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Weld Size Calculator

Work out weld size instantly with clear inputs, formula shown and shareable results.

About Weld Size Calculator

A fillet weld carries load across its throat, which for an equal-leg weld is 0.707 times the leg size. Required throat is load divided by allowable stress times effective length, and the leg follows. Codes also impose a minimum leg for the plate thickness so the weld cools slowly enough to avoid cracking.

Formulas

Throat required

Throat = load / (allowable stress x effective length)

Leg size

Leg = throat / 0.707, subject to the code minimum for plate thickness

Worked example

With the default values below, Weld Size Calculator returns Fillet leg size required of 5.66. Change any input above and every figure here is recalculated in your browser.

Inputs used

Thinner plate thicknessminimum 1 mm
10 mm
Design load on weldminimum 0.1 kN
120 kN
Effective weld lengthminimum 5 mm
200 mm
Allowable weld stressminimum 10 MPa
150 MPa

Results

Fillet leg size required
5.66 mm
Throat thickness required
4.00 mm
Minimum leg for this thickness
5.0 mm
Utilisation at chosen leg
100.0% %

Step by step

  1. 1Values used
  2. 2Throat required
  3. 3Leg size
  4. 4Fillet leg size required= 5.66 mm
  5. 5Throat thickness required= 4.00 mm
  6. 6Minimum leg for this thickness= 5.0 mm
  7. 7Utilisation at chosen leg= 100.0 %

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Weld design and procedure must be verified by a qualified welding engineer against the governing structural welding code.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Weld design and procedure must be verified by a qualified welding engineer against the governing structural welding code.

Frequently Asked Questions

How is the Weld Size figure worked out?

Throat = load / (allowable stress x effective length). A fillet weld carries load across its throat, which for an equal-leg weld is 0.707 times the leg size.

Why is there a minimum leg size?

A small weld on thick plate cools very fast, hardening the heat affected zone and risking cracking. The minimum leg ensures adequate heat input for the section.

What values do I need to enter?

This calculator takes 4 inputs: Thinner plate thickness, Design load on weld, Effective weld length, Allowable weld stress. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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Weld Size Calculator – Calculate Fillet leg size required