What Is a Fillet Weld Throat Size Calculator?

A fillet weld's cross-section is (ideally) a right triangle formed by the two leg faces. The leg size is easy to measure with a fillet gauge, but weld strength calculations actually depend on the throat — the perpendicular distance from the weld root to the outer (hypotenuse) face.

This calculator finds the effective throat using the standard geometric formula for the altitude to the hypotenuse of a right triangle, which works for both equal-leg and unequal-leg fillet welds.

How to Read Your Results

Effective Throat Size

This is the critical dimension used in fillet weld strength calculations. For an equal-leg weld, it's always about 70.7% of the leg size; for unequal legs, the formula adjusts automatically.

Leg 1 & Leg 2

The two leg measurements you entered, shown back for confirmation. For a standard equal-leg fillet weld these should match.

Throat ÷ Smaller Leg

This ratio shows the throat as a percentage of the smaller leg — useful for a quick sanity check against the familiar 70.7% figure for equal-leg welds.

Real-World Example

A standard 1/4-inch equal-leg fillet weld (leg1 = leg2 = 0.25 in):

Quantity	Value
Leg size	0.250 in
Effective throat	0.250 × 0.250 ÷ √(0.250² + 0.250²) = 0.1768 in
Throat ÷ leg	0.1768 ÷ 0.250 = 70.7%

This confirms the well-known rule of thumb: for an equal-leg fillet weld, the effective throat is always 0.707 times the leg size.

Tips for Fillet Weld Sizing

Weld strength scales with throat, not leg size — always design and inspect against the throat dimension.
Convex or concave weld profiles can shift the actual throat slightly from this idealized flat-face calculation.
Unequal-leg fillet welds are sometimes used deliberately to match unequal base metal thicknesses — this calculator handles that case directly.
Check your governing code (such as AWS D1.1) for minimum fillet weld sizes based on base metal thickness before finalizing a design.

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Note: This calculator assumes an idealized flat-face fillet weld profile. Actual welds with convex or concave reinforcement, undercut, or incomplete fusion may have an effective throat that differs from this geometric calculation. Always verify against your applicable welding code.

Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team

Fillet Weld Throat Size Calculator

Fillet weld leg size