Work out the force a bend actually needs, in kN per metre and in tonnes across your bend length, then check it against your machine capacity and your tooling's rated load. Formula shown, no sign-up.
Single bend, full length in one hit. Updates as you type.
Rm is tensile strength in N/mm². 1.42 is the constant published by most tooling makers for air bending; expect real-world agreement within roughly 10–15%. Bottoming and coining multipliers are rough approximations — they move with punch radius, die angle and how hard the machine is set to press.
| Result | Value |
|---|---|
| Force per metre | |
| Total force | |
| Machine load | |
| Tooling load | |
| Inside radius (air bend) | |
| Minimum flange |
Guidance only. Check the rating stamped on your tooling and the load limits in your machine manual before bending. Full disclaimer.
Need the flat pattern too? Bend allowance calculator →
Rm 410 N/mm², using the usual V-die for each thickness. Generated by the same code as the calculator, so the two can never disagree.
| Thickness | V-die | Air bend kN/m | Air bend t/m | Bottoming t/m | Coining t/m |
|---|---|---|---|---|---|
| 1 mm | V6 | 97 | 9.9 | 30–49 | 99+ |
| 1.5 mm | V10 | 131 | 13.4 | 40–67 | 134+ |
| 2 mm | V12 | 194 | 19.8 | 59–99 | 198+ |
| 2.5 mm | V16 | 227 | 23.2 | 70–116 | 232+ |
| 3 mm | V22 | 238 | 24.3 | 73–121 | 243+ |
| 4 mm | V26 | 358 | 36.5 | 110–183 | 365+ |
| 5 mm | V32 | 455 | 46.4 | 139–232 | 464+ |
| 6 mm | V40 | 524 | 53.4 | 160–267 | 534+ |
| 8 mm | V50 | 745 | 76.0 | 228–380 | 760+ |
| 10 mm | V63 | 924 | 94.2 | 283–471 | 942+ |
| 12 mm | V80 | 1048 | 106.8 | 320–534 | 1068+ |
Read across: a 3 mm bend over a V22 die needs about 24 tonnes per metre air bending — but around 73–121 t/m bottoming, which is where tooling and machines get hurt.
Force scales with the square of thickness. Going from 3 mm to 6 mm does not double the tonnage — it roughly quadruples it, before you even account for the wider die you would normally move to. This is the single most common reason a job that ran fine yesterday stalls the machine today.
Force is inversely proportional to the V opening. Open the die up and the tonnage drops — but the inside radius grows with it, at roughly 16% of the die opening, and springback increases. That trade-off is the whole craft of die selection: the narrowest die that gives you the radius you want, without exceeding the load your tooling and machine can take.
Air bending only touches three points — punch tip and two die shoulders — so it needs the least force, and the formula above applies directly. Bottoming presses the material into the die and needs roughly three to five times that force. Coining crushes the material at the bend, effectively removing springback, and needs on the order of ten times or more. Those multipliers are approximations, and they are why a coining job on thick plate can exceed a machine that handles the same bend comfortably in air.
The machine's rated tonnage is not the only ceiling. Tooling has its own rated load per metre — commonly in the region of 80 t/m for standard precision-ground sections and higher for heavier styles, but it is printed in the supplier's catalogue and you should use their figure, not a guess. Exceed it and the punch or die fails, not the sheet. Short bends concentrate load badly: 30 tonnes spread over 2 metres is routine, the same 30 tonnes over a 150 mm punch is 200 t/m and will damage tooling. The calculator flags this.
Air bending over a V22 die, about 24 tonnes per metre of bend — so roughly 60 tonnes for a 2.5 m bend. Bottoming the same bend takes three to five times that. Change the die opening and the answer changes.
Directly proportionally. Force scales linearly with tensile strength, so stainless 304 needs about 45% more force than mild steel of the same thickness, and abrasion-resistant plate can need three times or more. Nominal figures also understate reality — real coils commonly run above their minimum specified strength.
Usual suspects: the die is narrower than you think, the material is stronger than the nominal grade, you are bottoming rather than air bending, or the load is concentrated over a short length or off-centre. Check those four before assuming the machine has a fault.
It is the constant most tooling manufacturers publish for air bending, and it is what this page uses. You will also see 1.33 and 1.65 in various references depending on assumptions and units. All of them are empirical — treat the result as an estimate with roughly 10–15% uncertainty, and add margin.
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.
You are solely responsible for confirming that the load, thickness, material and tooling are within the rated capacity of your press brake, punches, dies and any adaptors; for complying with the machine manufacturer's manual and load limits; for complying with all applicable workplace health and safety obligations, standards and regulations in your jurisdiction; and for having any structural, safety-critical or certified work checked and approved by a suitably qualified person. Where a calculation affects safety or structural integrity, it must be independently verified by a competent professional engineer.
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