What Size Power Press Do I Need? Tonnage Sizing Guide

Work out exactly what tonnage your job needs — formulas, shear strength chart and a worked example. Then match it to the right press type. Free sizing help.

What Size Power Press Do I Need?

It's the question we get on WhatsApp more than any other, usually with a part drawing attached.

And it's the right question to ask early, because both wrong answers are expensive. Buy too small and you'll stall the press at the bottom of the stroke, slip and overheat the clutch, and eventually crack a frame or bend a crankshaft — failures that aren't economically repairable. Buy too big and you've parked lakhs of capital in headroom you'll never use.

This guide walks through the sizing maths, then shows you which type of press fits the result.

If you'd rather skip ahead, our press tonnage calculator does the arithmetic for you.

What Press Tonnage Means

Press tonnage is the maximum force a press delivers at the bottom of its stroke, in metric tons.

Every operation — punching, blanking, piercing, trimming, bending, drawing, forging — needs a certain force to shear or form the metal. Your press has to deliver more than that, with margin, on every stroke, all shift long.

One point that catches people out: a mechanical press only gives its rated tonnage near bottom dead centre. That's where the crank geometry has the most leverage. Work higher up the stroke and you have less force available than the nameplate suggests. It matters most for deep forming and drawing.

The Formula for Punching and Blanking

Cutting force depends on three things: how long the cut line is, how thick the material is, and how stubborn the material is.

Tonnage = (Cut Perimeter mm × Thickness mm × Shear Strength kg/mm²) ÷ 1000

Cut perimeter is the length of the cut edge, not the area.

• Round hole: 3.1416 × diameter

• Blank: the full outline length

• Multiple holes in one stroke: add all the perimeters together

Using area instead of perimeter is the single most common mistake we see. You aren't pushing the whole slug out — you're fracturing metal along its edge only.

Thickness — use the measured thickness, not the nominal spec. Sheet often arrives at the top of tolerance, and thickness affects tonnage directly.

Shear strength is not tensile strength. Tensile is how hard you can pull a material before it snaps; shear is how hard you must slide one section past another before it tears. Shear runs roughly 70–80% of tensile. Use the tensile figure by mistake and you'll calculate about 25% more tonnage than the job needs, then buy a press a size too big.

Shear Strength Chart

Working values for press shop calculations. Use mill certs where you have them.

Material	Shear Strength (kg/mm²)	Shear Strength (MPa)
Mild steel / low carbon	30–35	294–343
Medium carbon steel	45–55	441–539
HSLA	40–50	392–490
Spring steel	60–70	588–686
Stainless steel 304	52–60	510–588
Aluminium (soft)	7–10	69–98
Aluminium 6061-T6	20–22	196–216
Copper (soft)	15–18	147–176
Brass	22–28	216–275

The spread matters. Stainless needs roughly three times the force of soft copper for an identical cut — so a change of material can change which press the job can run on.

Worked Example

The job: punch a 20 mm hole in 5 mm mild steel.

Step	Value
Cut perimeter	3.1416 × 20 = 62.8 mm
Thickness	5 mm
Shear strength	35 kg/mm²
Calculation	62.8 × 5 × 35 ÷ 1000
Required tonnage	11 tons

Now add the margin — and there's a real reason for each part of it.

Why You Must Add a Safety Margin

Material variation. Shear strength shifts batch to batch, and sheet frequently arrives harder than specified.

Tool wear. You did the maths with a sharp punch. A dull punch can need 20% or more above that figure, and it will be dull for most of its service life.

Stripping force. Once through, the punch is gripped by the hole and the press has to pull it back out. Budget another 5–20%.

Rating point. As above — rated tonnage applies near BDC only.

The rule: select a press rated at least 30% above your calculated requirement.

So our 11-ton job goes onto a 30-ton press — comfortable, with capacity in hand for multi-hole dies later.

Running a press at 100% of rating every stroke shortens clutch, crankshaft and frame life. The headroom isn't waste; it's what stops the machine ageing prematurely.

Bending Is a Different Calculation

Don't reuse the cutting formula. Bending deforms metal rather than fracturing it, and generally needs far less force for the same length and thickness.

