Bending Guide

How to Calculate Press Brake Tonnage

The full air-bending formula, a worked example you can check your own numbers against, and the mistakes that most often throw the result off.

Press brake tonnage is the amount of bending force — measured in tons — that a machine has to apply to fold a piece of sheet metal to a given angle. Get it wrong on the low side and the brake stalls or the fold comes out short; get it wrong on the high side and you're overpaying for machine capacity you don't need, or worse, pushing a smaller machine past its rated limit. The good news is that for standard air bending, the number comes from four inputs you almost always already know: material, thickness, bend length, and V-die opening.

The air bending tonnage formula

For air bending — the most common method, where the punch forms the sheet against the die without pressing it fully home — bending force is calculated as:

Where:

• UTS — the material's ultimate tensile strength in megapascals (MPa). Mild steel S275 sits around 430 MPa; 304 stainless is closer to 620 MPa.
• T — sheet thickness in millimetres. This is squared in the formula, so it's the single biggest driver of tonnage.
• L — the total length of the bend in millimetres.
• V — the V-die opening width in millimetres, usually set using the 8×T rule.

The result comes out in kilonewtons. Since press brakes are rated in metric tons, divide by 9.81 (1 tonne-force ≈ 9.81 kN) to get the number you'll actually compare against a machine spec sheet.

Worked example

Take a common job: a 3mm mild steel panel (S275, ~430 MPa), a 2000mm bend, air-bent in a 24mm V-die (following the 8×T rule for 3mm stock).

That last number — 60.8 t — is the one to shop a machine against, not the raw 50.6. A shop running this job every day would want a press brake rated comfortably above that, both for headroom on thicker stock later and to avoid running any machine flat out as routine.

Why thickness matters more than anything else

Because T is squared in the formula, tonnage scales with the square of thickness, not linearly. Double the material thickness and you roughly quadruple the force needed — everything else held constant. Going from 3mm to 6mm mild steel on the same 2000mm bend takes the requirement from ~50.6 tons to over 200 tons. This is the single most common source of surprise on a shop floor: someone specs a machine for "similar" jobs without checking that the thickest job in the mix hasn't quietly 4x'd the tonnage need.

Bottoming and coining need a lot more force

The formula above is for air bending. If the job calls for bottoming (pressing the sheet fully against the die) or coining (permanently forming the exact die radius into the material), tonnage climbs sharply — typically around 5× for bottoming, and anywhere from 5-10× again for coining, depending on how tight the corner needs to be. See air bending vs bottoming vs coining for when each method is worth the extra force.

Common mistakes that throw the number off

• Using a generic tensile strength. "Mild steel" spans a range; a specific grade (S235 vs S275 vs S355) can shift UTS by 15-20%, which shows up directly in the result.
• Guessing the V-die width. Tonnage is inversely proportional to V — a narrower die dramatically increases force. Always use the die you're actually tooling with, not an assumed one.
• Skipping the safety margin. Running a machine at its calculated limit with zero headroom leaves no room for material variation or tooling wear.
• Applying air-bend numbers to a bottoming or coining job. These need a different multiplier entirely — see above.
• Forgetting the kN → ton conversion. A lot of manual calculations quietly stay in kilonewtons and get compared against a tonnage spec by mistake, understating the requirement by roughly 10x.

Frequently asked questions