K-factor calculator
Every other bend formula needs a K-factor, and the only accurate one is the one measured on your own press brake. Bend a test coupon, put a caliper on it, and this works the K-factor backwards from what you measured.
Measurements from your test bend
How to run the test bend
- Cut a strip of the exact material and thickness you will use for the real part. Length is not critical, but measure it precisely — 100 mm is convenient.
- Bend it once, roughly in the middle, using the same tooling and the same die opening the real job will run on. That last part matters: in air bending the die sets the radius.
- Measure the resulting inside radius. A radius gauge is ideal; failing that, take it as the die opening divided by about 6.5 for mild steel.
- Measure both outside flange dimensions with a caliper, and confirm the angle with a protractor.
- Enter all of it above. The K-factor that comes out is yours to reuse.
Repeat the test whenever the material, thickness or tooling changes. A K-factor measured on 2 mm mild steel says nothing useful about 5 mm stainless.
Typical K-factor values
Starting points only — use them until you have measured your own. R is the inside bend radius and T the material thickness.
| Material | Radius to thickness | Typical K |
|---|---|---|
| Mild steel | R < T | 0.33 – 0.38 |
| Mild steel | R ≈ T | 0.38 – 0.42 |
| Mild steel | R > 3T | 0.45 – 0.50 |
| Stainless steel | R ≈ T | 0.40 – 0.45 |
| Aluminium (soft) | R ≈ T | 0.33 – 0.40 |
| Copper / brass | R ≈ T | 0.35 – 0.42 |
The pattern behind the table: the tighter the bend, the more the inner surface is compressed, and the further the neutral axis migrates toward the inside face — so K falls. Open up the radius and the neutral axis drifts back toward the middle, approaching but never reaching 0.5. There is more on the reasoning in what the K-factor actually is.
Frequently asked questions
What is the K-factor in sheet metal?
The K-factor is where the neutral axis sits inside the material, expressed as a fraction of the thickness measured from the inside surface of the bend. A K-factor of 0.4 in 2 mm material puts the neutral axis 0.8 mm in from the inside face.
What is a typical K-factor value?
K-factors fall between about 0.33 and 0.5. A value of 0.4 is a common default for mild steel air-bent to a radius near the material thickness. Tighter radii push it lower, generous radii push it toward 0.5.
Can the K-factor be greater than 0.5?
No. The neutral axis cannot move past the middle of the material during bending — it only ever shifts toward the inside surface, so 0.5 is the theoretical ceiling.
How do I measure my own K-factor?
Cut a strip of known length, bend it once at a known angle, then measure the two outside flange dimensions. The difference between the sum of those flanges and the original strip length is the bend deduction, and the K-factor follows from it.
Put your K-factor to work
Set the thickness and K-factor once per part in the editor, and every bend on it unfolds with your numbers.
Open the editor