Sheet Metal Bend Allowance Calculator

How it works

Bend allowance = (π ÷ 180) × angle × (R + K·T) · Setback = (R + T)·tan(angle ÷ 2) · Bend deduction = 2·setback − BA · Flat length = flangeA + flangeB − BD

When sheet metal bends, the outside stretches and the inside compresses; somewhere between sits the neutral axis, whose position the K-factor captures (a fraction of thickness, typically 0.3–0.45). The bend allowance is the arc length of that neutral axis through the bend — the material actually consumed in the radius — so the flat blank equals the sum of the flat flanges plus each bend allowance. Fabricators more often work from the bend deduction, subtracting it from the sum of the outside flange dimensions to get the blank size to shear.

Worked example

For T = 2 mm, R = 2 mm, 90° bend, K = 0.33: bend allowance = (π ÷ 180) × 90 × (2 + 0.33 × 2) = 1.5708 × 2.66 = 4.18 mm. Setback = (2 + 2) × tan(45°) = 4.00 mm, so bend deduction = 2 × 4.00 − 4.18 = 3.82 mm. Two 50 mm outside flanges give a flat blank of 50 + 50 − 3.82 = 96.18 mm.

Frequently asked questions