Sheet Metal Bend Allowance Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Enter Values
How to use this calculator
- Enter the bend angle in degrees, the inside bend radius and the material thickness.
- Optionally set the K-factor (typically about 0.33 for steel; leave blank to use it).
- Read the bend allowance, setback and bend deduction.
How it works
The Sheet Metal Bend Allowance Calculator works out the extra length of material consumed in a bend so you can lay out an accurate flat pattern.
Bend allowance BA = (π/180)·angle·(R + K·t), where R is the inside radius, t the thickness and K the K-factor (where the neutral axis sits). Setback SB = (R + t)·tan(angle/2), and bend deduction BD = 2·SB − BA. The flat length is the sum of the flange lengths plus the bend allowance (or minus the bend deduction).
Worked example
90° bend, R3, t2, K0.33. BA = (π/180)×90×(3 + 0.33×2) ≈ 5.75 mm of material is taken up by the bend.
Frequently asked questions
What K-factor should I use?
It depends on the material, thickness and tooling — commonly 0.3–0.45, with about 0.33 a reasonable starting point for mild steel. Confirm with test bends for production parts.
What's the difference between bend allowance and bend deduction?
Bend allowance is added to the flange lengths measured to the bend lines; bend deduction is subtracted from the outside dimensions. Both give the same flat length when applied correctly.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.