Bend allowance calculator

Flat pattern length for sheet metal, with the K-factor that fits your press.

The formulas

BA = (π ÷ 180) × angle × (R + K × T)
OSSB = tan(angle ÷ 2) × (R + T)
BD = 2 × OSSB − BA
flat length = Σ flanges − Σ BD

Bend angle is not the included angle

This is the error that ruins more flat patterns than any other. Bend angle means the angle the material turns through. A square corner is a 90° bend, not a 270° one, even though the included angle inside the part is 90° too and the two happen to coincide. They stop coinciding the moment the bend is not square, and a 30° bend entered as 150° produces a part that is visibly wrong.

K-factor starting points

MaterialK
Soft aluminum0.33
Aluminum 60610.40
Copper / brass0.38
Mild steel0.446
Galvanized steel0.45
Stainless steel0.50

K-factor depends on the tooling and how the press is set, not only on the material. Bend a test coupon, measure the flat you actually needed, and work backwards before committing a batch.

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