The press brake tonnage calculator estimates the bending force needed for air bending sheet metal on a V-die. Enter material, thickness and bend length to get the required tonnage, a recommended die opening, the expected inside radius and the minimum flange length, then choose a press brake with enough capacity for the job.
Need a machine for this job? See our CNC press brakes or send your part drawing to an engineer.
Calculation formula
For air bending of a 90° bend on a V-die, the required force is:
| Symbol | Meaning | Unit |
|---|---|---|
| F | Required bending force | kN (÷ 9.81 for metric tons) |
| Rm | Ultimate tensile strength of the material | MPa (N/mm²) |
| S | Sheet thickness | mm |
| L | Bend length | m |
| V | Die opening width | mm |
The inside radius in air bending is set mainly by the die opening, not the punch. It is estimated as about 0.14–0.21 × V depending on the material. The minimum flange length is about 0.7 × V; a shorter flange falls into the die before the bend is complete.
How to use the calculator
- Choose a material. The typical tensile strength fills in automatically; overwrite it with the value from your mill certificate if you have one.
- Enter the sheet thickness and the total length of the bend line.
- Keep "Use recommended die opening" checked to get a standard V-die sized for your thickness, or uncheck it to enter the die you already own.
- Read the required force and the suggested press brake size, which includes a 25% safety margin.
Recommended die opening by thickness
| Thickness S (mm) | Die opening V | Typical use |
|---|---|---|
| 0.5 – 2.5 | 6 × S | Thin enclosures, HVAC duct, cabinets |
| 3 – 8 | 8 × S | Brackets, chassis, general fabrication |
| 9 – 10 | 10 × S | Structural parts, frames |
| Over 10 | 12 × S | Heavy plate, agricultural and construction equipment |
Important notes
- Results apply to air bending at 90°. Bottoming needs roughly 3–5 times more force, and coining 5–10 times more.
- Tensile strength varies between suppliers and batches. High-strength steels such as S700 or wear plate need a larger V-die and more tonnage than mild steel.
- Check the load limit of your tooling. Short, thick parts can exceed the tons-per-meter rating of the punch and die even when the machine has enough total capacity.
- When bending a short part on a long machine, keep it near the center of the bed to avoid uneven ram deflection.
Frequently asked questions
How much tonnage do I need to bend 3 mm mild steel?
With a 24 mm V-die, 3 mm mild steel (Rm 450 MPa) needs about 240 kN per meter, or roughly 24 metric tons for a 1 m bend. A 3 m bend needs about 73 tons, so a 100 ton press brake gives a comfortable margin.
Why does stainless steel need more force than mild steel?
Stainless steel has a higher tensile strength (around 520–750 MPa for 304) and work-hardens during bending, so the force rises roughly in proportion to its strength. It also springs back more, so you may need to overbend by a few degrees.
What happens if I use a smaller die opening?
Force goes up as V gets smaller: halving the die opening doubles the tonnage. A small V also gives a tighter inside radius but increases the risk of cracking and of marking the part.
Can I use this for laser-cut blanks?
Yes. Laser-cut blanks from a fiber laser have clean, burr-free edges, which helps consistent bending. Account for the bend allowance when you nest the flat pattern so the finished flanges come out to size.