Estimate the air-bending force for your job, and see the die opening and radius it implies

What does this calculator do?

This calculator estimates the air-bending force needed to bend a given sheet, using the standard air bending equation. Enter material, thickness, bend length and die opening to get the required tonnage, the recommended die opening and the approximate inner radius.

The result is a planning estimate for air bending. It excludes die friction, " "bottoming and coining, and applies no safety factor. Confirm against the rated capacity of the press and of the tooling.

What formula does it use?

It uses the standard air bending force equation: F in kilonewtons equals 1.42 times tensile strength in MPa, times bend length in mm, times thickness squared in mm, divided by 1000 times the die opening in mm. The result is divided by 9.81 to give tonnes.

The formula is F = 1.42 · σ_b · L · t² / (1000 · V), with F in kN, σ_b in MPa, L in mm, t in mm and V in mm. It describes air bending, where the sheet is not pressed fully to the die walls.

Related quantities used by this calculator:

• Recommended die opening: V = 8t as a starting point for mild steel.
• Inner radius: approximately 0.16 × V in air bending.
• Minimum flange: approximately V/2 + t.
• Unit conversion: 1 tonne-force = 9.81 kN.

How accurate is the result?

It is a planning estimate accurate to roughly the same order as the published tonnage charts, typically within about ten per cent for plain air bending of common materials. It is not a substitute for the machine builder's own capacity data or the tool maker's load rating for a specific punch.

Accuracy falls away in three situations. Bottoming or coining needs several times the air-bending force. Narrow punches and short bend lengths concentrate load at the tip, so the tool rating rather than the average tonnage becomes limiting. Long bends deflect the ram and bed, which is a crowning problem rather than a tonnage problem.

What safety margin should I apply?

Keep the calculated force comfortably below the rated capacity of both the press and the tooling. A common practice is to size the operation so it stays under roughly 80 per cent of the rated tonnage, and to check the tool maker's own load limit whenever a narrow punch is used.

Check the narrowest point of the tooling as well as the machine. A small punch tip concentrates the full bend load across a short contact area, and it is usually the tool, not the press, that fails first.

Frequently asked questions

These are the questions buyers ask most often about press brake tonnage calculation.

What is the formula for press brake tonnage?

F in kilonewtons = 1.42 x tensile strength (MPa) x bend length (mm) x thickness squared (mm) / (1000 x die opening in mm). Divide by 9.81 to convert kilonewtons to tonnes. It applies to air bending.

How many tonnes does it take to bend 1 mm mild steel?

About 8 tonnes per metre with a die opening of eight times the thickness, because the V = 8t rule and the tonnage equation together give roughly 8 tonnes per metre for mild steel at 450 MPa.

Why does thinner sheet sometimes need more tonnage than expected?

Because the die opening usually shrinks with the thickness. Tonnage per metre falls with thickness, but a narrower die opening raises the force required, and a narrow punch tip concentrates that load, so the tool rating becomes the limit rather than the machine.

Do I need to add a safety factor to the calculated tonnage?

Yes. Keep the calculated force under roughly 80 per cent of the rated tonnage of the press, and separately check the load rating of the tooling, especially for narrow punches where tip loading is concentrated.