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K-Factor Calculator

Use our K-factor calculator to determine the position of the neutral axis in the sheet of metal you are planning to bend.

0.33 for a tight bend, 0.5 for a generous one.
Clear
Bend allowance0.28274 inthe arc length along the neutral axis
Bend deduction0.21726 insubtract this from the sum of the outside flanges
Setback0.25 in(R + T) × tan(angle ÷ 2)
Neutral axis position0.055 infrom the inside surface — K × T
Neutral axis radius0.18 in
Flat blank length3.78274 in2 + 2 − 0.2173 deduction
By the bend-allowance route3.78274 inthe two flat legs plus the bend allowance — the same answer, arrived at differently
Do the two methods agree?yes
Inside bend radius0.125 inratio to thickness 1
Minimum radius rule of thumb0.125 inbending tighter than one thickness risks cracking on the outside
K-factor used0.44measure it from a test bend for real accuracy — it varies with material and tooling

The formula

BA = (π ÷ 180) × angle × (R + K × T); the neutral axis sits at K × T

Metal stretches on the outside

Bend a sheet and the outside surface is in tension while the inside is in compression. Somewhere between them is a plane that neither stretches nor shortens — the neutral axis — and the K-factor says where: 0.44 means 44% of the way in from the inside face.

That position is what makes flat blank development possible. The bend allowance is the arc length along the neutral axis, so adding the flat legs to it gives the length of material you have to start with.

K is not truly constant. It shifts with the radius-to-thickness ratio, the material, and the tooling, and the only reliable value is one measured from a test bend in your own shop. Published values are a starting point.