Bend Allowance and K-Factor Calculator

BA = π ÷ 180 × A × (R + K × T), where A is the bend angle in degrees, R the inside bend radius, T the sheet thickness and K the K-factor. For 2 mm sheet, 2 mm inside radius, 90° and K = 0.40, BA ≈ 4.398 mm.

The K-factor is the distance from the inside surface to the neutral axis divided by the sheet thickness, usually between 0.3 and 0.5. As a start, use 0.33 for R/T below 1, 0.40 for R/T 1–3 and 0.50 above 3. Confirm it with a test bend for critical parts.

The bend allowance (BA) is the arc length added to the straight flange lengths. The bend deduction (BD) is subtracted from the sum of the outside dimensions: BD = 2 × OSSB − BA. If the drawing is dimensioned to the outside, working with BD is more practical.

The A in the formulas is the angle bent from the flat sheet. A bend with a 90° included angle is a 90° bend; a bend with a 120° included angle is a 180 − 120 = 60° bend. If you select the included angle in the calculator, the conversion is automatic.

Bend a test piece of known flat length with the same material, thickness and tooling, then measure the outside dimensions of both flanges and the inside radius. In "Find K from a measured flat length" mode the K-factor is calculated as K = (BA × 180 ÷ (π × A) − R) ÷ T.

The actual sheet thickness, the inside radius produced by the V-die, springback and the material batch all affect the result. The formula gives a starting value; verify it with a test bend before series production and record it for that material, thickness and tooling.