The one-line definition
The K-factor is the position of the neutral axis inside the material, measured from the inside surface of the bend, expressed as a fraction of the material thickness. A K-factor of 0.4 in 2 mm sheet puts the neutral axis 0.8 mm in from the inside face.
What the neutral axis is
Bend a strip of metal and the outside of the curve has further to travel than the inside, so the outer fibres stretch and the inner fibres compress. Between them, one layer travels exactly its original length — it is neither stretched nor squashed. That layer is the neutral axis.
This matters because a flat pattern is a length question. Every other layer changes length through the bend, so measuring any of them tells you nothing useful about how much flat stock you need. The neutral axis does not change length, which makes its arc through the bend exactly the amount of flat material the bend consumes. That arc is the bend allowance.
Why it moves inward
Before you bend anything, the neutral axis sits at the geometric middle — K = 0.5. As the bend forms, the material on the inside has nowhere to go. It is being crushed into a shorter arc, and metal resists compression more stubbornly than it resists stretching. The material accommodates this by letting the outside thin slightly and by shifting the balance point inward.
The tighter the bend relative to the material thickness, the more severe the compression and the further the axis migrates. This gives the rule that governs every K-factor table you will find:
- Tight radius (R < T) — heavy compression, axis well inside, K around 0.33.
- Radius near thickness (R ≈ T) — the common case, K around 0.40.
- Generous radius (R > 3T) — gentle forming, axis near the middle, K approaching 0.50.
It follows that the K-factor can never exceed 0.5. The axis starts in the middle and only ever moves one way.
Why published tables disappoint
Tables are averages across mill batches, forming methods and tooling. The K-factor for your part depends on things a table cannot know: whether the shop air bends or bottoms the material, the die opening they happen to have set up, the grain direction relative to the bend, and the exact alloy and temper.
The error is small per bend — commonly a few tenths of a millimetre — but it accumulates. Four bends on an enclosure, each off by 0.3 mm, and a panel that should drop into a 500 mm opening does not.
Measuring your own, once
The whole procedure takes ten minutes and pays for itself on the first job. Cut a strip of known length from the actual material, bend it once at a known angle on the tooling the real job will run on, and measure the two outside flanges. The difference between their sum and the original length is the bend deduction, and the K-factor falls out of it by algebra — the K-factor calculator does the inversion for you.
Redo it whenever the material, thickness or tooling changes. A K-factor measured on 2 mm mild steel tells you nothing about 5 mm stainless.
Frequently asked questions
What is the K-factor in simple terms?
The K-factor is how far the neutral axis sits from the inside surface of a bend, as a fraction of the material thickness. K = 0.4 in 2 mm sheet means the layer that neither stretches nor compresses is 0.8 mm in from the inside face.
Why can the K-factor never exceed 0.5?
Before bending, the neutral axis is at the middle of the material, which is K = 0.5. Bending compresses the inside more than it stretches the outside, so the axis only ever moves inward. It approaches 0.5 for very generous radii but never passes it.
Does the K-factor change with bend angle?
Only slightly. It is dominated by the ratio of inside radius to material thickness and by the material itself. Treating it as constant across angles for a given material, thickness and tooling is accurate enough for fabrication work.