Throat thickness, capacity and minimum leg size for a fillet weld from the leg length and allowable stress.
Page last changed 2026-10-19 · based on n/a — standard engineering relationships, no single governing revision
Quick Answer
The throat of a fillet weld is t = 0.707 × leg, because the critical section runs at 45° through the weld. A 1/4 in fillet therefore has a 0.177 in throat — only 71% of the leg, which is why weld capacity is quoted on the throat, not the leg.
Fillet Weld Capacity
The Formulas Used
Throat: t = 0.707 × leg (the theoretical throat through the 45° section) Capacity: P = t × L × τallow Minimum leg (AWS D1.1 guidance): leg ≥ tmin ÷ 4 Maximum leg for a square edge: leg ≤ tmin
Capacity Is Based on the Throat, Not the Leg
A fillet weld fails in shear along its throat — the plane running at 45° from the root to the face. That plane is the shortest path through the weld, and its length is the leg times cos 45°, or 0.7071 × leg.
Two practical consequences. A weld measured on the leg has 41% more area than its throat suggests, so quoting leg sizes as though they were thicknesses overstates capacity by that factor. And because the throat plane is in shear while the load may be in tension, the allowable stress for a fillet weld is a shear allowables, typically 0.30 times the tensile strength of the electrode — appreciably lower than for a full-penetration groove weld.
Frequently Asked Questions
How is a fillet weld throat calculated?
Throat = 0.707 × leg, because the critical section runs at 45° through the weld. A 1/4 in fillet has a 0.177 in throat. Note that some codes use the actual throat measured to the weld face rather than the theoretical throat to the root, which is slightly larger.
Why is fillet weld capacity lower than groove weld capacity?
Two reasons. The weld is loaded in shear rather than tension, and the allowable shear stress is typically 30% of the electrode's tensile strength. And the critical section is the throat, which is only 71% of the leg — so a fillet weld carries much less load than a full-penetration groove weld of the same nominal size.
What is the minimum fillet weld size?
It depends on the thickness of the thinner part being joined. As a guide, the leg should be at least a quarter of that thickness; AWS D1.1 tabulates the actual minimums by thickness, ranging from 1/8 in for material up to 1/4 in thick up to 5/16 in for material over 3/4 in thick.
What is the maximum fillet weld size?
For material with a square edge, the leg cannot exceed the thickness of the thinner part, because the weld would otherwise overhang the edge. For material with a bevelled edge, larger welds are permitted. Codes specify the limits by detail.
Does weld length add capacity directly?
Nominally yes — capacity is proportional to length. But the ends of a fillet weld are less effective than the middle because of the stress concentration at the termination, which is why codes require returns around corners and specify minimum lengths. Very short welds do not achieve the calculated capacity.
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