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Bend allowance calculator

Enter your outside dimensions, your angles and your inside radius: you get the flat length to cut before bending. The K-factors offered are the ones the Koventor configurator applies, and you can replace them with your own.

K-factor by material

Mild steel (S235): 0.38
S355 steel: 0.4
Galvanised steel: 0.38
304 stainless: 0.33
Aluminium: 0.42

These are the values the configurator applies. Your real K depends on your tooling: the field below takes yours.

MaterialThickness (mm)K-factor

Outside dimensions, segment by segment

A profile with 2 bends has 3 segments.

Bends

P1Angle (°)Rayon int. (mm)

P2Angle (°)Rayon int. (mm)

The formula, no mystery

Longueur de fibre neutre

L = π / 180 × angle × (Ri + k × e)

Sheet does not stretch or compress through its whole thickness: there is a fibre that keeps its length. The K-factor says where it sits, as a proportion of the thickness. The more ductile the material, the further out it shifts.

Retrait par pli

retrait = 2 × (Ri + e) × tan(angle / 2) − L

That is what has to come off the sum of the outside dimensions. On a 90° bend in 2 mm steel with a 2 mm inside radius, the deduction is around 3.5 mm: ignored across five bends, the error passes a centimetre.

Material	K-factor applied
Mild steel (S235)	0.38
S355 steel	0.40
Galvanised steel	0.38
304 stainless	0.33
Aluminium	0.42

These are the values the Koventor configurator applies, not an average picked up elsewhere. A workshop that has measured its own K-factor should use that one.

Going further

Bend radius chartV width, resulting radius and force needed, from 1 to 10 mm and by material.Minimum bend radiusThe values by material, and why the V decides the radius, not you.SpringbackWhy a 90° bend does not come out at 90, and the three ways to compensate.Bending forceTonnage before the run: thickness squared, and the three mistakes that cost a sheet.Minimum distance to edgeThe half-V rule, holes too close to the line, and bends that will not go.What the quote must sayThe wording that prevents almost every dispute after delivery.

What if your customers ran this calculation themselves?

That is what Koventor does: the customer draws their profile, the flat length is worked out with YOUR factors, and you receive the quote, the DXF and the bend sheet. You retype nothing.

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