How to calculate press brake tonnage
Before bending sheet metal on a press brake you need to know the force required, so you can pick a machine with enough capacity — and avoid overloading the tooling or the ram. Too little tonnage gives poor, inconsistent angles; too much risks cracking dies or damaging the machine.
The air bending formula
This calculator uses a 1.54 air-bending force coefficient. UTS is tensile strength in MPa; T is sheet thickness, L is bend length and V is die opening, all in mm. Divide force in kN by 9.80665 for metric tonnes:
Because thickness is squared, doubling the thickness roughly quadruples the force. A wider V-die acts like a longer lever and reduces tonnage, while a narrower die increases it.
Choosing the V-die
The standard starting point is a V-die opening of 8 times the material thickness. It balances required tonnage against springback and bend quality. The "Suggest 8×T" button fills this in for you. Going narrower sharpens the radius but raises force; going wider lowers force but opens the radius.
Bending method matters
Air bending — where the punch doesn't force the metal fully into the die — needs the least force. Bottoming needs roughly 4–5× as much, and coining, where the punch presses the material hard into the die for a precise radius, can need 6–10× or more. Always keep a safety margin and never run a press brake at its rated limit.
Material strength
Tonnage is proportional to the material's tensile strength. Mild steel sits around 360–430 MPa, aluminium is lower (around 210–290), and stainless is much higher (around 700) — so stainless needs roughly 1.5× the force of mild steel and work-hardens as it bends.
Worked example
Suppose you're air bending a 3 mm mild steel (S275, 430 MPa) panel with a 2,000 mm bend line, using a 24 mm V-die (the 8×T recommendation):
With a 20% safety margin that's about 61 tonnes, so an 80-tonne press brake comfortably handles the job while staying well clear of its rated limit. Switch the same part to 304 stainless and the force jumps to roughly 82 tonnes, because stainless is far stronger — which is exactly the kind of check that prevents an overloaded machine or a cracked die.
Press brake tonnage chart (mild steel, air bending)
Approximate tonnage per metre of bend for mild steel at the recommended 8×T V-die. Use the calculator above for your exact material, length and die.
| Thickness | V-die (8×T) | Tonnes / metre | Tonnes @ 2m bend |
|---|---|---|---|
| 1 mm | 8 mm | 8.4 | 16.9 |
| 2 mm | 16 mm | 16.9 | 33.8 |
| 3 mm | 24 mm | 25.3 | 50.6 |
| 4 mm | 32 mm | 33.8 | 67.5 |
| 6 mm | 48 mm | 50.6 | 101.3 |
View the full press brake tonnage chart for mild steel, stainless steel and aluminium.
Frequently asked questions
How is press brake tonnage calculated?
For air bending, force (kN) = 1.54 × UTS (MPa) × thickness (mm)² × bend length (mm) ÷ (V-die opening (mm) × 1000). Divide by 9.80665 for tonnes. Per metre, this is approximately 0.157 × UTS × thickness² ÷ V.
What V-die opening should I use?
Start with about 8 × material thickness for general air bending. This is a rule of thumb: check the material, inside radius, minimum flange and tooling limits before choosing the die.
How much more tonnage does bottoming or coining need?
The calculator uses illustrative multipliers of 4.5 for bottoming and 8 for coining. Actual force depends on material and tooling; confirm the required load with the tooling supplier.
Why does stainless steel need more bending force?
Force scales with tensile strength. The calculator presets use 700 MPa for stainless 304 and 430 MPa for mild steel S275, giving about 1.63 times the force for otherwise identical inputs. Use the actual material strength where available.