Work out the bending force a job actually needs before you specify a machine or load a program. Covers mild steel, stainless, aluminum and high-strength steel, and returns the tonnage with a safety margin applied.
Required bending force for air bending
Formula: F = (C × t² × L) / V gives force in kN, where C = 575 for mild steel; the result is divided by 9.81 to convert to tonnes-force. A material factor is applied for stainless, aluminum and high-strength steel. Air bending only.
Air bending force scales with the square of material thickness, linearly with bend length, and inversely with V-die opening. That squared term is why a small jump in thickness produces a large jump in required tonnage.
The 575 constant belongs to the kilonewton form of this equation — it comes from 1.42 × 405 MPa, the tensile strength of mild steel. Treating that raw output as tonnes overstates the required force by a factor of 9.81. This calculator converts for you and reports both figures.
The calculator multiplies the mild steel constant by a factor for other materials:
| Material | Factor | Effect vs mild steel |
|---|---|---|
| Mild Steel (Grade 250) | 1.0 | Baseline |
| Stainless Steel (304) | 1.4 | 40% more force |
| High-Strength Steel (Grade 350) | 1.2 | 20% more force |
| Aluminum (5052) | 0.5 | Half the force |
V-die opening sits in the denominator, so widening the die reduces required tonnage — but it also increases the inside bend radius and springback. The usual working range is 6–8× material thickness, and the calculator suggests 7× as a starting point. Going tighter than 6× drives tonnage up sharply and risks cracking high-tensile material.
Never size a press brake to the bare calculated figure. This calculator adds a 25% margin, and that result is what you should compare against a machine's rated capacity — running a press at 100% of rating shortens ram and frame life.
3 mm mild steel, 1 m bend length, 18 mm V-die:
So the bend itself needs about 29 tonnes, and you should specify against roughly 37 tonnes once margin is included. A 40-tonne press covers this comfortably; a 30-tonne machine does not.
This is also a useful gut-check on any tonnage figure: if a calculator tells you a 3 mm bend over one metre needs several hundred tonnes, it is reporting kilonewtons with the wrong label.
Rise Tek sells CNC press brakes, plate rolls and fiber laser cutting machines across Canada, with installation and operator training included.