How to Use the Sheet Metal Bend Allowance Calculator
When you bend sheet metal, the finished part needs more flat material than simply adding the two outside leg lengths (A+B). Material compresses on the inside of the bend and stretches on the outside, effectively redistributing along the bend zone. This calculator uses the standard K-factor bend formula to compute the exact flat pattern length — how much material to cut before bending.
The calculation has two steps. First, setback (SB) = (radius + thickness) × tan(angle/2) gives the length lost at the corner. Then bend allowance (BA) = angle (radians) × (radius + K × thickness) gives the actual material consumed in the bend zone. The final flat length is leg A + leg B − 2 × SB + BA.
The K-factor represents where the neutral axis sits within the material thickness. Mild steel commonly uses 0.44, but the true value can range from about 0.3 to 0.5 depending on material, thickness, and the radius-to-thickness ratio. For production runs, verify K-factor with an actual test bend on your press brake, and double-check any mismatch between drawings and shop-floor measurements.
Frequently Asked Questions
You need to know how much flat stock to cut before bending. Simply adding the finished leg lengths (A+B) ignores the extra material consumed at the bend, giving a blank that's too short.
0.44 is the most common default for mild steel. Softer or thicker materials, and tighter radius-to-thickness ratios, tend to shift the K-factor lower — test bends on your actual press brake for the most accurate value.