Fillet weld geometry in plain numbers — effective throat and cross-sectional area from leg size, for equal-leg and unequal-leg welds. The shapes underneath the drawing callout.
Throat = leg × 0.707
Equal-leg 90° fillet — leg ÷ √2
Throat ≈ (a × b) ÷ √(a² + b²)
Unequal-leg fillet — shortest distance from root to face
In a 90° fillet the weld cross-section is a right triangle. The effective throat is the shortest distance from the root of the joint to the face of the weld — the dimension that carries the load. For equal legs it falls at 0.707 × leg; for unequal legs it's the perpendicular from the root to the hypotenuse.
| Variable | Meaning | Typical |
|---|---|---|
| Leg size | Weld leg along each member | 3–12 mm |
| Throat | Load-carrying depth | 0.707 × leg |
| Area | Cross-section of weld metal | leg² ÷ 2 |
These are pure geometric relationships. Codes and WPS documents can define their own effective throat rules — see the notes below.
Quick reference for the sizes that show up on everyday shop drawings — equal-leg, 90° fillets.
| Leg | Throat (0.707) | Area (leg² ÷ 2) | Typical Use |
|---|---|---|---|
| 6 mm (1/4") | 4.2 mm | 18 mm² | Light framing, sheet |
| 8 mm (5/16") | 5.7 mm | 32 mm² | General structural |
| 10 mm (3/8") | 7.1 mm | 50 mm² | Heavy structural |
| 12 mm (1/2") | 8.5 mm | 72 mm² | Heavy plate, base plates |
A 10 mm fillet is not "twice the weld" of a 5 mm fillet — the throat scales with the leg, but the area (and the weld metal needed) scales with the square.
Strength calculations are sized on the effective throat, not the leg. If a drawing says "6 mm fillet," it's the leg being called out — the throat is what carries the load underneath it.
An unequal-leg fillet shows up on skewed joints or where one member is thicker. The throat here is an approximation from right-triangle geometry — confirm against the code's definition for the actual joint.
These values are geometric estimates. Whether a weld is acceptable under AWS, ISO, ASME, or any other code is decided by the code, the WPS, and the inspection — not by this calculator.
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