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
The widely used industry formula for air bending gives the tonnage in metric tons from the thickness, bend length, material strength and die opening:
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 tons, 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 tons, 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) | Tons / metre | Tons @ 2m bend |
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Frequently asked questions
How is press brake tonnage calculated?
For air bending: force (kN) = 1.54 × tensile strength (MPa) × thickness² × bend length (mm) ÷ (V-die × 1000), then divide by 9.81 for metric tons. Force rises with the square of thickness and falls as the V-die widens.
What V-die opening should I use?
Start with 8 × the material thickness — the standard balance of tonnage, springback and bend quality. Narrower needs more force; wider needs less but opens the radius.
How much more tonnage does bottoming or coining need?
Bottoming needs roughly 4–5× air-bending tonnage; coining 6–10× or more, since the punch forces the material fully into the die.
Why does stainless steel need more bending force?
Tonnage scales with tensile strength. Stainless (~700 MPa) is far stronger than mild steel (~430), needing about 1.5× the force, and it work-hardens as it bends.