How is fillet weld strength calculated?

The allowable shear stress on the weld is 0.30 times the electrode tensile strength, applied to the effective throat. For an equal-leg fillet the throat is 0.707 times the leg. Multiplying allowable stress by throat and weld length gives the load the weld can carry.

What is the effective throat of a fillet weld?

The effective throat is the shortest distance from the root to the theoretical face, which for an equal-leg fillet equals 0.707 times the leg size, since the throat lies on the 45 degree diagonal of the weld triangle. Strength is governed by this throat, not the leg directly.

Why does AWS specify a minimum fillet size?

A weld that is too small relative to a thick plate cools too fast and can crack, regardless of the calculated load. AWS D1.1 sets a minimum leg by the thickness of the thicker part joined, so for thick steel the minimum can govern even when the load needs less.

What is the difference between ASD and LRFD here?

This tool uses allowable stress design, with an allowable of 0.30 times electrode tensile. Load and resistance factor design instead applies a resistance factor of 0.75 to a nominal weld strength of 0.60 times electrode tensile. Both should give a comparable, safe leg size.

Should I round the leg size up?

Yes. Always round the governing leg up to the next standard increment that your fabricator can lay down reliably, such as the next millimetre or sixteenth of an inch. Specifying an oversized weld wastes filler and adds distortion, so do not round up excessively.

Fillet Weld Size & Strength Calculator

A fillet weld carries load through its throat in shear. This calculator finds the leg size that delivers enough throat area to carry a given load, then checks it against the AWS minimum size set by plate thickness, and reports which one governs. It works in either US or SI units with the allowable-stress method.

How it works

The capacity of a fillet weld comes from the allowable shear stress on its effective throat:

F_allow = 0.30 x F_EXX           electrode tensile strength
throat  = 0.707 x leg            equal-leg fillet
P       = F_allow x throat x L   weld capacity

Rearranging for the leg size needed to carry a load P over length L:

leg_required = P / (0.707 x 0.30 x F_EXX x L)

That is then compared to the AWS D1.1 minimum fillet size, which depends on the thicker part joined, for example 5 mm for plate from 6 to 12 mm and 6 mm from 12 to 20 mm. The larger of the two governs.

Example and notes

For a 40 kip load over 12 inches of E70 weld: allowable stress is 0.30 x 70 = 21 ksi, and the required leg is 40 / (0.707 x 21 x 12) = 0.225 in. If the thicker plate is 0.5 in the AWS minimum is 0.1875 in, so the load governs and you would round up to 1/4 in.

When the minimum governs, the plate thickness, not the force, is sizing the weld.
This is the static allowable-stress check. Add fatigue checks for cyclic loading and confirm base-metal capacity separately.
Both legs of a weld group share the load; enter the total effective length, and for intermittent welds use the summed weld length only.