Bending Guide
K-Factor Explained
What it actually represents, typical values by material, and how to stop guessing and measure your own.
K-factor is the single number that has the biggest effect on how close a calculated flat pattern comes to the real, physical part. Get it right and your blanks come out the correct size on the first try; get it wrong and you're trimming or scrapping parts after the fact. It's also one of the most misunderstood values in sheet metal fabrication — not because the concept is complicated, but because most people use a textbook default instead of their own measured number.
Measure yours, don't guess it
Switch the calculator to "Solve for K-factor," enter a test bend, and get your exact value back.
What K-factor actually represents
When sheet metal bends, the neutral axis — the internal layer that neither stretches nor compresses — shifts slightly toward the inside surface of the bend. K-factor is that shift, expressed as a fraction of the material thickness measured from the inside face:
A K-factor of 0.5 would mean the neutral axis sits exactly in the middle of the material — the simplest theoretical case. In practice it almost always sits closer to the inside surface than that, which is why K-factor values in the real world typically run lower, between about 0.3 and 0.5.
Typical K-factor values by material
| Material / use case | Typical K-factor |
|---|---|
| Steel (mild/carbon) | ≈ 0.33 |
| Aluminium | ≈ 0.38 |
| General / "Lockheed" default | ≈ 0.44 |
| Soft, more ductile materials | 0.40 – 0.50 |
| Harder, less ductile materials | 0.28 – 0.35 |
These are starting points, not fixed constants — actual K-factor shifts with bend radius, thickness, tooling and even bend direction relative to the material's grain.
Why the generic value isn't always good enough
A generic K-factor gets you in the right neighbourhood, which is fine for a one-off part where a small trim afterward is no big deal. It's a different story on repeat production: a K-factor error of even 0.05 compounds across every bend in a multi-fold part, and across every part in a production run. That's wasted material, wasted machine time, and parts that need rework.
How to solve for your exact K-factor
The reliable fix is to measure it directly from your own material, thickness and tooling, rather than trust a table:
- Cut a flat test coupon of a known length.
- Bend it to a known angle (a simple 90° bend is easiest).
- Measure the finished flange lengths on the bent part.
- Back-calculate the actual bend deduction from the difference between your starting flat length and the finished flange dimensions.
- Solve the bend allowance formula for K using that measured deduction.
That's exactly what "Solve for K-factor" mode does automatically — enter your measured flat length and the test bend's geometry, and it returns the K-factor to use on your real parts going forward. See how to calculate bend allowance for the formula this is built on.
Turn a test bend into your real K-factor
One test coupon is all it takes — plug in the measurement and reuse the result on every future job in that material.
Frequently asked questions
What is K-factor in sheet metal bending?
It's the position of the neutral axis within the material during a bend, expressed as a fraction of thickness measured from the inside surface — typically between 0.3 and 0.5.
What K-factor should I use for steel?
0.33 is a common starting value, though it varies with bend radius, thickness and tooling. Measuring your own from a test bend is more accurate.
What K-factor should I use for aluminium?
0.38 is a common starting value. Aluminium's different ductility compared to mild steel tends to shift the neutral axis slightly, reflected in a different default.
How do I find my exact K-factor?
Bend a test coupon, measure the flat blank you started with and the finished flange dimensions, then back-calculate K from the actual bend deduction using a solve-for-K tool.