Guides — Tonnage

How press brake tonnage is calculated

By Jon Chacon · October 22, 2026

Every bend needs a certain amount of force to form. Too little and the part won't come to angle. Too much and you're into the tooling, the machine, or both — dies crack, rams get overloaded, and parts come off the brake as scrap.

So the number matters. And in most shops it still comes off a wall chart that's been hanging there for twenty years, or out of the last guy's notebook, or from whoever set the job up last time and wrote it on cardboard.

This page is how that number actually gets figured — the same math we built into Precision Form, with a real bend at the end to check it against. Rather run your own numbers? Use the free tonnage calculator.

The short version

In air bending, the force you need comes down to four things:

• Material. What it is and how strong it is. Harder, higher-tensile material takes more force to bend. That's the whole story behind stainless taking more than mild steel, and aluminum taking less.
• Thickness — and this is the big one. Thickness is squared in the math. Double the thickness and you need roughly four times the force, not two. A small step up in gauge is never a small step in tonnage.
• V-die opening. A wider die opening spreads the work out and drops the force. A narrow die concentrates it and the tonnage climbs fast.
• Bend length. Force is figured per foot of bend, then multiplied over the full length of the part.

The classic shop formula for mild steel puts it together like this:

Know what that formula assumes, though. It's an estimate built around mild steel at roughly 60,000 psi tensile. Run a stronger material through it without adjusting and you'll under-call the tonnage. And whatever number it gives you is the requirement, not the plan — you still want a safety factor over it. We plan at 1.5×, and we'll get to why in the FAQ.

If you just want the numbers, the press brake tonnage chart has tons per foot for the common thicknesses and die openings, figured from this same formula.

A worked example, start to finish

Take a common job: 1/4-inch A36 over a 2-inch V — the 8× die for quarter-inch, and the same size of part as the homepage proof bend. Run it through the formula by hand:

Thickness squared: 0.250 × 0.250 = 0.0625. Times the constant: 575 × 0.0625 = 35.9. Divided by the opening: 35.9 ÷ 2 = 18.0 tons per foot. That’s the same number the chart page reads for quarter-inch in a 2-inch die — the chart is just this math, done ahead of time.

Now give it a length. Say the bend runs 5 feet: 18.0 × 5 = 90 tons total to form the part. And that’s the requirement, not the plan — at the 1.5× safety factor from the short version above, you’d want roughly 135 tons of machine under it before you called the setup good.

A real example

Here's a bend we ran through Precision Form and then ran on an actual brake, the same day, on the same part. The job: quarter-inch A36 plate, 61 inches wide, bent to 90 degrees, 2.375-inch flange, .38-inch inside radius.

The app called it at 82.4 tons total — 16.2 tons per foot — with a 3/8-inch radius punch. For the die, it picked our #37 die, a 1.960-inch opening, as the best fit at 84.1 tons. It also flagged the other die we could have grabbed, the #35 with a 1.260-inch opening: that one would have needed 130.8 tons, at 25.7 tons per foot. Same part, same bend — the die choice alone was the difference between a comfortable setup and a high-pressure one. And we knew it before anybody touched the machine.

Then we bent the part and watched the gauge. It matched the call. There's a section on the homepage — "Proof, not promises" — with the setup, the prediction, and the bend itself, if you want to see it instead of taking our word for it.

Where the wall chart gets you in trouble

Wall charts aren't wrong, exactly. They're just answering a narrower question than the one you're asking at the machine.

• They assume one material. The chart is drawn for mild steel. The part in your hands might not be.
• They don't know your dies. A chart gives you a number for a standard opening. It doesn't know which dies are actually in your rack, which one the last job left in the machine, or which one fits your flange.
• They don't warn you about concentration. A short part on a narrow die can put a lot of tons per foot through a small section of tooling even when the total tonnage looks fine. Totals don't crack dies — pressure does, and that's per foot.
• They don't know your machine. Your brake has a limit, and where the load sits on the ram matters too. The chart on the wall has never met your press brake.

Questions we get asked