The K-factor is the one number you should measure, not guess
Every bend formula leans on K — the fraction of material thickness, measured from the inside surface, where the neutral axis sits. Published tables get you close, but your real K depends on your material, your tooling, and how hard you form. The reliable way to nail it is to bend a test coupon, measure what came out, and back-calculate. That is exactly what this tool does.
How to measure your K-factor
Method 1: from a test bend
Cut a flat blank of known length, form a single bend at a known angle with a known inside radius, then measure the sum of the two outside legs of the formed part. The difference between those is the bend deduction, and from there the math returns the true bend allowance and K-factor for your setup.
Method 2: from a known bend allowance
If you already have a measured bend allowance, switch methods and enter it directly. K = (BA x 180 / (pi x A) – R) / T, where A is the bend angle, R the inside radius and T the thickness.
What is a normal K-factor?
| Forming condition | Typical K |
|---|---|
| Soft material, tight radius, air bending | 0.30 – 0.35 |
| Medium radius and tooling | 0.38 – 0.42 |
| Hard forming, bottoming or coining | 0.42 – 0.50 |
K stays between 0 and 0.5; a result outside that range usually means a mistyped measurement, the wrong leg reference, or the angle entered as the included angle instead of the angle of bend.
Why bother measuring?
A K-factor off by 0.05 can shift a flat blank by a millimetre or more per bend. Across a multi-bend part that stacks into parts that will not line up. Measuring K once for a given material and gauge pays for itself on every job that follows.
FAQ
Does K-factor change with bend angle?
In practice it drifts a little with angle and radius, which is why measuring at the angle you actually run gives the best flat patterns. For most work a single measured K per material and thickness is plenty accurate.
Inside or outside leg measurements?
Use outside legs for the test-bend method here — measure to the outside mold lines of the formed part and sum them.