# Fillet Weld Strength Calculator

A 6 mm leg is not a 6 mm throat — capacity follows the 4.2 mm throat.

- Page: https://jianzhujisuan.com/en/fillet-weld
- Category: Steel Design (https://jianzhujisuan.com/en/c/steel-design)
- Chinese version: https://jianzhujisuan.com/zh/fillet-weld.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Leg size s 6 mm, Weld length L 200 mm, Steel grade S275, Ultimate strength fu 430 MPa): **Weld design resistance 188.0 kN**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Leg size s | number | 6 | mm |  |
| Weld length L | number | 200 | mm |  |
| Steel grade | select | S275 | — |  |
| Ultimate strength fu | number | 430 | MPa |  |

## Formula

- **Throat**: a = 0.7·s (equal leg)
- **Design strength**: fvw,d = fu/(√3·βw·γM2), γM2 = 1.25
- **Correlation factor**: βw: S235 0.80 / S275 0.85 / S355 0.90 / S420, S460 1.00
- **Effective length**: Leff = L − 2a, and ≥ max(6a, 30 mm)
- **Long weld factor**: βLw = 1.2 − 0.2Lj/(150a) for Lj > 150a, 0.6 ≤ βLw ≤ 1.0
- **Resistance**: Fw,Rd = fvw,d·a·Leff·βLw

## Worked example

S275 at 430 MPa, 6 mm leg, 200 mm long: throat 4.20 mm, design strength 233.7 MPa, effective length 191.6 mm, 188.0 kN resistance, 981 kN/m.

## Questions

### How is fillet weld throat thickness calculated?

For an equal-leg fillet, a = 0.7s where s is the leg size. A 6 mm leg gives a 4.2 mm throat and an 8 mm leg gives 5.6 mm. Unequal legs use the smaller leg. Deep penetration welds may count the extra throat from penetration under cl. 4.5.2(3) and Figure 4.4, but only where tests show it is consistently achievable — without that, 0.7s is both conservative and defensible.

### Is weld strength the same as base metal strength?

No, considerably lower. The design shear strength is fu/(sqrt(3) beta_w 1.25), so S275 at 430 MPa gives 234 MPa, about 54%. The sqrt(3) converts tensile to shear by von Mises and 1.25 is the joint material factor. A fillet weld therefore cannot make a full-strength connection; that needs a full-penetration butt weld.

### How much length is deducted from a weld?

By default one throat thickness at each end for start and stop craters: a 200 mm weld with a 4.2 mm throat has 191.6 mm effective, a 4% loss. But cl. 4.5.1(1) says the overall length may be reduced that way — if the weld is full size throughout, the deduction is not required. The hard rule is separate: below max(6a, 30 mm) the weld must not be designed to carry load (cl. 4.5.1(2)). Long welds add a third rule: beyond 150a, multiply by beta_Lw = 1.2 - 0.2 Lj/(150a), not less than 0.6.

### What is the minimum fillet weld size?

The hard limit is on throat, not leg: cl. 4.5.2(2) requires the effective throat to be at least 3 mm, which for an equal-leg fillet means a leg of about 4.3 mm. So a 3 mm leg is wrong — it gives only a 2.1 mm throat. The advisory table by plate thickness (a = 3 mm up to 10 mm plate, 4 mm to 20, 5 mm to 30, 6 mm above) refers to throat thickness and is guidance, not a requirement. There is an upper limit too: along a plate edge the leg should not exceed the thickness, otherwise the edge burns away or undercuts. Oversized legs also widen the heat-affected zone, which is bad news in fatigue.

## Related tools

- [Steel Beam Span Calculator](https://jianzhujisuan.com/en/steel-beam-span): The depth is not looked up in a table — it falls out of the deflection limit.
- [Section Properties Calculator](https://jianzhujisuan.com/en/section-modulus): Inertia governs deflection, section modulus governs strength — they are not interchangeable.
- [Base Plate Design Calculator](https://jianzhujisuan.com/en/base-plate): Thicker is not the answer — check the bearing area first, then the cantilever.
- [Column Buckling Calculator](https://jianzhujisuan.com/en/column-buckling): Half the effective length means four times the Euler load — but the design resistance differs by less, and how much depends on slenderness.

---

Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.