Press Brake Tonnage Calculator
Estimate the force for a 90 degree air bend from material, thickness, bend length and V-die opening, in US tons or kN. The calculator also gives the inside radius the die will form, the shortest flange that still spans the die, and a starting-point minimum bend radius for the material, so you can check a flange or a radius callout before the part goes to the brake.
Air bend tonnage
The baseline for the tonnage rule. Thin CR sheet often bends tighter than 1t.
Enter thickness and bend length to get the air-bend force. Leave the die opening blank to use 8 x thickness.
The rules behind the numbers
- Tonnage: tons per foot = 575 x t² / V x (tensile / 60 ksi), with t and V in inches. This is the long-standing air-bend rule for 60 ksi mild steel, scaled by tensile strength for other materials.
- Metric: the same rule converted is about 67.4 x t² / V metric tons per meter (t and V in mm, 414 MPa steel). 1 US ton = 8.896 kN, 1 metric ton = 9.807 kN.
- Inside radius formed: a percentage of V. 15.6 percent for 60 ksi cold-rolled steel, 21 percent for 304 annealed, 14 percent for 5052-H32. Materials without a published figure use 15.6 percent x tensile / 60 ksi, marked as an estimate.
- Minimum flange: 0.77 x V for a 90 degree bend. At the bottom of the stroke each flange sits at 45 degrees and still has to reach past the die shoulder, (V / 2) / cos 45 = 0.71 x V; the rest is margin so the flange does not slip into the die.
- Die opening: 8 x thickness by default; 6x to 12x is the normal air-bending range.
These are rules of thumb for sizing a press and choosing tooling. Real tonnage moves with the actual tensile on the mill cert, the punch radius, the bend angle and friction, so leave headroom and let the shop confirm against its own brake charts.
Material table
Tensile strength sets the tonnage factor against mild steel. Minimum radii are starting points for a 90 degree bend across the rolling direction; bending with the grain, thicker stock and harder tempers need more.
| Material | Tensile | Tonnage vs mild steel | Air-bend radius | Suggested min. radius |
|---|---|---|---|---|
| Mild steel, cold rolled | 60 ksi / 414 MPa | 1.00x | 15.6% of V | 1t |
| A36 hot rolled | 70 ksi / 483 MPa | 1.17x | 18.2% of V (est.) | 1t to 1.5t |
| 304 stainless, annealed | 90 ksi / 621 MPa | 1.50x | 21.0% of V | 1t to 2t |
| 5052-H32 aluminum | 33 ksi / 228 MPa | 0.55x | 14.0% of V | 1t |
| 6061-T6 aluminum | 45 ksi / 310 MPa | 0.75x | 11.7% of V (est.) | 1.5t to 3t |
| 4130 alloy steel, normalized | 97 ksi / 669 MPa | 1.62x | 25.2% of V (est.) | Ask the shop |
Typical figures. A36 allows 58 to 80 ksi and 304 annealed runs about 75 to 90 ksi, so use the mill cert value (Custom in the calculator) when you have it.
Air bend tonnage quick reference (V = 8t)
Tons per foot of bend at a die opening of eight times thickness, from the same rule. At V = 8t the rule reduces to about 71.9 x t tons per foot for mild steel, so tonnage climbs in step with thickness.
| Thickness | V-die (8t) | Mild steel | Mild steel (kN/m) | 304 stainless | 5052-H32 |
|---|---|---|---|---|---|
| 16 ga (0.0598 in / 1.52 mm) | 0.478 in / 12.2 mm | 4.3 t/ft | 125 kN/m | 6.4 t/ft | 2.4 t/ft |
| 14 ga (0.0747 in / 1.90 mm) | 0.598 in / 15.2 mm | 5.4 t/ft | 157 kN/m | 8.1 t/ft | 3.0 t/ft |
| 12 ga (0.1046 in / 2.66 mm) | 0.837 in / 21.3 mm | 7.5 t/ft | 219 kN/m | 11.3 t/ft | 4.1 t/ft |
| 10 ga (0.1345 in / 3.42 mm) | 1.076 in / 27.3 mm | 9.7 t/ft | 282 kN/m | 14.5 t/ft | 5.3 t/ft |
| 3/16 in (0.1875 in / 4.76 mm) | 1.500 in / 38.1 mm | 13.5 t/ft | 393 kN/m | 20.2 t/ft | 7.4 t/ft |
| 1/4 in (0.2500 in / 6.35 mm) | 2.000 in / 50.8 mm | 18.0 t/ft | 524 kN/m | 27.0 t/ft | 9.9 t/ft |
| 3/8 in (0.3750 in / 9.52 mm) | 3.000 in / 76.2 mm | 27.0 t/ft | 787 kN/m | 40.4 t/ft | 14.8 t/ft |
| 1/2 in (0.5000 in / 12.70 mm) | 4.000 in / 101.6 mm | 35.9 t/ft | 1,049 kN/m | 53.9 t/ft | 19.8 t/ft |
US tons per foot. Gauge thicknesses are Manufacturers' Standard Gauge for uncoated steel, from the sheet metal gauge chart. Plate over 1/2 in usually runs a 10x to 12x die, which lowers the figure.
Have a formed part to make? Send the drawing or STEP file and U.S. sheet metal shops quote it, or browse sheet metal fabricators. Work out the flat blank with the bend allowance calculator.
Frequently asked questions
- How do you calculate press brake tonnage for air bending?
- The common rule for a 90 degree air bend in 60 ksi mild steel is tons per foot = 575 x t² / V, with thickness t and die opening V in inches. Multiply by the bend length in feet for the total, and by tensile strength / 60 ksi for other materials. For 1/8 in mild steel over a 1 in die that is 575 x 0.015625 / 1 = 8.98 tons per foot.
- What is the metric press brake tonnage formula?
- Converting the same rule to millimeters and metric tons gives about 67.4 x t² / V metric tons per meter for 60 ksi (414 MPa) steel, with t and V in mm. Multiply by 9.81 for kN per meter. Other published metric formulas start from tensile strength in MPa and differ by around 10 percent, so treat any of them as an estimate.
- What V-die opening should I use?
- Eight times the material thickness is the usual starting point for sheet, and air bending generally stays between 6x and 12x. Thicker plate goes wider: a common guide is 8x up to about 1/2 in (12 mm), 10x from there to about 1 in (25 mm) and 12x above that. A narrower die raises the tonnage and tightens the radius; a wider one lowers the tonnage but needs longer flanges.
- What inside radius does air bending produce?
- In air bending the radius comes from the die opening, not the punch, as long as the punch radius is smaller. A widely used rule puts it at about 15.6 percent of V for 60 ksi cold-rolled steel, 20 to 22 percent for 304 stainless and 13 to 15 percent for 5052-H32. Use the formed radius, not the punch radius, in the bend allowance.
- What about bottoming and coining?
- This calculator covers air bending only. Bottom bending presses the sheet into the die and takes several times the air-bend force; coining takes more again. If a drawing calls for a radius or angle that only bottoming can hold, ask the shop what their brake and tooling can do.
- Can I run a short bend at the press brake’s full rated tonnage?
- Not safely. A brake’s rating assumes the load is spread along much of the bed. A short, heavy bend concentrates it and can deflect or damage the ram, bed and tooling. Check the builder’s maximum load per foot (or per meter) and the tooling’s rated load before you run a short bend near capacity, and leave headroom on any rule-of-thumb estimate.
