Most people shopping for a press brake ask the wrong question first. They ask “how many tons?” when the real question is how many tons, over what length, in what material, with which die.
Change the die opening and the tonnage requirement can move by half. Switch from mild steel to stainless and it jumps 50%. Bend 10 feet instead of 4 and it nearly triples.
Run your actual job through the calculator, then read on for the four things it can’t tell you.
Your bend
The length of the bend itself, not the width of the sheet.
Air bending is the default for most shops — lowest tonnage, most flexible, one die set covers many angles.
A wider die needs less tonnage but produces a larger inside radius. Narrower is the reverse.
Tonnage required
Machine class to shop
Setup details
| Recommended die opening (V) | — |
| Resulting inside radius | — |
| Minimum flange length | — |
| Minimum bed length | — |
How this is calculated. Air bending tonnage uses the standard industry formula for 60,000 psi tensile mild steel: tons per foot = (575 × T²) ÷ V, where T is material thickness and V is die opening, both in inches. Total tonnage is scaled by bend length and by a material factor (stainless ≈ 1.5×, 5052 aluminum ≈ 0.5×, 6061-T6 ≈ 0.65×). Bottoming multiplies air-bend tonnage by roughly 3×; coining by roughly 8×. Inside radius is estimated at 0.156 × V and minimum flange at 0.63 × V.
Treat this as a planning estimate, not a certification. Actual results vary with punch radius, die angle, grain direction, material certification, temper, and machine condition. Always confirm against the tooling manufacturer’s chart and your machine’s rated capacity before committing to a job — and never exceed your press brake’s rated tonnage or its distributed-load limits. Short bends concentrated at one end of the bed can damage a machine well below its rated capacity.
The four specs that actually decide your machine
1. Tonnage — but per foot, not total
Press brakes are rated in tons, and that rating assumes the load is spread evenly across the full bed. The number that matters for your job is tons per foot, multiplied by how much bend you’re making.
The trap: a 10-foot machine rated at 175 tons does not mean you can put 175 tons on 12 inches of bed. Concentrating full tonnage on a short section will crown the ram and bend the bed — permanently, and well under the rated capacity. Every manufacturer publishes a distributed-load limit. Read it before you buy a machine to do short, heavy parts.
Rule of thumb: size to your heaviest realistic job, then add 20%. Shops almost never regret extra tonnage. They regret buying exactly enough.
2. Bed length and distance between housings
Bed length has to exceed your longest bend — but the spec that trips people up is the distance between the housings (the side frames). That’s what limits how far a formed part can stick out sideways. If you’re making deep boxes or long channel, check this number, not just the bed.
3. Throat depth
How far back the ram can reach into the machine. This decides the maximum flange depth you can form. Cheap on the spec sheet, expensive to discover you don’t have enough.
4. Hydraulic vs. mechanical vs. CNC
- Manual / mechanical — lowest cost, fine for repetitive simple bends and light gauge. You’re setting stops by hand.
- Hydraulic — the shop standard. Smooth, controllable, full tonnage anywhere in the stroke, and you can stop mid-bend to check an angle.
- CNC with back gauge — pays for itself the moment you’re running production, multi-bend parts, or anything where setup time is the real cost. If you’re bending the same part 50 times, a back gauge is not a luxury.
Air bending vs. bottoming vs. coining
Air bending is what most shops do most of the time. The punch pushes the material into the die without touching the bottom. Lowest tonnage, and one die set covers a wide range of angles — you control the angle by how deep you press. Downside: more springback, so angle repeatability depends on consistent material.
Bottoming presses the material fully into the die. Roughly 3× the tonnage of air bending, but much tighter angle control and less springback. You need a die for each angle.
Coining forces the material into the die radius hard enough to plastically deform it through the whole thickness. Roughly 8× the tonnage. Tightest tolerance and smallest inside radius, but the loads get out of hand fast — it’s really only practical on thin material.
If you’re not sure: air bend. It’s the flexible default, and it’s why the calculator starts there.
The die opening rule
The V-die opening is the single biggest lever on your tonnage number, and it’s the one people ignore.
Standard starting point: V = 8 × material thickness.
| Die opening | Tonnage | Inside radius | Notes |
|---|---|---|---|
| 6 × T | Higher | Tighter | Risks cracking the outside of the bend |
| 8 × T | Baseline | Standard | Where to start |
| 10–12 × T | Lower | Larger | Easier on the machine, less crisp bend |
Two more numbers fall out of the die opening:
- Inside radius ≈ 0.156 × V. With a 1" die you get roughly a 5/32" inside radius regardless of punch, as long as the punch radius is smaller than the natural radius.
- Minimum flange ≈ 0.63 × V. Any flange shorter than that will slip into the die and won’t form. If your print calls for a short flange, you need a narrower die — and that means more tonnage.
Material matters more than people expect
| Material | Tonnage vs. mild steel | Watch out for |
|---|---|---|
| Mild steel A36 / 1018 | 1.0× (baseline) | — |
| Stainless 304 / 316 | ~1.5× | Work-hardens, springs back hard — plan to overbend |
| Aluminum 5052 | ~0.5× | Forgiving, bends well |
| Aluminum 6061-T6 | ~0.65× | Cracks on tight bends. Bend across the grain, use a generous radius |
Grain direction is real. Bending parallel to the rolling direction is how parts crack — especially in aluminum and higher-strength steel. When the print allows it, bend across the grain.
Common mistakes we see
- Buying exactly enough tonnage. The job you buy for is never the last job. 20% margin is the minimum.
- Forgetting freight and install. A 90-ton press brake is several thousand pounds. Confirm your floor, your door, and how it comes off the truck before you order — our freight guide covers all of it.
- Ignoring tooling in the budget. The machine is the beginning. Punches, dies, and a holder system are a real line item, and the wrong tooling makes a good machine useless.
- Sizing to the machine instead of the part. Start with your heaviest and longest realistic part, then work backwards to the machine. Not the other way around.
Talk it through with someone
Send us the print, the material, and the part you’re trying to make. We’ll spec the machine and the tooling, tell you honestly if a smaller machine will do the job, and quote it with freight included.