Bend Allowance Calculator
Calculate the bend allowance (BA), bend deduction (BD), outside setback and flat blank length for a sheet metal bend. Enter thickness, inside radius, bend angle and K-factor in any consistent unit.
Bend allowance formulas
The bend allowance is the length of the neutral axis through the bend. The K-factor places that neutral axis at K × T from the inside surface; see the K-factor calculator to derive K from a test bend and for a K-factor chart by material. The outside setback is the distance from the bend tangent line to the mold line (where the outside surfaces of the flanges would intersect), and the bend deduction is what you subtract from the sum of the outside flange dimensions to get the flat blank. Bend angle here is the angle through which the sheet is bent (90° for an L-bracket), not the included angle between flanges.
Worked examples
90° bend: T = 2 mm, R = 2 mm, K = 0.44. BA = 1.5708 × (2 + 0.88) = 4.524 mm; OSSB = tan 45° × 4 = 4.000 mm; BD = 8.000 − 4.524 = 3.476 mm. Flanges of 50 and 30 mm (outside) need a flat blank of 80 − 3.476 = 76.52 mm.
45° bend in inches: T = 0.060 in, R = 0.060 in, K = 0.42. θ = 0.7854; BA = 0.7854 × (0.060 + 0.0252) = 0.0669 in; OSSB = tan 22.5° × 0.120 = 0.0497 in; BD = 0.0994 − 0.0669 = 0.0325 in.
Bend allowance table
| T | R | Angle | BA | BD | OSSB |
|---|---|---|---|---|---|
| 0.5 mm | 0.5 mm | 45° | 0.565 | 0.263 | 0.414 |
| 0.5 mm | 0.5 mm | 90° | 1.131 | 0.869 | 1.000 |
| 0.5 mm | 0.5 mm | 135° | 1.696 | 3.132 | 2.414 |
| 1 mm | 1 mm | 45° | 1.131 | 0.526 | 0.828 |
| 1 mm | 1 mm | 90° | 2.262 | 1.738 | 2.000 |
| 1 mm | 1 mm | 135° | 3.393 | 6.264 | 4.828 |
| 1.5 mm | 1.5 mm | 45° | 1.696 | 0.789 | 1.243 |
| 1.5 mm | 1.5 mm | 90° | 3.393 | 2.607 | 3.000 |
| 1.5 mm | 1.5 mm | 135° | 5.089 | 9.396 | 7.243 |
| 2 mm | 2 mm | 45° | 2.262 | 1.052 | 1.657 |
| 2 mm | 2 mm | 90° | 4.524 | 3.476 | 4.000 |
| 2 mm | 2 mm | 135° | 6.786 | 12.528 | 9.657 |
| 3 mm | 3 mm | 45° | 3.393 | 1.578 | 2.485 |
| 3 mm | 3 mm | 90° | 6.786 | 5.214 | 6.000 |
| 3 mm | 3 mm | 135° | 10.179 | 18.792 | 14.485 |
K = 0.44, R = T. Results in mm.
Tips
- Use the actual inside radius produced by your tooling: in air bending it is roughly 15–16% of the V-die opening for mild steel.
- Keep all inputs in one unit (mm or inches). The angle is always in degrees.
- For parts with several bends, subtract one bend deduction per bend from the sum of outside dimensions.
Frequently asked questions
What is bend allowance in sheet metal?
Bend allowance is the arc length of the neutral axis through a bend. It is added to the flat lengths of the flanges (measured to the bend tangent lines) to get the blank length.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to inside-tangent flat lengths; bend deduction is subtracted from the sum of outside mold-line flange dimensions. Both produce the same flat pattern.
What K-factor should I use?
0.44 is a common starting point for air-bent mild steel with R ≈ T. Softer materials and tight radii are lower (0.33–0.40); large radii approach 0.5. Test bends give the best value.
Is the bend angle the included angle?
No. Use the angle the sheet is bent through. A 90° L-bracket is a 90° bend; a part with a 135° included angle is a 45° bend.
Can I use inches?
Yes. Any length unit works as long as thickness, radius and flange lengths all use the same unit.