Enter thickness, die opening and angle. You get the flat length, bend allowance, bend deduction, K‑factor, air bend force and minimum flange — calculated to DIN 6935, with every formula printed so you can check it before you cut.
Single bend, two flanges measured outside-to-outside. Updates as you type.
| Result | Value |
|---|---|
| Inside radius used | |
| DIN 6935 factor k | |
| K-factor (k/2) | |
| Bend allowance | |
| Outside setback | |
| Bend deduction | |
| Air bend force | |
| Total force on machine | |
| Minimum flange |
Guidance only. Verify every result against your machine, tooling ratings and material certificate before bending. Full disclaimer.
Generated by the same code as the calculator above, so the two can never disagree. Tonnage column is mild steel at Rm 410 N/mm².
| Thickness | Usual V-die | Natural inside radius | Minimum flange | Force kN/m | Tonnes per metre |
|---|
V-die selection here follows the common workshop rule of roughly 6–8 × thickness. Your tooling list, material and required radius can all move it — treat this as a starting point, not a specification.
When you bend sheet, the outside of the bend stretches and the inside compresses. Somewhere between them is a layer that does neither — the neutral axis. The flat blank has to equal the length of that neutral layer, not the length of the finished part measured around the outside. That difference is the bend deduction, and it is why a part cut to the sum of its flanges always comes out too long.
The K-factor says where the neutral axis sits, as a fraction of thickness from the inside face. Tighter bends push it inwards. DIN 6935 handles this with a correction factor k that depends on the ratio of inside radius to thickness:
k = 0.65 + 0.5 · log₁₀(r/t), capped at 1 once r/t exceeds about 5.
The K-factor is half of that, so a typical 90° bend in 3 mm over a 22 mm die lands near K = 0.34 — not the 0.5 that a lot of drawing-office spreadsheets still assume. That difference is around 0.6 mm on a single bend, and it compounds on every bend in the part.
For air bending, force scales with the square of thickness and inversely with the die opening:
F (kN per metre) = 1.42 · Rm · t² / V
Rm is the material's tensile strength in N/mm², t and V in millimetres. It is an empirical formula, and 1.42 is the constant most tooling makers publish — expect real-world agreement within roughly 10–15%. Two things worth noticing: doubling thickness quadruples the force, and opening the die up reduces force but increases the radius you get. Bottoming and coining need several times more force than this formula gives, which is why the number here is labelled air bend.
A flange has to reach across the die shoulder or it will not form — it lifts out and the bend never completes. The practical limit is about 0.65 · V + t. If your flange is shorter than that, you need a narrower die, special tooling, or a different bend sequence.
There isn't one number. It depends on radius-to-thickness ratio, which is why the calculator derives it rather than letting you pick. For common 90° air bends it usually falls between 0.33 and 0.42. Using a flat 0.5 will make every flat pattern too short.
No, and mixing them up is the most common flat-pattern error. Bend allowance is the developed length of the bend itself, added to the two flange lengths measured to the tangent points. Bend deduction is what you subtract from the outside-to-outside dimensions. This page gives both, and uses the deduction for the flat length because outside dimensions are what most drawings carry.
Springback. The material recovers elastically when the punch retracts, so an air bend overbends slightly to land on angle. Higher tensile strength and larger die openings both increase it.
In air bending, no — not primarily. The radius is set mainly by the die opening, roughly 0.16 × V, because the sheet spans the die and forms a natural arc. The punch tip only dictates the radius when you bottom or coin.
Yes — pick the material in the dropdown, which changes tensile strength and so the force. The geometry maths is material-independent, but be aware that aluminium cracks more readily on tight radii and stainless springs back harder.
Please read this before using any figure from this site in production.
Every output on this site 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 tonnage distribution, temperature, and batch-to-batch variation in material properties. Actual tensile strength commonly differs from the nominal figures listed here. Figures from this site must be treated 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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