Bending force calculator

Press brake tonnage: what the number does and does not tell you

The force needed to bend a sheet is dominated by one term, and it is not the one people expect. Force rises with the square of the material thickness: a 4 mm sheet does not need twice the tonnage of a 2 mm sheet, it needs roughly four times. That is why a job that ran comfortably yesterday can overload the machine today after what looked like a small change in specification. Length matters too, but only linearly — double the bend length and you double the force. So when a bend will not go, look at thickness first and length second.

The V-opening is the dial nobody thinks to turn

The die opening sits in the denominator, which means a wider V reduces the required force in direct proportion. The standard starting point is a V-opening of about eight times the material thickness, and it is a genuine compromise rather than a law: a wider V lowers tonnage and is gentler on the tooling, but produces a larger inside radius and needs more flange to sit on. A narrower V gives a tighter radius at the cost of much more force and more marking on the surface. If a bend is at the edge of your machine's capacity, opening up the V is usually the cheapest fix available, and it costs nothing but a tool change.

Air bending, bottoming and coining are three different jobs

The calculation here is for air bending, where the sheet touches only the punch tip and the two die shoulders and never reaches the bottom of the V. That is how the vast majority of production bending is done, and it is the reason one set of tooling covers many angles. Bottoming presses the material into the die and typically needs three to five times the force. Coining squeezes the material until it flows plastically through its full thickness and can need eight to ten times, or more. If someone quotes a tonnage that seems wildly higher than this calculator, check which of the three they meant before assuming either number is wrong.

Springback, and why the angle you bend is not the angle you get

Metal is elastic before it is plastic, so every bend relaxes slightly when the punch lifts. Bend to 90 degrees and you will measure 91 or 92, and the harder and thicker the material the more it recovers — high-tensile grades spring back dramatically compared with mild steel, and aluminium sits somewhere in between and varies by alloy. This is normal and it is compensated by overbending: aim a degree or two past your target and let it settle where you want it. The exact amount is a property of your material batch, your tooling and your machine, which is why the first bend of any job should be a test piece rather than a part.

Grain direction and the crack that appears on the outside

Rolled sheet has a grain, and it is not decorative. Bending across the grain is the safe direction; bending along it puts the fibres in tension lengthwise and can crack the outer face, especially in aluminium and in higher-strength steels. If you have a choice when nesting parts, orient bends across the grain. When you cannot, the fixes are a larger inside radius, or a different material temper. Also watch the edge condition: a laser or plasma edge is hardened and slightly notched, and a bend line running through a rough or nicked edge will find that notch and start a crack there. Deburring before bending is not cosmetic.

From flat pattern to finished part

A bend consumes material, which is why a flat blank is always shorter than the sum of the finished sides. The material on the outside of the bend stretches, the inside compresses, and somewhere between them sits a neutral line that neither does. That is what bend allowance and bend deduction describe, and getting it wrong is the most common reason a part comes off the brake with correct angles and the wrong dimensions. Modern CAD calculates it for you from a K-factor, but the K-factor is an assumption about your tooling, not a universal constant. The reliable habit on a new material or a new die is to bend one test strip, measure the actual result, and correct the flat pattern from reality rather than from theory.

How bending force is calculated

For air bending, the standard rule of thumb is F (kN/m) = (K × Rm × S²) / V — with K a die-geometry constant (1.42 here, sources range 1.3–1.6), Rm the material's tensile strength, S the sheet thickness and V the die opening. Multiply by the bend length for the total force; divide kN by 9.81 for the metric tons shown on most press brake gauges. Note the S²: doubling the thickness quadruples the force — that's why thick plate needs such heavy machines.