Press Brake Tonnage & Sheet Metal Bend Calculator
Work out air-bending tonnage and the right V-die opening, then calculate bend allowance, bend deduction, flat blank length and your K-factor from a test bend.
Press brake tonnage (air bending)
Air bending only. Bottoming typically needs 3 to 5 times this force and coining more again. Inside radius is about 16% of the die opening in mild steel; stainless forms a slightly larger radius. Size your press brake with at least 20% reserve over the calculated tonnage.
Bend allowance, bend deduction & flat length
K-factor from a test bend
Bend a measured blank, then enter the outside flange lengths you get.
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How the calculator works
Air-bending tonnage uses the industry formula: tons per foot = 575 × thickness² ÷ die opening, with thickness and die opening in inches. The 575 constant is for mild steel with about 60 ksi tensile strength, so the result is scaled by your material's tensile strength ÷ 60.
Die opening is usually 8 times the material thickness up to 1/2 in, and about 10 times for thicker plate. A wider die lowers tonnage but makes a larger inside radius and needs a longer minimum flange.
Bend allowance is the length of the neutral axis through the bend: angle (radians) × (inside radius + K × thickness). Bend deduction is 2 × outside setback − bend allowance, where outside setback = tan(angle ÷ 2) × (inside radius + thickness). Flat blank length = flange A + flange B − bend deduction.
Bending sheet in production? See RAGOS CNC press brakes and panel benders, or use our section modulus calculator and tube weight calculator.
Frequently Asked Questions
How do you calculate press brake tonnage?
For air bending mild steel, tons per foot = 575 × thickness² ÷ V-die opening, in inches. Multiply by the bend length in feet for total tonnage, and scale by tensile strength for other materials. For example, 1/8 in mild steel on a 1 in die needs about 9 tons per foot.
What V-die opening should I use?
A common rule is 8 times the material thickness for material up to 1/2 in thick, and about 10 times for thicker plate. Stainless and high-strength steel often use a slightly wider die to keep tonnage and springback under control.
What is a K-factor?
The K-factor is the position of the neutral axis as a fraction of the material thickness. It is usually between 0.33 and 0.5: around 0.33 for tight radii, about 0.4 for radii between one and three times thickness, and 0.5 for large radii. The most accurate value comes from a test bend with your own material and tooling.
What is the difference between bend allowance and bend deduction?
Bend allowance is the length of material in the bend itself, added to the inside flange lengths. Bend deduction is the amount subtracted from the sum of the outside flange lengths to get the flat blank. Both describe the same bend from different dimensions.
How much more tonnage does bottoming need than air bending?
Bottoming typically needs 3 to 5 times the air-bending tonnage, and coining can need 5 to 10 times or more. Most modern CNC press brakes air bend, which is why this calculator focuses on it.
Sizing a press brake or panel bender?
Tube Form Solutions supplies RAGOS CNC press brakes from 25 to 640 tons and automatic panel benders, with sales and service from Elkhart, Indiana.