Work out sheet metal bend allowance instantly with clear inputs, formula shown and shareable results.
Metal stretches on the outside of a bend and compresses on the inside, and the neutral axis that changes length least sits at the inside radius plus the K-factor times thickness. Bend allowance is the arc length along that neutral axis, and bend deduction, twice the setback minus the allowance, is what is subtracted from the sum of the outside flange dimensions to get the flat blank.
Bend allowance
BA = angle in radians x (R + K x T)
Bend deduction
Setback = tan(angle/2) x (R + T); BD = 2 x setback - BA
BA = angle in radians x (R + K x T). Metal stretches on the outside of a bend and compresses on the inside, and the neutral axis that changes length least sits at the inside radius plus the K-factor times thickness.
About 0.44 for mild steel at typical radii, dropping toward 0.33 for tight radii and rising toward 0.50 for generous ones. Measuring a test bend on your own press beats any published value.
This calculator takes 4 inputs: Material thickness, Inside bend radius, Bend angle, K-factor. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.