, minimum fillet is 3 mm. For plates 6-13 mm, minimum is 5 mm. For plates over 13 mm, minimum is 6 mm (per AWS D1.1).
Why is the throat thickness less than the leg size?
The weld fails along its weakest plane, which cuts diagonally through the fillet. This diagonal distance (the throat) is shorter than the leg by a factor of cos(45) = 0.707.
Do I need to account for the weld return?
Weld returns (wrapping the weld around corners) add modest capacity and help reduce stress concentrations. Many codes require returns of at least 2 times the weld size at ends of members.
Ready to run the numbers?
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