Sheet metal bend allowance calculator
The flat blank is the flanges’ outside dimensions less one bend deduction per bend: BD = 2·OSSB − BA. A 90° bend in 2 mm sheet at a 2 mm inside radius, with 40 and 30 mm outside flanges and K = 0.325, has a bend allowance of 4.163 mm, a setback of 4.000 mm and a deduction of 3.837 mm: the blank is 66.163 mm. Enter your sheet, radius and flanges, add as many bends as the part has, and the drawing shows the neutral axis and the blank it unfolds to.
Sheet and bend
- Flat blank L
- K-factor (DIN 6935)
- Bend allowance BA
- Outside setback OSSB
- Bend deduction BD
How it’s calculated
A is the bend angle (180° − the included angle). BA = (π/180)·A·(R + K·t); OSSB = tan(A/2)·(R + t); BD = 2·OSSB − BA. Blank = Σ outside dimensions − Σ BD, or Σ legs + Σ BA, each leg its dimension less the setback at each bend (R·tan(A/2) for inside dimensions). Dimensions run to the virtual sharp at every angle. Blank K uses DIN 6935: k = 0.65 + 0.5·log₁₀(R/t), 1 past R/t = 5, and K = k/2.
Typical K-factors
| Method | Material | R ≤ t | t < R ≤ 3t | R > 3t |
|---|---|---|---|---|
| Air bending | soft (aluminium) | 0.33 | 0.40 | 0.50 |
| Air bending | medium (steel) | 0.38 | 0.43 | 0.50 |
| Air bending | hard (stainless) | 0.40 | 0.45 | 0.50 |
| Bottoming | soft (aluminium) | 0.42 | 0.46 | 0.50 |
| Bottoming | medium (steel) | 0.44 | 0.47 | 0.50 |
| Bottoming | hard (stainless) | 0.46 | 0.48 | 0.50 |
| Coining | soft (aluminium) | 0.38 | 0.44 | 0.50 |
| Coining | medium (steel) | 0.41 | 0.46 | 0.50 |
| Coining | hard (stainless) | 0.44 | 0.47 | 0.50 |
What is the K-factor?
When sheet bends, the outside stretches and the inside is squeezed. One layer between them, the neutral axis, keeps its length, so the blank is measured along it. The K-factor says where it runs, as a share of the thickness from the inside face: K = 0.5 is mid-thickness, and tight bends pull it nearer the inside. The bend allowance is that layer’s arc, BA = (π/180)·A·(R + K·t), with A the angle the sheet is bent through (180° less the included angle).
Leave K blank and the calculator takes it from R/t with the DIN 6935 correction factor, k = 0.65 + 0.5·log₁₀(R/t), which is twice the K-factor CAD uses. Or load a typical value from the table by bending method and material. A test bend in your own material on your own tools beats both.