For air bending:

Force (kN per metre) = (1.42 × Tensile Strength MPa × Thickness²) ÷ V-die opening
Tons per metre = kN per metre ÷ 9.81

Thickness and V-opening in mm. As a starting point, V-die opening ≈ 8 × material thickness.

Note the thickness squared — double the thickness and bending force goes up four times.

The critical exception: if the die bottoms or coins the bend, force multiplies 3 to 5 times. Size a press for air bending when the print actually calls for a coined radius and you'll be undersized by 400%. If you're unsure which your die does, send us the die design and we'll factor it in.

Hot Forging Is Sized Differently Again

If you're forging rather than stamping, tonnage alone won't answer the question. Friction screw presses and forging screw presses are sized on energy per blow and the number of blows the part needs — which depends on material, forging temperature, part complexity and die design.

For that, send us the part drawing. It isn't a formula you can run in isolation.

Matching Tonnage to the Right Press

Calculated requirement	Recommended machine	Typical operations
Up to 75 ton	C-Frame Power Press (30–150 ton)	Piercing, small blanking, bending
75–300 ton	H-Frame Power Press (30–300 ton)	Heavy blanking, trimming, progressive dies
50–600 ton (forging)	Friction Screw Press, down-stroke	Hot forging, coining, embossing, sizing
Forging, up-stroke	Forging Screw Press, up-stroke	Hot forgings, brass and non-ferrous parts
High-speed stamping	Cross Shaft / Pneumatic Press	Continuous stamping with coil feed lines

C-frame or H-frame? A C-frame gives you three-side access to the die area, which makes loading and maintenance easier — good for smaller work. An H-frame is stiffer and resists deflection under off-centre loading, which is why heavy blanking and progressive dies belong on one.

Warning Signs You're Running Over Capacity

• The press stalls or hesitates at bottom dead centre

• The clutch slips or runs hot

• Burr height climbs across a production run

• Punches break or chip more often than they should

• The flywheel takes longer to recover between strokes

• The sound at the bottom of the stroke gets sharper

Any of these on a running job means stop and recalculate, not push through.

Get an Exact Recommendation

A formula gets you close. Forty years of building presses gets it right.

Send us your part drawing, material and production rate. Our engineers will confirm the tonnage, frame type and bed size you need — free of charge, no obligation.

WhatsApp +91 98147-15935 · info@birsonforgings.com

→ Try the Press Tonnage Calculator

Birson Forgings has manufactured mechanical, pneumatic and friction screw presses in Ludhiana, Punjab since 1985. ISO 9001:2015 certified, 30 to 600 ton, exported worldwide with pre-shipment load testing and documentation.

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Frequently Asked Questions

What size power press do I need?

Calculate required tonnage using cut perimeter × thickness × shear strength ÷ 1000, then select a press rated at least 30% above that figure. An 11-ton requirement should go on a 30-ton press.

How do I calculate press tonnage for punching?

Multiply cut perimeter in mm by thickness in mm by shear strength in kg/mm², then divide by 1000. Add a 30% safety margin and choose the next standard press size up.

What tonnage press punches a 20 mm hole in 5 mm mild steel?

Perimeter 62.8 mm × 5 mm × 35 kg/mm² ÷ 1000 = 11 tons. With margin, a 30-ton press handles it comfortably.

Is shear strength the same as tensile strength?

No. Shear strength is roughly 70–80% of tensile strength. Tonnage calculations use shear strength; using tensile overstates the requirement by about 25%.

Is press tonnage the same for cutting and bending?

No. Bending needs far less force than cutting the same length and thickness — unless the die bottoms or coins the bend, which multiplies force 3–5 times.

What happens if the press has too little tonnage?

It stalls at bottom dead centre, the clutch slips and overheats, and repeated overloading can crack the frame or bend the crankshaft. These failures are not economically repairable.

Should I choose a C-frame or H-frame press?

C-frame gives three-side die access and suits lighter work. H-frame is stiffer under off-centre loading and suits heavy blanking and progressive dies.

Can Birson Forgings size the press for my part?

Yes, free of charge. Send your part drawing on WhatsApp +91 98147-15935 or email info@birsonforgings.com and our engineers will calculate tonnage and recommend a machine.