Bend Allowance Calculator
Flat Stock Doesn't Stay Flat Once You Bend It
Sheet metal stretches slightly on the outside of a bend and compresses slightly on the inside, and the neutral axis in between shifts depending on the bend angle, radius, and material thickness. Bend allowance quantifies that shift, which is exactly what's needed to lay out a flat pattern that folds up to the correct final dimensions instead of coming out too long or too short.
The Formula
The K-factor represents where the neutral axis sits within the material thickness as a proportion from the inside surface, and it shifts with bend radius, material, and tooling — 0.33 to 0.50 is a typical range for air bending, but shop-specific press brake data gives a more precise value.
Worked Examples
| Inside Radius | Thickness | K-Factor | Bend Allowance | Bend Deduction |
|---|---|---|---|---|
| 0.125 | 0.075 | 0.33 | 0.2352 | 0.1648 |
| 3 | 2 | 0.44 | 6.0947 | 3.9053 |
Results are unitless multiples of whatever unit you enter for radius and thickness — use consistent units throughout (inches or millimeters), and the output comes out in that same unit.
Where This Calculation Matters
- Flat pattern development — calculating the correct starting blank length so a formed part hits its final bent dimensions accurately.
- Press brake programming — setting up bend sequences for sheet metal fabrication where flange length tolerances matter.
- Design for manufacturability — checking whether a specified bend radius and thickness combination is achievable and predictable before releasing a drawing.
How to Use This Calculator
- Enter Bend Angle (degrees, 0-180).
- Enter Inside Radius and Material Thickness, using consistent units.
- Enter a K-Factor if known, or leave the default of 0.33.
- Select Calculate to get both the bend allowance and the estimated bend deduction.
Related Calculations
Before setting up the brake, confirm it has enough force with the Press Brake Tonnage Calculator.