Weld Size Calculator
Find the right weld size for the job. Enter the plate thickness and the load, and get the required fillet leg, the effective throat, the code minimum and maximum sizes, and a clear recommended size, with a throat diagram, a weld symbol, and an intermittent-weld option.
Weld metal and safety
What size weld do I need?
Two things set the size of a fillet weld, and you take the larger. The first is strength: the weld throat has to carry the load without exceeding the allowable stress. The second is the welding code minimum, which depends on the thickness of the thicker part being joined; a minimum size ensures enough heat to fuse thick plate without cracking. On lightly loaded thick plate the code minimum usually wins, while on heavily loaded thin plate the strength requirement governs.
The calculator works out both, reports the larger as the recommended size, and warns if it exceeds the maximum fillet that thickness allows.
Fillet weld leg size vs throat size
A fillet weld is measured by its leg, the length along each plate, but it carries load across its throat, the shortest distance through the weld. For a standard 45-degree fillet the throat is 0.707 times the leg. The throat, not the leg, is what resists the load, which is why doubling the leg does not double the strength in the way you might expect.
| Leg size | Effective throat | Throat area per inch of weld |
|---|---|---|
| 1/8 in | 0.088 in | 0.088 in² |
| 3/16 in | 0.133 in | 0.133 in² |
| 1/4 in | 0.177 in | 0.177 in² |
| 5/16 in | 0.221 in | 0.221 in² |
| 3/8 in | 0.265 in | 0.265 in² |
| 1/2 in | 0.354 in | 0.354 in² |
Minimum fillet weld size by thickness
Typical code minimum fillet size for the thicker part joined (AWS D1.1 style):
| Thicker part | Minimum fillet leg |
|---|---|
| Up to 1/4 in (6 mm) | 1/8 in (3 mm) |
| Over 1/4 to 1/2 in | 3/16 in (5 mm) |
| Over 1/2 to 3/4 in | 1/4 in (6 mm) |
| Over 3/4 in (19 mm) | 5/16 in (8 mm) |
Frequently asked questions
The maximum fillet along an edge is the plate thickness if it is under 1/4 inch, or the thickness minus 1/16 inch for thicker plate, so the edge is not melted away. Above that, use a groove weld.
For fillet welds the throat is checked in shear. A common allowable is 0.30 times the electrode tensile strength, for example about 21 ksi for E70 electrodes, which the calculator applies through the electrode strength and factor of safety.
When the required throat is smaller than the minimum continuous size allows, stitch welds save filler and reduce distortion. Turn on the intermittent option and the calculator gives the segment length and pitch.
No. A full-penetration groove weld develops the full thickness and matches the base metal. Fillet welds are sized on their throat, which is why leg size and throat size are different numbers.