Bend Allowance Calculator
The flat length of sheet metal that becomes a bend — the bend allowance from thickness, inside radius, bend angle and K-factor — with the bend deduction, the setback, and the flat blank length for a part with two flanges.
When sheet bends, the outside stretches and the inside compresses; somewhere between is a neutral axis that keeps its length, and the K-factor says where — as a fraction of the thickness from the inside.
How the bend allowance calculator works
When sheet bends, the outside stretches and the inside compresses; somewhere between is a neutral axis that keeps its length, and the K-factor says where — as a fraction of the thickness from the inside. The arc length of the neutral axis through the bend is the bend allowance, the material the bend consumes. Flange lengths are measured to the outside corner, so the flat blank is the flanges minus the bend deduction, which is twice the setback less the allowance.
Formula: BA = (π/180) × angle × (R + K·T); setback = (R + T) tan(angle/2); BD = 2 × setback − BA; flat = flange₁ + flange₂ − BD
Worked examples
| Inputs | Bend allowance (mm) | Note |
|---|---|---|
| 2 mm sheet, 2 mm radius, 90° | 4.4611 | allowance 4.46 mm |
| A sharp bend | 1.8221 | a smaller allowance |
| A 45° bend | 2.2305 | half the arc |
FAQFrequently asked questions
What is bend allowance?
The length of material in the bend itself, measured along the neutral axis. Add it to the straight portions to get the flat length; it is what a bend consumes.
What is the K-factor?
The position of the neutral axis as a fraction of the thickness from the inside face — 0.5 would be the middle. Real bends compress the inside more than they stretch the outside, so the axis moves inward: about 0.33 for tight bends, 0.42 to 0.45 for typical, 0.5 for large radii.
What is bend deduction?
The amount to subtract from the sum of the outside flange lengths to get the flat. It is twice the setback minus the bend allowance, and it is what most shop drawings quote.
What inside radius should I use?
At least one material thickness for steel and two for aluminium, to avoid cracking. The radius the tooling actually forms is what matters; air bending gives a larger radius than the punch tip.
Do these numbers replace a bend test?
No — they are the standard first estimate. Shops measure a test bend in the real material on the real tooling and back out their own K-factor, which is what goes in the field.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.