Press-brake tonnage for air bending increases with bend length, material tensile strength, and the square of thickness, while a wider V-opening reduces required force. The estimate must still be checked against the machine, tooling, and application.
Imperial baseline for 60,000 psi steel: tons = [575 × thickness² ÷ V-opening ÷ 12] × bend length, with all dimensions in inches.
Adjust for material tensile strength and bending method only when the factor source and application are valid.
Air-bending tonnage relationship
Thickness is squared, so a small thickness change can produce a large force change. V-opening is in the denominator: narrowing the die increases force. Bend length is linear. A tensile-strength factor can be expressed as actual tensile strength ÷ 60,000 psi for this baseline.
Worked imperial example
Air-bend 0.125 in material over a 1.0 in V-opening for a 24 in bend, using the 60,000 psi baseline:
- Square thickness:
0.125² = 0.015625. - Multiply:
575 × 0.015625 = 8.9844. - Divide by V and 12:
8.9844 ÷ 1.0 ÷ 12 = 0.7487 ton/in. - Multiply by bend length:
0.7487 × 24 = 17.97 US tons.
This is a planning baseline for the specified conditions, not an authorization to run the bend.
Material, method, and tooling factors
Different tensile strengths change force. Bottoming and coining require substantially more tonnage than air bending, and special tooling such as offsets, hems, or multiple bends can introduce additional factors. Do not apply a generic multiplier without verifying it for the exact tooling supplier and method.
V-opening also influences the achieved inside radius and minimum flange. Changing it to reduce tonnage can therefore change the part geometry and flat pattern.
Check more than the machine nameplate
- Machine capacity over the actual bend length and stroke position.
- Punch, die, adapter, holder, and clamping load ratings per unit length.
- Concentrated-load and off-center-load limits.
- Minimum flange support over the selected V-opening.
- Material tensile strength, thickness tolerance, grain direction, and actual method.
- Tooling condition, alignment, crowning, and manufacturer instructions.
When formulas disagree, compare their reference material, force units, length basis, and assumptions. A metric tonne-force and a US short ton are not the same unit.
Technical references
Published and formula-checked by MfgWorkbench. Last reviewed September 15, 2026. See our editorial method.
The same example in metric units and line load
To avoid mixing different empirical models, convert the Bystronic 60,000 psi air-bending baseline used above rather than silently substituting a different metric coefficient. With t, V and L in millimeters, the equivalent expression is F(kN) = 0.660748 × t² × L / V for that same baseline. This coefficient includes the inch and US short-ton conversions; it is not a material-independent constant.
| Input or result | Equivalent value |
|---|---|
| Thickness | 0.125 in = 3.175 mm |
| V-opening | 1 in = 25.4 mm |
| Bend length | 24 in = 609.6 mm |
| Calculated force | 17.96875 US short tons ≈ 159.86 kN ≈ 16.301 metric tonne-force |
The average line load is 159.86 / 0.6096 = 262.2 kN/m. This is a second check, not the machine’s total force. For a deliberately hypothetical tool rating of 250 kN/m, the calculation exceeds that rating even if the machine’s nameplate total is much higher. The 250 kN/m number is an invented comparison, not a rating for any tool.
Inspect the rating and permitted loading conditions for every actual punch, die, holder and adapter. Segment arrangement, concentrated loads and off-center loading may impose additional limits. Do not average force over the entire bed when only a short segment is loaded.
Unit reminder: 1 US short ton-force = 8.89644 kN; 1 metric tonne-force = 9.80665 kN. If another chart disagrees, compare material strength, bend method and units before adjusting the machine. Use the tonnage calculator for arithmetic and the equipment documentation for permission to operate.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.