The short answer
K-factor is the number that tells you how much sheet metal stretches on the outside of a bend and compresses on the inside, so you can work out how big to cut the flat pattern before it goes under the brake. Get it wrong and your bent part comes out too long or too short - sometimes by several millimetres over a run of bends.
It isn't a fixed constant - it changes with material, thickness, bend radius and bending method. Worth understanding if you're designing your own brackets and enclosures. If you're sending your files to BPT to bend, it's one less thing you need to calculate - we've measured the real K-factor for the materials and thicknesses that go across our press brake, rather than working off a generic default.
What K-factor actually is
When you bend a flat sheet, the outside face of the bend stretches and the inside face compresses. Somewhere between the two, there's a line within the material thickness that does neither - it's neither stretched nor compressed. That's the neutral axis.
K-factor is simply the ratio of the neutral axis position to the material thickness:
K = t / T
where t is the distance from the inside face of the bend to the neutral axis, and T is the total material thickness.
In a perfectly symmetrical bend the neutral axis would sit dead centre, giving K = 0.5. In practice it almost never does - the neutral axis shifts toward the inside of the bend as the radius tightens, which is why real-world K-factors typically fall between 0.3 and 0.5.
Why it matters
Bend allowance: K-factor feeds directly into the bend allowance calculation - the length of material consumed by the bend itself. Get the bend allowance wrong and your flat pattern is the wrong size before you've even touched the brake.
Part accuracy: a K-factor that's off by even 0.05 can shift a finished leg length by a millimetre or more on a tight-radius bend, and that error compounds across multiple bends on the same part. On a bracket with four bends, a small K-factor error in the wrong direction is enough to turn a clean fit into a part that needs re-cutting.
Material use: an accurate flat pattern means you're not adding "safety margin" to every blank to cover for uncertainty - which matters when you're nesting parts on a sheet and every millimetre of width affects yield.
What changes the K-factor
- Material: ductility and tensile strength affect how much the material stretches before it yields. Mild steel, stainless and aluminium all behave differently under the same bend geometry.
- Thickness: thicker material generally pulls the neutral axis further toward the inside face, giving a lower K-factor than the same bend in thin sheet.
- Bend radius: tighter radii concentrate more strain near the inside face and push the K-factor down.
- Bending method: air bending, bottoming and coining each apply force differently, and each settles the neutral axis in a slightly different place even with identical material and radius.
Rule-of-thumb values — and their limits
Most CAD packages default to K = 0.44 for general sheet steel, and DIN 6935 offers a simpler split: roughly K = 0.33 for bend radii under twice the material thickness, and K = 0.5 for anything looser. Both are useful starting points for early-stage design.
| Bend radius vs. thickness | Typical K-factor | Notes |
|---|---|---|
| Tight (r < 2T) | ~0.33 | Neutral axis pulled toward the inside face |
| Generous (r ≥ 2T) | ~0.42–0.5 | Closer to the theoretical centre of the material |
| CAD software default | 0.44 | A reasonable general-purpose average, not a measured value |
Defaults like these will get a first prototype close. They won't get a production run of precision brackets consistently right — for that, the only reliable method is bending a test piece in your actual material and thickness, measuring the resulting leg lengths, and back-calculating the real K-factor for that specific setup. That's exactly the testing we've already done for the materials that run across our own brake.
Let us handle it — bending & CAD design at BPT
Alongside our welding tables, we run CNC laser cutting and CNC press-brake folding in-house from our Somerset workshop, and we hold measured K-factors for the materials and thicknesses we fold most often rather than relying on generic CAD defaults. Send us your flat DXF plus a folding drawing (or a step/stp file) and we'll quote the complete formed part - typically to ±0.5mm on formed dimensions as standard.
Don't have a DXF? CAD design and draughting is part of the service. Send a dimensioned sketch, a PDF, or even a clear photo of a hand drawing, and we'll draw it up for a small drafting charge - you don't need a CAD licence or a bend allowance table to get an accurate formed part off our brake.
Folding is quoted per bend, typically £5–£15 depending on complexity, with no minimum order quantity - one bracket or a full production run. Full details on our laser cutting, bending & CAD design page, or email your files straight to james@bptables.co.uk.
Key takeaway: K-factor depends on material, thickness, radius and bending method - there's no single correct value. If you're bending in-house, measure a test piece rather than trusting a generic default beyond a first prototype. Or send your files to BPT - we've already done that measurement for the materials that run across our brake, so your formed parts come back right first time.
Get a quote for your bent parts
Whether you'd rather understand the maths and bend in-house, or hand the whole job to a workshop that's already done the measuring, both are reasonable calls. If it's the latter: send us your DXF - or a sketch, if that's all you have - along with material and thickness, and we'll quote the job within 24–48 hours. Laser cut, folded, and welded if you need the finished assembly.
Get a bending & CAD design quote →
Questions about your specific setup? Get in touch - James will give you a straight answer.
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