Bend Allowance Calculator

Calculate bend allowance, bend deduction, and K-factor to develop accurate flat patterns for any bend.

When you fold sheet metal, the bend consumes material along the neutral axis. To cut the right flat blank, you account for that consumed length — the bend allowance — at every bend.

The K-factor places the neutral axis inside the thickness. Calibrate K from a test bend in your own material and tooling for the best accuracy.

Formula: BA = (π / 180) × Angle × (R + K × T)

Worked Example

  1. A 90° bend, inside radius R = 0.125 in, thickness T = 0.060 in, K = 0.44.
  2. BA = (π/180) × 90 × (0.125 + 0.44 × 0.060) = 1.5708 × 0.1514 = 0.238 in.
  3. Add the two flat flange lengths plus this 0.238 in to get the blank length.

Frequently Asked Questions

What is bend allowance?

Bend allowance is the arc length of the neutral axis through a bend. It is added to the flange lengths to find the flat blank length.

What is a typical K-factor?

K-factor commonly ranges from 0.33 to 0.50 depending on material and bend radius; 0.44 is a common starting point for mild steel air bending.

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