Inputs
Often set equal to the punch tip radius. A common rule of thumb is r ≈ t.
The angle the flange is bent through — a square corner is a 90° bend.
Fraction of thickness to the neutral axis. Change it to see the bend allowance update.
Results
Skip the hand maths
Bendline unfolds a 3D STEP part into a cut-ready DXF flat pattern in your browser — K-factor, bend allowance and every blank dimension worked out for you.
What is the K-factor?
When sheet metal is bent, the outside of the bend stretches and the inside compresses. Somewhere between the two is a layer that neither stretches nor compresses — the neutral axis. The K-factor is the ratio of the distance from the inside surface of the bend to that neutral axis, divided by the material thickness. It always falls between 0 and 0.5, and it is the single number that tells you how much material a bend actually consumes.
Because the neutral axis is what stays constant in length, the arc it traces through the bend is the bend allowance — the amount of flat material you need to form that bend. Get the K-factor right and your flat blank comes out the correct size; get it wrong and every bent part is off.
Typical K-factor values
K-factor depends on the material, its temper, and the ratio of bend radius to thickness. These are common starting points — always confirm against a test bend for production work:
| Material / condition | Typical K-factor |
|---|---|
| Soft / dead-soft copper, brass, aluminium | 0.35 – 0.40 |
| Mild steel (cold-rolled), r ≈ t | 0.38 – 0.45 |
| Aluminium (half hard to hard) | 0.40 – 0.45 |
| Stainless steel | 0.40 – 0.45 |
| Large radius (r > 2t) | up to 0.50 |
| Very tight radius (r < t) | 0.30 – 0.38 |
A widely used default for mild steel air bending is 0.38 – 0.42. As the inside radius grows relative to thickness, the neutral axis shifts toward the centre of the material and K rises toward 0.5.
How the calculator works
The bend allowance is the neutral-axis arc: BA = θ × (r + K·t), where
θ is the bend angle in radians. The neutral-axis offset
K·t tells you how far inside the material that arc sits. To turn a
bend allowance back into a flat length, subtract the bend deduction from the sum
of the outside leg lengths — that is what the
bend allowance & deduction
and flat-length
calculators do.