What is bend allowance?
Bend allowance is the arc length of the neutral axis (the layer of material that neither stretches nor compresses) within a bend. Fabricators use it to calculate the flat pattern length needed before bending sheet metal to a finished shape.
The formula
Bend Allowance = θ × (R + K × T), where θ is the bend angle in radians, R is the inside bend radius, T is material thickness, and K is the K-factor, a value between 0 and 1 describing where the neutral axis sits within the material thickness (commonly around 0.44 for many materials and bend setups).
The K-factor varies with material type, thickness, bend radius, and tooling, so shops often determine it empirically or from material-specific tables for precise work. This calculator is for general fabrication reference and educational purposes.