Under-size the press and you stall mid-stroke or never reach the target angle; over-size it and you risk crashing tooling and overloading the ram off-center. Calculating bending tonnage up front tells you whether a job fits the machine, which V-die to set, and how the load distributes across the bed.
For the overwhelming majority of shop work — air bending in a V-die — tonnage comes down to four variables: material thickness, the die opening (V-width), the bend length, and the material’s strength.
The classic shop rule for mild steel is: Tonnage per foot ≈ (575 × T²) / V, where T is material thickness in inches and V is the V-die opening in inches. Multiply by the bend length in feet for total tonnage.
The 575 constant bundles a ~60,000 psi tensile mild steel with the geometry of air bending. For other materials, scale by the tensile ratio: stainless runs roughly 1.5× mild steel, and most aluminum alloys roughly 0.5×. Our Press Brake Tonnage calculator does this scaling for you when you pick a material.
Notice that thickness is squared — doubling thickness quadruples tonnage — while a wider V-die reduces tonnage proportionally. That trade-off is the heart of die selection.
A common starting point is a V-opening of 6 to 8 times material thickness for gauges up to about 1/2 inch. Wider openings cut tonnage and ease the bend, but they also increase the inside radius the part forms to and the minimum flange you can hold.
If a print calls for a tight inside radius, you may be forced into a narrower V — and higher tonnage — or into a different process such as bottoming or coining, which can demand several times the air-bend tonnage.
Bend 0.125 in mild steel, 4 ft long, in a 1.0 in V-die. Tonnage per foot ≈ 575 × 0.125² / 1.0 = 575 × 0.015625 = 9.0 tons/ft. Total ≈ 9.0 × 4 = 36 tons.
Confirm that 36 tons is within the press rating, and that the load is centered on the bed. Then verify the resulting inside radius and springback before committing — wider dies and higher-strength material both increase springback, which you compensate by overbending.
Forgetting to scale for material: running the mild-steel number on stainless under-sizes the press by ~50%. Ignoring bend length: tonnage is per foot, so a long bend adds up fast. And confusing air bending with bottoming/coining: those set the angle by forcing the material into the die and need far more force.
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