Press Brake Tonnage Calculator

Tonnage is how hard the press brake has to squeeze to fold your sheet. Too little and the bend comes out open and inconsistent. Too much for the machine and you damage the tooling, the bed, or both.

Four things drive it, and a fifth that multiplies all of them:

• How strong the material is. Force is directly proportional to tensile strength. It is the first term in the formula and the first box on the form. The same job in 304 stainless needs about half as much force again as mild steel; in S700MC it very nearly doubles.
• Thickness. Force goes up with the square of thickness, in the same die, double the thickness and you need four times the force. In practice you open the die up too, and on the 8× rule the tonnage only doubles: 3mm in a 24mm V is 24 tonnes per metre, 6mm in a 48mm V is 49. The square only bites when the die cannot open any further, which is exactly what happens when you are stuck with the tooling already in the shop.
• How long the bend is. Twice the length, twice the force. Straightforward.
• The V-die you bend it in. A wider die needs far less force. This is the lever most people forget, and it is the one that decides whether the machine you already own can do the job.
• And how you are bending it. Air bending, bottoming or coining: the multiplier on the button, and it is a big one.

Here is the whole thing. It is the standard air-bending formula used across the industry, nothing invented, nothing hidden. This page does air bending only, on purpose: it is what nearly everyone is doing, and the multipliers for the other two methods are rough enough that putting them behind a button would suggest a precision they do not have.

Two formulas, and they do not quite agree

American charts use a different one: 575 × S² ÷ V ÷ 12 tons per inch, on a 60,000 psi baseline. Put both on the same material and the American one is 12.5% higher, that 575 works out to a metric constant of 1.60 against our 1.42.

But the two are not usually compared on the same material. The American charts assume 60,000 psi, which is 414 N/mm², and this page assumes 450. Those two differences pull in opposite directions, so on a real job the answers land within about 3.5% of each other. Neither is wrong; they are two empirical fits to the same messy reality.

Why published charts disagree by about 10% anyway

Because they assume different material. Mild steel is not one thing. The same grade can run anywhere from 400 to 500 N/mm². A chart built on 400 and a chart built on 450 will differ by 12% and both are honest.

This calculator uses 450 N/mm² for mild steel, which is the figure European press brake and tooling charts are built on. Change it in the box if you know better.

Always leave a margin. The number here is what the bend needs on a good day. Blunt tooling, cold material, a mill certificate at the top of tolerance: all push it up. That is why the machine we point you at carries 20% in hand.

Pick the die first, then the tonnage follows. The published rule is a single one, eight times thickness, and everything else is a sensible drift either side of it.

The material matters as much as the thickness. Mild steel sits on 8×. Stainless wants 10 to 12×, soft aluminium 5 to 6×, and high-tensile steel 12× and upwards. Past about 10× the inside radius grows faster than most drawings allow, check the radius before you commit to a die.

Never go below 5 × thickness. And the first thing to fail is not the tooling. It is the sheet. The floated radius drops below what the grade will take and the outside of the bend cracks. Check the radius this page prints against the material's minimum bend radius before you commit: roughly 1 × thickness for mild steel and 304, 1 to 2 × for half-hard aluminium, and 0.8 to 1.6 × for the 700 MPa grades on SSAB's own figures. If the die you picked floats a radius tighter than that, go up a size, not down.

For mild steel, 6 × is the sensible working floor: any tighter and you also compromise angular accuracy, and the die flanks carry stress they were not designed for.

Working it backwards: the smallest return decides the die

Most of the time the part decides, not the tooling. If a bracket has a 15mm return on it, that return has to sit on the die shoulders: so the die can be no wider than 15 ÷ 0.77 = 19mm. That is the ceiling, whatever the tonnage chart would prefer.

Put your shortest flange into the box marked smallest return and the calculator works that out for you, drops the die to the biggest one that will hold it, and puts the tonnage up to match.

Sometimes the two cannot both be satisfied. A 20mm return on 6mm plate caps the die at 25mm, but 6mm needs at least 30mm of opening to bend safely. That part cannot be made in one hit with standard tooling, and it is far better to find that out now than with the material cut. The calculator says so, and tells you the return you would need instead.

What the die opening gives you

• Inside radius ≈ 16% of the V opening, in mild steel. Air bending a 24mm V floats an inside radius of roughly 3.8mm, whatever radius is on the punch. You cannot get a tighter radius by fitting a sharper punch, you change the die. Other materials float differently: stainless runs 20 to 22% of the opening, soft aluminium about 13 to 15%. Same die, different radius.
• Smallest flange ≈ 77% of the V opening. Shorter than that and the flange drops into the die instead of sitting on the shoulders. The bare geometry says 70.7%, the sheet lies at 45° across the two shoulders, and 77% is that plus the margin for the shoulder radius and enough material to actually rest on it. You will see 67% quoted elsewhere; that is the internal leg, measured from the inside of the bend rather than the outside. We use 77% because it is the one that keeps the part on the die.
• Wider die, less force, bigger radius, longer minimum flange. It is always that trade. There is no die that wins on all three.
• And springback. Every air bend opens up when the ram lifts, so the machine has to overbend to land on the angle. Mild steel wants a degree or two; 304 stainless several times that. SSAB publish about 4° for Domex 355MC, close to 7° for Strenx 700MC and 15° or more for Hardox. It climbs with both the strength of the steel and the die-to-thickness ratio, so the wide die that saved you tonnage costs you here. It also means an 88° or 85° die rather than a 90° one: you cannot reach 90 degrees in high-tensile steel out of a 90 degree die. None of this changes the tonnage, but it decides whether you can hit the angle at all. Prove it with a test bend on the actual coil.