Sheet Metal Bend Allowance Calculator
Enter material thickness, bend radius, and angle to get the correct flat blank length for sheet metal parts. Supports up to 8 bends in a single part with material-specific K-factors. Nothing uploaded.
Cross-Section Diagram
Per-Bend Breakdown
| Bend | Angle | Radius | BA ? | BD ? | OSSB ? |
|---|
Learn more: sheet metal bending and K-factors
Why flat blank calculations are easy to get wrong
Sheet metal does not simply fold - it stretches and compresses through the bend zone. The neutral axis (the layer that neither stretches nor compresses) shifts towards the inside of the bend. The K-factor describes this shift as a fraction of material thickness. Use the wrong K-factor for your material and every flat blank you cut will be slightly wrong. The calculator includes material presets: mild steel (K=0.44), stainless steel (K=0.45), aluminium (K=0.41), and copper, with the option to enter a custom K-factor.
Understanding bend allowance and bend deduction
Bend Allowance (BA) is the arc length of material consumed by the bend through the neutral axis - it is added to the straight leg lengths to get the total flat blank. Bend Deduction (BD) is how much shorter the flat blank is compared to the sum of the outer dimensions. Different CAD systems and fabricators prefer one or the other depending on whether they measure from inner or outer faces.
FAQ
What K-factor should I use for copper, steel, or aluminium?
The K-factor is the ratio of the neutral axis position to the material thickness during bending, and it changes by material. The calculator's presets are K=0.44 for mild steel, K=0.45 for stainless steel, K=0.41 for aluminium, and K=0.43 for copper. These are starting points, not guarantees - actual K-factor also shifts with bend radius and thickness, so test-bend and adjust if your parts come out slightly long or short.
What is the difference between bend allowance and bend deduction?
Bend Allowance (BA) is the length of material consumed by the bend - the arc length along the neutral axis. Bend Deduction (BD) is the amount you subtract from the total flat blank length to account for the material pulled into the bend. Your CAD system typically uses one or the other.
Can I calculate flat blank length for parts with multiple bends?
Yes. The calculator supports up to 8 bends in one part. Add each bend with its angle, radius, and K-factor. The total flat blank length is the sum of all straight sections plus all bend allowances.