Convert between the four numbers that describe one bend — K-factor, bend allowance, outside setback and bend deduction — for any thickness, inside radius and angle. Enter a measured deduction and it works backwards to the K-factor your machine is really producing.
Enter the bend. Leave the radius blank to use the natural air-bend radius for the die.
θ is the included angle you read off the part — 90° for a square corner. K is the neutral-axis position as a fraction of thickness: 0.5 means the neutral axis sits dead centre, and it drops below that as the radius gets tight because the inside face compresses. The automatic option follows the DIN 6935 correction factor and is the same code used by the flat pattern pages on this site, so the numbers always agree.
| Result | Value |
|---|---|
| Inside radius used | |
| K-factor | |
| DIN 6935 factor | |
| Bend allowance (BA) | |
| Outside setback (OSSB) | |
| Flat length A + B − BD | |
| Bend line from edge A |
Guidance only. An uncalibrated development is good to roughly ±0.3–0.5 mm per bend. Measure a test part. Full disclaimer.
More than one bend? Flat pattern & layout calculator →
Using the usual V-die for each thickness and the radius that die naturally produces (about 0.16 × V). Generated by the same code as the calculator above.
| Thickness | V-die | Inside r (mm) | K | BA (mm) | OSSB (mm) | BD (mm) |
|---|---|---|---|---|---|---|
| 1 mm | V6 | 0.96 | 0.321 | 2.01 | 1.96 | 1.91 |
| 1.5 mm | V10 | 1.60 | 0.332 | 3.30 | 3.10 | 2.90 |
| 2 mm | V12 | 1.92 | 0.321 | 4.02 | 3.92 | 3.82 |
| 2.5 mm | V16 | 2.56 | 0.328 | 5.31 | 5.06 | 4.81 |
| 3 mm | V22 | 3.52 | 0.342 | 7.14 | 6.52 | 5.90 |
| 4 mm | V26 | 4.16 | 0.329 | 8.60 | 8.16 | 7.72 |
| 5 mm | V32 | 5.12 | 0.328 | 10.62 | 10.12 | 9.62 |
| 6 mm | V40 | 6.40 | 0.332 | 13.18 | 12.40 | 11.62 |
| 8 mm | V50 | 8.00 | 0.325 | 16.65 | 16.00 | 15.35 |
| 10 mm | V63 | 10.08 | 0.326 | 20.95 | 20.08 | 19.21 |
| 12 mm | V80 | 12.80 | 0.332 | 26.36 | 24.80 | 23.24 |
Change the die and every column changes with it — which is why a shop-wide "deduct 2 × t" habit only works for the one die and thickness it was calibrated on.
Bend allowance is the developed length of the material through the bend, measured along the neutral axis. You add it to the two inside flange lengths to get the flat. Bend deduction is what you subtract from the two outside dimensions — the ones actually on the drawing — to get the same flat. Both describe one physical bend; they differ only in which dimensions you started from. Mixing them up is the single most common flat pattern error, and it always makes the blank too long by roughly two thicknesses.
Outside setback is pure trigonometry: how far the outside corner sits beyond where the two flat faces would meet. It depends only on radius, thickness and angle. The K-factor is the part that depends on what the metal does — where the neutral axis ends up once the inside face has been compressed and the outside stretched. Tight radii push K down towards 0.33 or lower; generous radii leave it near 0.5.
Bend a test piece, measure the finished outside dimensions and the flat blank you started with, and the difference is your true bend deduction. Feed it into the third mode above and you get the K-factor your machine, tooling and material are actually producing. Once you have that number for a given thickness and die, your flat patterns stop being estimates.
The amount you subtract from the sum of the two outside flange dimensions to get the flat blank length. It equals twice the outside setback minus the bend allowance, so it grows with thickness, radius and bend angle.
Bend allowance is added to inside dimensions; bend deduction is subtracted from outside dimensions. They describe the same bend from different starting measurements, and for a 90° bend they differ by about two material thicknesses.
For a 90° air bend at a normal radius, K commonly lands between 0.40 and 0.45 — the DIN 6935 value at r/t = 1 is around 0.45. But it moves with r/t, so calculate it rather than assuming 0.33 or 0.5, and calibrate against a test bend when the tolerance is tight.
Yes, on very small angles or very large radii, where the bend allowance exceeds twice the setback. A negative deduction means the blank is longer than the sum of the outside dimensions, which is correct, not a fault in the arithmetic.
Please read this before using any figure from this page in production.
Every output here is an indicative calculation produced from the values you enter. Real results vary with springback, grain direction, tooling condition and geometry, friction and lubrication, machine calibration and load distribution, temperature, and batch-to-batch variation in material properties. Actual tensile strength commonly differs from the nominal figures listed. Treat these numbers as a starting point to be proven on a test piece — never as a verified value for a production run, a load calculation, or a safety-critical component.
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