Sheet metal K-factor and bend allowance: flat length calculation
Published October 25, 2026 — the K-factor, bend allowance (BA), outside setback (OSSB) and bend deduction (BD), a worked example, measuring K from a test bend and a calculator.
In short: when bending, only the neutral axis keeps its length; it sits K·T from the inside face (T: thickness, K between 0 and 0.5). The length of sheet in the bend is BA = π · θ / 180 · (R + K · T). With flanges dimensioned to the outside, the flat length is L1 + L2 − BD, where BD = 2 · tan(θ/2) · (R + T) − BA. K depends on the sheet, the tooling and the radius: measure it on a test bend.
1. Flat length calculator
One bend between two flanges dimensioned to the outside. The angle θ is the bend angle (90° for a bracket). The preloaded K of 0.4 is the example's, not a recommended value: enter your own.
2. The K-factor and the neutral axis
- Definition: K = t / T, where t is the distance from the inside face of the bend to the neutral axis, and T the thickness.
- Range: the neutral axis moves towards the inside of the bend, where the material is compressed; K stays between 0 and 0.5. It equals 0.5 when the neutral axis is at mid-thickness, which only happens with large radii.
- What changes it: the R/T ratio (the smaller the radius compared with the thickness, the lower K), the material and its temper, the process (air bending, bottoming, coining) and the tooling. A K-factor published elsewhere remains an assumption until checked on your parts.
3. Formulas and an example
T: thickness; R: inside radius; θ: bend angle in degrees; K: K-factor. All lengths in the same unit.
Example: 2 mm sheet bracket, 2 mm inside radius, bent to 90°, flanges dimensioned to the outside 50 and 30 mm, K = 0.4 (value chosen for the example). Lengths in mm.
4. Measuring your K-factor on a test bend
Cut a strip of known length, bend it to 90° with the production sheet, tooling and radius, then measure both flanges on the outside. Example: 100 mm strip, 2 mm sheet, 2 mm radius, flanges measured at 61.8 and 41.8 mm.
The calculation uses T, R and θ from the calculator in section 1. Measure carefully: at this scale, a 0.1 mm error on one flange shifts K by about 0.03. Bend several strips and keep the average.
5. Flat pattern pitfalls
- Bend angle or included angle: the formula takes the angle the sheet has turned through. Flanges 120° apart mean θ = 60°.
- Inside or outside dimensions: L1 + L2 − BD only applies to flanges dimensioned to the outside apex. With straight segments, use a + b + BA.
- Actual thickness: a sheet ordered by gauge does not have the round thickness you might expect, see the sheet metal gauge chart. Use the measured thickness.
- Tolerances: every bend adds its deviation to the flat length; on a part with several bends, treat the dimensions as a tolerance stack-up.
- In CAD: sheet metal software, SOLIDWORKS included, unfolds parts from a K-factor, a bend allowance, a bend deduction or a bend table. Enter the values measured in your shop rather than a default.
Printable version: the formulas, example and K measurement as a PDF (A4, no sign-up), to keep near the press brake.
6. FAQ
What is the K-factor in sheet metal?
It is the position of the neutral axis within the sheet thickness, as a fraction of the thickness: K = t / T, where t is the distance from the inside face of the bend to the neutral axis. The neutral axis neither stretches nor shrinks during bending, so its length is what ends up in the flat pattern.
Why is the K-factor below 0.5?
During bending, the inside face is compressed and the outside face stretched. The neutral axis shifts towards the inside of the bend, all the more so as the radius is small compared with the thickness. It therefore stays in the inner half: K lies between 0 and 0.5.
How do you calculate the flat length of a bent sheet?
With outside dimensions: L = L1 + L2 − BD, where BD = 2 · tan(θ/2) · (R + T) − BA. With the straight segments outside the bend: L = a + b + BA, where BA = π · θ / 180 · (R + K · T). For several bends, add one term per bend.
Which angle goes into the formula?
The bend angle θ, the angle the sheet has turned through, not the included angle between the flanges. A 90° bracket gives θ = 90°; a part whose flanges are 120° apart was bent by θ = 60°.
How do I find my shop's K-factor?
Bend a strip of known flat length with the actual sheet, tooling and radius, measure both outside flanges, then apply BD = L1 + L2 − L, BA = 2 · OSSB − BD and K = (BA · 180 / (π · θ) − R) / T. The calculator in section 4 does this.