How fillet weld strength is calculated
A fillet weld fails by shearing through its throat. The throat of an equal-leg fillet is 0.707 times the leg size, so a 6 mm weld has a 4.24 mm throat. Under the allowable stress method in AWS D1.1, the weld metal can carry 0.30 times the electrode's tensile strength in shear.
Capacity per millimetre = 0.30 × FEXX × 0.707 × leg. Multiply by the weld length and the number of weld lines carrying the load to get the allowable force.
Base metal check and minimum size
The plate next to the weld can also govern, especially on thin sections. The calculator compares weld metal capacity with the base metal shear capacity (0.40 × Fy × thickness) and reports the lower value. It also compares your leg size with the AWS D1.1 minimum fillet size for the thicker part, and the maximum size allowed along plate edges.
Choosing a leg size
Enter the service load to see utilisation, safety factor and the leg size that exactly carries it. Round up to the next standard size, and remember that doubling the leg size quadruples the weld volume, so lengthening the weld or welding both sides is often cheaper than a larger fillet.
Frequently asked questions
How do you calculate fillet weld strength?
Multiply the effective throat (0.707 × leg size) by the allowable shear stress of the weld metal (0.30 × electrode tensile strength under AWS D1.1 ASD), then by the weld length and the number of weld lines. For an E70XX electrode (482 MPa) the allowable shear is 145 MPa (21 ksi), so a 6 mm fillet carries about 0.61 kN per millimetre of weld.
What is the throat of a fillet weld?
The throat is the shortest distance from the root of the joint to the face of the weld. For an equal-leg fillet with a flat face it equals 0.707 times the leg size, which is why the weld's strength depends on the throat and not on the visible leg.
What is the minimum fillet weld size?
AWS D1.1 Table 5.8 sets the minimum from the thicker part joined: 3 mm (1/8″) up to 6.4 mm (1/4″), 5 mm (3/16″) up to 12.7 mm (1/2″), 6 mm (1/4″) up to 19 mm (3/4″), and 8 mm (5/16″) above that. The calculator flags any leg size below this.
Does the base metal ever govern instead of the weld?
Yes. On thin plate or with high-strength filler on mild steel the base metal can be the weaker element, because the allowable shear on the fusion face is 0.40 × the base metal yield strength. The tool checks both and tells you which governs.
Can I use this for structural design?
Use it for quick sizing and checking. Final design must follow the governing code (AWS D1.1, AISC 360, EN 1993-1-8) with the correct load combinations, directional strength increase and resistance factors, checked by a qualified engineer.
More welding tools
- Welding Calculator - Settings, heat input, filler weight, cost, carbon equivalent, duty cycle and unit converter.
- Shielding Gas Selector - Pick the right gas blend for your process, material and priority.
- Welding Electrode Selector - Compare stick electrodes by position, power source and application, with amp ranges.
- Welding Amperage Chart - Settings tables by material thickness for every process, in mm and inches.