Fillet weld size, explained

A fillet weld is specified by its leg and works through its throat — and most weld-sizing confusion is those two numbers trading places. The leg is the gauge-measurable face dimension; the throat is the failure plane.

The 0.707

throat a = 0.707·z (equal legs) · unequal: a = z₁z₂ / √(z₁² + z₂²) · capacity = σ_design · a · L

The throat is the height of the inscribed triangle — leg over √2 for the common equal-leg right-angle fillet. Design stress comes from the electrode: ASD uses 0.30 × FEXX, LRFD 0.45 × FEXX with factored loads. The weld throat calculator runs both bases, deducts root-opening allowance from the effective throat, and inverts to the required leg; the fillet weld size calculator covers the AWS/AISC sizing side.

Worked example — 6 mm E70 fillet, 100 mm long

a = 0.707 × 6 = 4.24 mm · σ = 0.30 × 483 = 145 MPa · capacity = 145 × 4.24 × 100 ≈ 61.5 kN

That is 615 N per millimeter of weld. Against a 50 kN load the weld runs at 81% — and the calculator's inverse says a 4.9 mm leg would have sufficed, which is the difference between one pass and two.

The shop shortcut: kips per sixteenth

E70 ASD: 0.928 kip/in per 1/16 of leg · LRFD: 1.392

The classic AISC mental math, and it is just the formula folded up: 0.30 × 70 ksi × 0.707 ÷ 16 = 0.928. A 1/4 in fillet carries 3.71 kip/in; a 5/16 fillet both sides of a 10 in connection plate is 2 × 5 × 0.928 × 10 ≈ 93 kips. Estimate with the shortcut, confirm with the calculator, and remember both assume the weld metal governs — the base-metal shear path alongside the weld is its own check.

Common mistakes

• Stressing the leg instead of the throat. Using z where a belongs overstates capacity by 41% — the most expensive substitution in weld math.
• Ignoring the root gap. Fit-up opening comes straight out of the effective throat; a 1 mm gap on a 6 mm fillet is a quarter of the strength gone (the calculator's g input).
• Skipping the code minimums and edge maximums. AWS D1.1/AISC set minimum fillet sizes by the thicker part joined (cooling-rate driven, not strength) and cap legs along plate edges at t − 1/16 in — check the current tables rather than memory.
• Sizing the weld and forgetting the heat. Leg squared is deposited metal; the heat-input guide and amperage calculator carry the process side.

Frequently asked questions

What is the difference between weld leg and throat?

The leg is what you see and measure with a gauge; the throat is what carries the load — the shortest distance through the weld, 0.707 × leg for an equal-leg right-angle fillet. All strength math runs on the throat: a 6 mm fillet works through a 4.24 mm throat.

How do you calculate fillet weld strength?

Capacity = design stress × throat × length. With E70 electrode on the ASD basis (0.30 × FEXX = 21 ksi on the throat), a 6 mm fillet carries about 615 N per mm of length — 61.5 kN over 100 mm. The LRFD basis uses 0.45 × FEXX with factored loads.

What is the 0.928 kips per sixteenth rule?

The classic AISC shortcut: an E70 fillet carries 0.928 kip per inch of length per sixteenth of leg (ASD), 1.392 LRFD. A 1/4 in fillet (4 sixteenths) is 3.71 kip/in; a 5/16 in fillet both sides of a 10 in plate is 2 × 5 × 0.928 × 10 ≈ 93 kips. It is exactly 0.30 × 70 ksi × 0.707 ÷ 16.

Is a bigger fillet weld always stronger?

Capacity rises with leg, but so do heat input, distortion and cost — roughly with leg squared for deposited metal. Codes also cap the leg along a plate edge (t − 1/16 in on edges 1/4 in and thicker) and set minimum sizes by the thicker part joined. Two smaller welds usually beat one oversized one.