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Fillet Weld Strength Calculator

§ Code basis: EN 1993-1-8 (Europe / Commonwealth) — The design equations differ, so the values are not interchangeable - recalculate with your own code.

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

Also known as fillet weld strength · weld throat thickness · fillet weld calculation

Quick answer

With the default inputs: Leg size s 6 mm · Weld length L 200 mm · Steel grade S275 etc.Weld design resistance 188.0 kN

Use your own dimensions ↓

Inputs

MPa

Result

Weld design resistance
188.0kN
Throat thickness a4.20mm
Design shear strength fvw,d233.7MPa
Effective length Leff191.6mm
Minimum effective length30mm
Long weld factor βLw1.000
Resistance Fw,Rd188.0kN
Capacity per metre981kN/m
Per mm of throat0.234kN/mm²
Length checkOK
How this is worked out

Leg size and throat thickness are different numbers: for an equal-leg fillet the throat is 0.7s, so a 6 mm leg has a 4.2 mm throat and carries load on 4.2 — designing on the leg over-predicts by 43%. Weld strength is not base metal strength either: fvw,d = fu / (sqrt(3) beta_w 1.25), where the sqrt(3) converts von Mises stress to shear and 1.25 is the gamma_M2 factor. For S275 at 430 MPa the design strength is 234 MPa, about 54% of the parent metal, which is why a full-strength connection cannot be made by lengthening the weld — the intensity is capped. Add weld size, or switch to a full-penetration butt weld, which needs no such reduction. Three detailing rules matter. First, effective length: cl. 4.5.1(1) allows the overall length to be reduced by twice the throat to cover craters at the start and stop, but if the weld is full size throughout that deduction may be waived — this tool always deducts, which is conservative. Second, minimum length: cl. 4.5.1(2) says a fillet weld shorter than max(6a, 30 mm) must not be designed to carry load — 6a and 30 mm, not 4a and 40 mm. Short welds are not discounted, they are zero. Third, long welds: beyond 150a the stress no longer distributes evenly and the resistance is multiplied by beta_Lw = 1.2 - 0.2 Lj/(150a), not less than 0.6. For a 6 mm leg (a = 4.2) the threshold is 630 mm, which ordinary details never reach but long stiffener or continuous welds easily do.

Next up: Steel Beam Span Calculator · Section Properties Calculator

Formula

Throata = 0.7·s (equal leg)
Design strengthfvw,d = fu/(√3·βw·γM2), γM2 = 1.25
Correlation factorβw: S235 0.80 / S275 0.85 / S355 0.90 / S420, S460 1.00
Effective lengthLeff = L − 2a, and ≥ max(6a, 30 mm)
Long weld factorβLw = 1.2 − 0.2Lj/(150a) for Lj > 150a, 0.6 ≤ βLw ≤ 1.0
ResistanceFw,Rd = fvw,d·a·Leff·βLw
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.

Basis

The codes below are listed for comparison. This calculator returns EN 1993-1-8 values only.

FAQ

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.

What to calculate next

These jobs are done back to back on site — follow the steps to work through the whole sequence.

Steel: span to connectionStep 4 of 4

  1. 1Steel Beam Span Calculator
  2. 2Section Properties Calculator
  3. 3Column Buckling Calculator
  4. 4Fillet Weld Strength Calculator

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Fillet Weld Strength Calculator. 2026-10-25. https://jianzhujisuan.com/en/fillet-weld

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.