Bend allowance (BA) is the amount of material consumed by a bend, measured along the neutral axis — the layer inside the material that is neither stretched nor compressed. When you develop a flat pattern, you add the bend allowance to the flat lengths of the flanges to get the total blank length.
BA depends on the bend angle, the inside bend radius, the material thickness, and the K-factor (which locates the neutral axis). For a 90° bend: BA = (π / 2) × (R + K × T), where R is the inside radius, T is thickness, and K is the K-factor.