Fillet Weld Calculator

Convert between Leg Length (what the welder measures) and Theoretical Throat (what the engineer calculates for strength).

The measured length from the root to the toe of the fillet weld.
The minimum cross-sectional distance from the root to the face.
Weld Geometry Results
Leg Size (z)

--

in
Theoretical Throat (a)

--

in
Cross-Sectional Area

0.0

sq in

Used to estimate wire consumption and heat input.

Weld Geometry Reference
Leg (z) Leg (z) Throat (a) Root
The Design Disconnect

Engineers calculate the required strength of a weld based on the Throat (a), because it is the narrowest point of the weld cross-section (the weakest link). However, welders cannot easily measure the throat. Instead, they use a fillet gauge to measure the Leg (z).

Standard 90° Conversions
Throat = Leg × 0.7071
Leg = Throat × 1.414
Effective Throat vs Theoretical: This calculator determines the Theoretical throat assuming zero root penetration. Deep penetrating processes (like Submerged Arc) create an Effective throat that is larger than the theoretical one.