What is the K-factor in sheet metal bending?

The K-factor is the single number that tells you where the neutral axis sits inside a sheet metal bend. Get it right and your flat blanks come out the correct length on the first try; get it wrong and every part is short or long by a predictable, frustrating amount. It is the key input to the bend allowance and bend deduction that drive every flat-pattern layout.

Why the neutral axis moves

When a strip of metal is bent, the material on the inside of the bend compresses and the material on the outside stretches. Somewhere between the two there is a layer whose length does not change at all — the neutral axis. Because metal resists compression more than it resists being stretched, this neutral layer does not sit in the middle of the thickness: it shifts toward the inside of the bend.

That shift is exactly what the K-factor measures. It is the fraction of the way through the material thickness at which the neutral axis lies, measured from the inside surface:

K = t / T

where:

• t — the distance from the inside surface of the bend to the neutral axis.
• T — the material thickness.

A K-factor of 0.5 would put the neutral axis dead-centre; the real value is almost always less, because the axis has migrated inward. Since the neutral axis sits a distance K·T out from the inside surface, and the inside surface is at the bend (inside) radius R, the radius of the neutral axis is:

R_neutral = R + K · T

How the K-factor sets the bend allowance

The neutral axis is the one layer that keeps its length through the bend, so the arc length of the neutral axis is the material consumed by the bend. That arc length is the bend allowance (BA):

BA = (π / 180) · angle · (R + K · T)

Here angle is the bend angle in degrees (the amount the metal sweeps through, equal to 180° minus the included angle of the finished part). The (π / 180) · angle factor simply converts that swept angle to radians, and multiplying by the neutral-axis radius R + K·T gives the true arc length. This is why the K-factor matters so much: it is buried inside the term that decides how much of your flat blank disappears into every bend. Add up the bend allowances and the remaining flat segments and you have the blank length.

Typical K-factor values

For sheet metal the K-factor almost always lands between 0.33 and 0.50. A reasonable starting point for air-bent mild steel is around 0.42–0.45. The value is not fixed, though — it depends on the geometry and the forming method:

• Tight inside radii (small R relative to T) push the neutral axis further inward, giving a lower K-factor.
• Generous inside radii (large R/T) keep the neutral axis closer to the centre, so K rises toward 0.5 as R/T grows.
• Forming method matters. Air bending, bottoming and coining stress the material differently, so a K-factor calibrated for one will not be exact for another.

Deriving the K-factor from a test bend

Published tables are a starting point, not gospel. The reliable way to get the K-factor for your material, thickness and tooling is a test bend:

1. Cut a blank of known length and bend it to a known angle with your production tooling.
2. Measure the finished flat dimensions of the part and add them up.
3. The difference between the original blank length and the sum of the measured flats is the bend allowance the bend actually consumed.
4. Plug that measured BA, your radius, thickness and angle into BA = (π/180)·angle·(R + K·T) and rearrange to solve for K.

For any production work, a test bend per material/thickness/tooling combination is strongly recommended — it converts a generic estimate into a number you can trust.

Worked example

Take 2 mm steel air-bent to a 90° bend over a 2 mm inside radius, with a K-factor of 0.44:

R_neutral = R + K · T = 2 + 0.44 · 2 = 2.88 mm

BA = (π / 180) · 90 · 2.88 ≈ 4.52 mm

So this single 90° bend eats about 4.52 mm of flat material. Subtract the bend allowance from the sum of the outside dimensions and you have the bend deduction; you can confirm both numbers with the bend deduction calculator. To go straight from angle, radius, thickness and K-factor to bend allowance, setback and blank length, use the bend allowance calculator.

Frequently asked questions

What is a typical K-factor value?

For most sheet metal the K-factor falls between 0.33 and 0.50, and a value around 0.42–0.45 is a common starting point for air-bent mild steel. The number rises toward 0.5 as the inside radius grows relative to thickness, and drops for very tight bends. Bottoming and coining shift it as well, so treat any table value as a starting point and confirm it with a test bend.

How do I find the K-factor for my exact setup?

Bend a known blank, then measure the finished flat length and back-solve. The bend consumes material equal to the bend allowance BA, so you can compute BA from your measured dimensions and rearrange BA = (π/180)·angle·(R + K·T) to solve for K. Doing this once for each material, thickness and tooling combination is far more reliable than any generic table.

Does the K-factor change with bend angle?

The K-factor itself is mostly a property of the material, the inside radius and the forming method, not the angle. The angle enters the bend allowance separately through the (π/180)·angle term. That said, very sharp or very shallow angles and different forming methods can shift the effective neutral-axis position, which is one more reason to validate with a physical test bend.

Last reviewed: 2026-05-29.