Bend Allowance Calculator

What bend allowance actually is

When sheet metal bends, the outside surface stretches and the inside compresses. Somewhere between them is the neutral axis — the layer that neither stretches nor compresses. Bend allowance is the arc length measured along that neutral axis, and it is the amount of material a bend consumes.

BA = (π / 180) × (R + K × t) × A

BA — bend allowance in mm
R — inside bend radius in mm
K — K-factor (where the neutral axis sits)
t — material thickness in mm
A — bend angle in degrees

Flat blank length

Add the bend allowance for every bend to the sum of your flat flange lengths:

Flat length = Σ flange lengths + Σ bend allowances

Bend deduction — the other way to do it

Many shops work from outside dimensions instead. Bend deduction is subtracted from the sum of the two mold-line (outside) lengths, and the calculator returns it alongside bend allowance:

BD = 2 × OSSB − BA
where OSSB = (R + t) × tan(A / 2)

Choosing a K-factor

K-Factor	Use for
0.33	Sharp bends or soft material
0.38	Mild steel — the standard default
0.41	Stainless steel
0.44	Aluminum
0.50	Large radius bends where R > 2t

Calibrate against a test bend

K-factor is a model, not a constant of nature. It shifts with tooling, material batch and bend method. For tight-tolerance work, bend a test coupon, measure the actual flat length, and back-calculate the K-factor your specific setup produces.

Frequently Asked Questions

What is the formula for bend allowance?

BA = (π / 180) × (R + K × t) × A, where R is the inside bend radius, K is the K-factor, t is material thickness, and A is the bend angle in degrees. The result is the arc length along the neutral axis, in mm.

What K-factor should I use for mild steel?

0.38 is the standard starting value for mild steel. Stainless steel typically runs around 0.41 and aluminum around 0.44. For large-radius bends where the inside radius exceeds twice the material thickness, 0.50 is common.

What is the difference between bend allowance and bend deduction?

Bend allowance is added to the sum of your flat flange lengths to get the flat blank length. Bend deduction is subtracted from the sum of the outside mold-line dimensions to reach the same answer. They are two routes to the same flat pattern — use whichever matches how your drawing is dimensioned.

Why is my flat pattern still coming out wrong?

The most common cause is a K-factor that does not match your actual tooling and material. K-factor varies with die opening, bend method and material batch. Bend a test coupon, measure the real flat length, and back-calculate the K-factor your setup actually produces.