Formula
Air-bend force per metre F = 1.42 x Rm x thickness squared / V-die opening, in kN/m, where Rm is tensile strength in MPa and thickness and V are in mm. Total force = F x bend length in metres. Tonnes-force = kN / 9.81.
Standard / source
Air-bend tonnage formula as summarised in the MetalCalz tool-expansion research (2026-10-27), with the press-brake maker rule that bottoming needs about 4 to 5 times and coining about 6 to 10 times the air-bend force. Those multipliers are ranges, not single values, so this calculator covers air bending only.
Worked example
Rm 450 MPa, thickness 3 mm, V-die 24 mm, bend length 2 m, machine capacity 1000 kN: F = 1.42 x 450 x 9 / 24 = 239.625 kN/m, shown as 239.6 kN/m. Total = 239.625 x 2 = 479.25 kN, shown as 479.3 kN, about 48.9 tonnes-force, which is 47.9% of 1000 kN. These are the calculator defaults.
Safety note
Safety-critical: overload gate and qualified-operator gate. Never exceed the machine rated capacity or the tooling load limit, and keep a margin near capacity. Bottoming and coining need much higher force than the air-bend figure shown here. A qualified operator must check machine capacity, tooling rating and maker data before bending.