K-Factor Calculator

The K-factor is the ratio that locates the neutral axis inside a bent sheet. When metal is bent, the inside compresses and the outside stretches; somewhere between them is the neutral axis, which neither stretches nor compresses. The K-factor is the distance from the inside face to that neutral axis, divided by the material thickness.

It runs from about 0.33 for a tight bend in soft metal to 0.50 for a large radius, and it is what makes the flat blank length come out right. A typical value for mild steel is around 0.38 to 0.45.

For example, mild steel with the bend radius equal to the thickness uses a K of about 0.38, putting the neutral axis 0.0475 inch into a 0.125 inch sheet. This calculator picks a K from your material and R-over-T ratio, or takes your own value.

The flat, or blank, length is the length of sheet you cut before bending so the finished part comes out to size. For a single bend using outside flange dimensions, it is Leg A plus Leg B minus the bend deduction. The bend deduction accounts for the material the bend effectively consumes.

The two legs are measured to the outside of the bend (the mold lines), and the bend deduction is worked out from the angle, radius, thickness and K-factor.

For example, a 90 degree bend in 0.125 inch mild steel at a 0.125 inch radius, with 2 inch and 3 inch outside legs, has a bend deduction of about 0.229 inch, so the flat length is 2 plus 3 minus 0.229, which is 4.771 inches. This calculator computes the flat length from your legs and bend.

Bend allowance is the arc length of the neutral axis through the bend — the amount of material actually used up in the curved part of the bend. It is the bend angle in radians times the sum of the inside radius and the K-factor times the thickness.

You add the bend allowance to the flat portions of the flanges (measured to the start of the bend) to get the total flat length when working from inside or tangent dimensions.

For example, a 90 degree bend in 0.125 inch steel at a 0.125 inch radius with a K of 0.38 has a bend allowance of about 0.271 inch. This calculator reports the bend allowance for your bend.

Bend deduction is how much shorter the flat blank is than the sum of the outside flange dimensions. It is twice the outside setback minus the bend allowance, and it is the figure you subtract from Leg A plus Leg B to get the flat length.

The outside setback is the distance from the bend’s tangent point to the apex where the two outside faces would meet — the tangent of half the angle, times the radius plus the thickness.

For example, a 90 degree bend in 0.125 inch steel at a 0.125 inch radius has an outside setback of 0.250 inch and a bend allowance of 0.271 inch, giving a bend deduction of 2 times 0.250 minus 0.271, which is 0.229 inch. This calculator gives the bend deduction directly.

As a starting point, use a K-factor that matches your material and the ratio of bend radius to thickness. Soft metals like aluminum sit lower, around 0.33 at a tight radius; mild steel is around 0.38; stainless and harder metals are around 0.40 and up. As the radius grows relative to the thickness, K rises toward 0.50 for all of them.

These are air-bending starting values. The real K depends on your press, tooling, die opening and the exact material lot, so treat a table value as a first guess.

For example, this calculator uses 0.38 for mild steel at a radius equal to the thickness, rising to 0.43 and then 0.50 as the radius grows. Always confirm with a test bend before committing a production blank.

They are two ways of accounting for the same bend, used with different measurements. Bend allowance is added: it is the arc length of the neutral axis, which you add to the flat flange portions (from inside or tangent dimensions) to get the total flat length.

Bend deduction is subtracted: it is twice the outside setback minus the bend allowance, which you subtract from the sum of the outside flange dimensions to get the flat length.

So if you dimension to the outside mold lines, use bend deduction; if you dimension to the bend tangents, use bend allowance. For a 90 degree bend in 0.125 inch steel the allowance is about 0.271 inch and the deduction about 0.229 inch. This calculator gives both.

The ratio of the inside bend radius to the material thickness — the R-over-T ratio — is the main driver of the K-factor. At a tight radius, where R is small compared with the thickness, the neutral axis shifts toward the inside and K is low, around 0.33 to 0.40. As the radius grows, the neutral axis moves toward the middle of the sheet and K rises toward 0.50.

This is because a sharper bend compresses the inside material more, pulling the neutral axis inward; a gentle bend stresses the section more evenly.

For example, mild steel goes from a K of 0.38 at an R-over-T of 1 up to 0.43 near an R-over-T of 2 and 0.50 at larger radii. This calculator adjusts K with the R-over-T ratio automatically.

Because the K-factor in any table is a typical starting value, not an exact property of your setup. The real neutral-axis position depends on the press, the die opening, the punch radius, the bending method and the specific material lot, so a table value can be off by enough to spoil a precise blank.

A test bend calibrates it: bend a sample of the actual material on the actual tooling, measure the result, and back-calculate the K-factor that makes the blank come out right. Use that value for the production run.

This calculator gives a solid air-bending starting point and lets you type your own measured K once you have it, so the flat blank length matches what your press actually produces.