Sheet Metal Bend Allowance Calculator
Bend allowance, bend deduction and flat pattern length from the K-factor.
Springback is not included. Air-bent mild steel typically springs back 1–3°, stainless more; the press is overbent to compensate and the amount is a property of your tooling.
How it works
Bent metal is longer flat than the sum of its legs, because the outside of the bend stretches and the inside compresses. Somewhere between them is a layer that does neither — the neutral axis — and the flat length is the length of that layer.
The K-factor is where the neutral axis sits, as a fraction of thickness measured from the inside face. It is never 0.5, because material moves outward under the punch: the neutral axis drifts toward the inside of the bend, so K sits between about 0.33 and 0.45 for ordinary sheet.
bend allowance = angle in radians × (radius + K × thickness)
outside setback = tan(angle ÷ 2) × (radius + thickness)
bend deduction = 2 × setback − bend allowance
and the flat is the legs measured to the outside, minus one deduction per bend.
K rises with the radius-to-thickness ratio, which is why the automatic setting reads it off that ratio rather than off the material alone. A tight bend where the radius equals thickness crushes the inside hard and K falls toward 0.33; a generous one past three times thickness barely deforms and K approaches 0.45.
None of this is exact until you have bent a test piece on your press with your tooling. Every shop's K-factor is measured, not looked up. Bend one strip, measure the flat you actually needed, and work K backward from it — then that number is right for that machine and that material for good.
Common questions
Allowance or deduction — which do I use?
Deduction, if your legs are dimensioned to the outside of the part, which is how drawings are normally made: flat = A + B − deduction. Allowance is for legs measured to the tangent point where the bend starts, which is rarer.
How do I find my real K-factor?
Bend a test strip of known length, measure the two legs afterwards, and solve backward. Ten minutes of that beats any table, because it captures your punch, your die and your material's actual temper.
Why does my bend crack on the outside?
The radius is too tight for the material. Aluminium in hard tempers is the usual culprit — 6061-T6 wants a radius of two to three times thickness and will crack at one. Bend across the grain if you can; along it is far worse.
What V-die should I use?
About eight times thickness for mild steel under 3 mm, rising to ten or twelve for thicker plate. A narrower die bends a tighter radius and needs much more tonnage; a wider one is gentler but limits how short a flange you can form.