Weld Size Calculator
Enter the thickness of the thicker plate, the weld length, the load, the electrode and whether you weld one side or both. The calculator gives the fillet leg size, the throat and the weld's capacity.
- Leg from load
- AWS minimum size
- Throat and capacity
Weld length is the effective length of each weld. Do not count the start and stop craters.
A fillet weld's strength is in its throat. The throat is about 0.7 of the leg size. Steel welds are allowed a shear stress of 0.3 times the electrode strength, so an E70 weld carries about 145 N per mm² of throat.
Fillet weld minimum size
AWS D1.1. Search for a thickness.
| Thicker plate (mm) | Thicker plate (in) | Minimum leg (mm) | Minimum leg (in) |
|---|---|---|---|
| Up to 6 | Up to 1/4 | 3 | 1/8 |
| 6 to 12 | 1/4 to 1/2 | 5 | 3/16 |
| 12 to 19 | 1/2 to 3/4 | 6 | 1/4 |
| Over 19 | Over 3/4 | 8 | 5/16 |
The minimum leg depends on the thicker plate, for proper fusion.
Load and weld size
100 mm welds both sides, E70. Slide the load.
The T joint is shown end on, with the fillet welds in colour.
How to size a fillet weld
Load, length, strength. Step through it.
Load
In shear.
Welds on both sides of a joint share the load and keep the parts square.
Fillet, butt or plug weld
Weld types. Compare them.
In the corner.
Plates edge to edge.
Through a hole.
Full penetration butt welds are as strong as the plate, so they need no size calculation.
Before welding
Weld checks. Tick each item.
Tick each item as you go.
Bigger welds are not always better. They add heat and distortion, so use the size you need.
What you get
Weld sizing. Each tile explains one result.
Standard.
Calculated.
For the plate.
0.707 × leg.
kN.
A 10 mm leg weld has twice the throat of a 5 mm leg, and so twice the strength per millimetre.
Frequently Asked Questions
No question matches that search.
How do I calculate fillet weld size?
Leg = load ÷ (0.707 × length × 0.3 × electrode strength × number of welds).
What is the throat of a fillet weld?
The shortest distance from the root to the face, about 0.7 times the leg size.
What is the minimum fillet weld size?
It depends on the thicker plate, from 3 mm for thin plate to 8 mm over 19 mm.
What electrode strength should I use?
E70 is the most common for structural steel.
How long should a fillet weld be?
At least four times the leg size, and long enough to carry the load.
Can a fillet weld be too big?
Yes. Oversized welds add distortion, cost and the risk of cracking.
Does welding both sides double strength?
Yes, two equal welds carry twice the load of one.
A weld sized for the job
Work out the throat the load needs, check the minimum size for the plate and weld both sides where you can. Strong, neat and not oversized.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.