Press brake bend force calculator

The formula and what each term means

P = C × Rm × T² / V

P

— bending force per metre of bend length (kN/m)

C

— empirical die-geometry constant (1.42, used in the calculator)

Rm

— tensile strength of the sheet (N/mm²)

T

— sheet thickness (mm)

V

— the die's V-opening (mm)

1.42 is the constant behind the tonnage tables published in European cold-forming practice. At the standard ratio V ≈ 8 × T, the formula reproduces those tables within a percent or two: for 3 mm steel on a V25 die it gives 261 kN/m against roughly 26 t/m published, for 5 mm on a V40 die it gives 453 kN/m against roughly 46 t/m published, and for 1 mm on a V8 die it gives 91 kN/m against roughly 9 t/m published. The constant is not attributed to any manufacturer.

In air bending, the usual V-opening is 6 to 12 times the sheet thickness, most commonly around 8 times. Within that window, the formula tracks reality closely. Outside it, the simplification stops holding, in a direction that depends on which side you're on — see the example below.

Shop-floor practice often quotes tons-force rather than kN: 1 tf ≈ 9.81 kN, so 100 kN/m is roughly 10.2 t/m.

Example from the UKB catalog

Mild steel, Rm 510 N/mm², 1 mm sheet, 90° bend. Source of catalog values: UKB GmbH.

The two figures show exactly where the formula stops applying. At 1 mm thickness, a V4 die is a V/T ratio of 4, and a V16 die is 16. Both fall outside the 6-to-12 window, on opposite sides of it.

Below a ratio of 6, the sheet is tightly constrained in the channel, friction rises, and the real force climbs far faster than the formula's 1/V term predicts. Above a ratio of 12, the situation reverses and the formula slightly overestimates. Between those bounds, where most applications actually work, the estimate stays close to published force tables.

The practical takeaway: within a V/T ratio of 6 to 12, use the calculator's number as a solid starting point. Outside it, treat it as an order of magnitude and confirm the tool's real rating with us. More on rated capacity and checking it for UKB tools: press brake tool maintenance.

Reference table: force in mild steel (Rm 510 N/mm²)

90° air bending, values in kN per metre of bend. Lighter cells fall outside the recommended 6-to-12 V/T range — use them only as an order of magnitude.

What the calculator doesn't tell you

• Springback. The formula gives force, not angle. Springback depends on material, thickness and radius, and compensation happens on the machine.
• Air bending, not coining. Coining (bottoming in the die) needs several times more force. The calculator covers air bending only.
• Sheet condition. Rusty, scaled or unlubricated sheet needs noticeably more force than the same sheet in normal condition.
• Real Rm, not catalog Rm. The default values are the upper end of a material category's range. If you have the material certificate, use its Rm.
• Press capacity and load distribution. A short bend applied over a small portion of the bed length loads the machine differently than one spread across its full length. Check the press documentation.