Fillet Weld Size Calculator (Throat, Stress & Capacity)
By Santaji GadepatilinUpdated November 3, 2026
A fillet weld transfers load through its throat, the shortest distance from the root to the weld face, which for an equal-leg weld is 0.707 times the leg size. This fillet weld size calculator finds the throat, the shear stress for a given load and weld length, and the load the weld can safely carry at an allowable stress.
How to use the fillet weld size calculator
Enter the leg size, the effective weld length, the load and the allowable shear stress on the throat. The calculator returns the throat thickness, the actual stress, the weld’s load capacity and its utilisation (below 100% passes). Clear the leg size and enter the load to find the fillet size you need.
Fillet weld formula
a = 0.707 × s · τ = F ÷ (a × L) · Fmax = τallow × a × L
Example
A 6 mm fillet, 100 mm long, made with E70 electrodes (τallow = 145 MPa), carries 50 kN:
- Throat a = 0.707 × 6 = 4.24 mm
- Stress τ = 50,000 ÷ (4.24 × 100) = 117.9 MPa
- Capacity = 145 × 4.24 × 100 = 61.5 kN; utilisation 81% — OK
Fillet weld capacity at 145 MPa (E70 electrode)
| Leg size | Throat | Capacity per mm of weld | Capacity of 100 mm |
|---|---|---|---|
| 3 mm | 2.12 mm | 0.308 kN/mm | 30.8 kN |
| 4 mm | 2.83 mm | 0.410 kN/mm | 41.0 kN |
| 5 mm | 3.54 mm | 0.513 kN/mm | 51.3 kN |
| 6 mm | 4.24 mm | 0.615 kN/mm | 61.5 kN |
| 8 mm | 5.66 mm | 0.820 kN/mm | 82.0 kN |
| 10 mm | 7.07 mm | 1.025 kN/mm | 102.5 kN |
| 12 mm | 8.49 mm | 1.230 kN/mm | 123.0 kN |
| 16 mm | 11.31 mm | 1.640 kN/mm | 164.0 kN |
Notes
- This simplified method treats the load as shear on the throat in any direction; it is conservative for welds loaded transversely.
- The parent metal next to the weld must also be checked.
- Compare the weld stress with your required factor of safety.
Frequently asked questions
How do you calculate fillet weld strength?
Capacity = allowable shear stress × throat × length, with throat a = 0.707 × leg size for an equal-leg 45° fillet. A 6 mm fillet 100 mm long at 145 MPa carries about 61.5 kN.
Why is the throat 0.707 times the leg?
The throat of a right-angled isosceles triangle is the leg times sin 45° = 0.7071. It is the smallest section through the weld, so the stress is calculated there.
What allowable stress should I use for fillet welds?
AWS D1.1 allows 0.30 × the electrode's tensile strength on the effective throat — about 145 MPa (21 ksi) for E70XX electrodes. Eurocode 3 uses a directional or simplified method based on the parent metal's strength.
Is there a minimum fillet weld size?
Yes. Codes set minimum leg sizes based on the thicker plate (in AWS D1.1: 3 mm up to 6 mm plate, 5 mm for 6–12 mm, 6 mm for 12–20 mm and 8 mm above) so the weld cools slowly enough to fuse properly, and a maximum leg along plate edges.