Hong Life CalculatorsUnderstanding the fillet weld strength calculator — concept and principle
A fillet weld joins two plates at an angle, usually 90°, and carries load mainly in shear through its smallest cross-section, the throat. This calculator converts the leg size you would specify on a drawing into the effective throat, then divides the load by the throat area to give the average shear stress. It suits quick checks of brackets, lugs and lap joints.
How it is calculated
- Throat: t = 0.707 × a, where a is the leg size in mm (equal legs, flat face).
- Shear stress on the throat: τ = F × 1000 ÷ (t × L_w), with F in kN and the weld length L_w in mm, giving MPa.
If the joint has welds on both sides or around several edges, enter the total effective length.
The principle
An equal-leg fillet weld has a right-triangle cross-section. The shortest distance from the root to the face is the altitude of that triangle, a × sin 45° = a × 0.7071. Failure surfaces in tests run close to this plane, so codes treat the throat area t × L_w as the area resisting load, regardless of the load direction. This simple method assumes stress is spread evenly along the weld; in reality the ends carry more, and transverse loads are somewhat stronger than longitudinal ones, which detailed code methods account for.
Worked example
- Inputs: leg 6 mm, weld length 100 mm, load 30 kN.
- Throat = 0.707 × 6 = 4.242 mm.
- Throat area = 4.242 × 100 = 424.2 mm².
- Shear stress = 30,000 N ÷ 424.2 mm² = 70.72 MPa.
| Leg size (mm) | Throat (mm) |
|---|---|
| 1.5 | 1.0605 |
| 3 | 2.121 |
| 4.5 | 3.1815 |
| 6 | 4.242 |
| 7.5 | 5.3025 |
| 9 | 6.363 |
| 12 | 8.484 |
How to read the result
Compare τ with the allowable shear stress of the weld metal. In the AWS D1.1 and AISC allowable-stress approach, the allowable shear on the effective throat is 0.30 × F_EXX, the electrode's tensile strength; for an E70 electrode (about 482 MPa) that is roughly 145 MPa, so 70.7 MPa leaves a comfortable margin. Limit-state codes such as EN 1993-1-8 use factored loads and a different check, so compare like with like. Doubling the leg size doubles capacity; doubling the length does the same, which is often cheaper than a bigger weld.
Common mistakes
- Using the leg size instead of the throat, which overstates capacity by about 41%.
- Counting the full nominal length when start and stop craters are not filled; many codes require deducting the ends or setting minimum lengths.
- Forgetting that the base metal next to the weld must also be checked.
- Ignoring bending or eccentric loads, which add stress at the weld ends.
Frequently asked questions
Is the throat always 0.707 times the leg?
Only for an equal-leg 90° fillet with a flat face. Unequal legs, skewed joints and deep-penetration welds have different throats.
What is the minimum fillet weld size?
Codes link it to plate thickness to ensure enough heat input; check the minimum-size table in the code you design to.
How do I add fatigue?
Fatigue depends on the weld detail category and stress range, not on static strength, and needs a separate check.
Sources: AWS D1.1/D1.1M, Structural Welding Code — Steel · ANSI/AISC 360, Specification for Structural Steel Buildings · EN 1993-1-8, Eurocode 3: Design of steel structures — Design of joints