What this calculator does
The K-factor calculator helps sheet metal fabricators determine the flat pattern length needed before bending, by estimating the K-factor (the ratio describing where the neutral bend line sits within the material thickness) along with bend allowance and bend deduction.
The formula
The K-factor typically ranges from about 0.33 for tight bend-radius-to-thickness ratios up to about 0.44-0.50 for larger ratios, since the neutral axis shifts outward as the radius increases relative to thickness. Bend allowance is calculated as the bend angle’s complement (in radians) multiplied by the sum of the inside radius and K-factor times thickness: BA = (π − angle) × (R + K×T). Bend deduction subtracts the bend allowance from twice the outside setback distance. The flat pattern length is the sum of the flange lengths minus the bend deduction.
- K-factor values vary by material and bending method; the estimate here uses commonly published rule-of-thumb ranges.
- For critical or production work, verify K-factor against your specific material, machine, and tooling through test bends.
This tool is for general estimating and educational purposes; always confirm dimensions before cutting production parts.