Punching Force Calculator
Work out the force to punch or blank round, square, rectangular or slotted holes from perimeter, sheet thickness and shear strength, in kN, US tons and tonnes.
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About this toolWhat is the Punching Force Calculator?
Punching a hole is a shearing operation. The punch pushes a slug out of the sheet, and the material resists along the whole cut edge, through the full thickness, until it fractures. So the force needed is the length of that edge times the thickness times the shear strength of the metal. That is all this calculator does, but it does it for the shapes and units people actually use.
Pick a round hole, a square, a rectangle, an oblong slot or a custom perimeter for any other blank shape, and enter its size and the sheet thickness. Choose a material, enter a shear strength, or estimate one from the tensile strength. If several punches hit in the same stroke, set the number of holes. The result is given in kilonewtons, US tons, metric tonnes and pounds-force, with a table of the same hole in other common materials.
- Round, square, rectangular, oblong and custom perimeters
- Shear strength presets, custom value or estimate from tensile strength
- kN, US tons, metric tonnes and lbf
- Multiple holes in one stroke and material comparison table
- Minimum hole size warning for slender punches
How to use the Punching Force Calculator
- Choose metric or inch units and the hole shape.
- Enter the hole dimensions: diameter, side, length and width, slot length and width, or the full cut perimeter for a custom shape.
- Enter the sheet thickness and the number of holes punched in the same stroke.
- Pick a material shear strength, type your own, or estimate it from tensile strength.
- Read the force per hole and in total, and compare materials in the table below.
Worked examples
Default: 20 mm hole in 3 mm mild steel
Input
Round, 20 mm, t 3 mm, 345 MPa
Output
65.03 kN, 6.63 t, 7.31 US tons
The cut perimeter is 62.83 mm and the sheared area 188.5 mm². The same hole in 304 stainless needs about 97.5 kN.
The classic 1 in hole in 1/4 in plate
Input
Round, 1 in, t 0.25 in, 50 ksi
Output
19.63 US tons (174.7 kN)
The rule of thumb 1 x 0.25 x 80 = 20 tons agrees within 2%.
Four slots in stainless at once
Input
Oblong 30 x 10 mm, t 2 mm, 304 at 517 MPa, 4 holes
Output
295.4 kN, 33.2 US tons
Each slot has a perimeter of 71.4 mm. Staggering the punch heights so they do not all cut together would cut the peak force to that of one slot.
Background and insight
The shear strength values are the typical figures used in punching charts: 50 ksi (345 MPa, or 25 tons per square inch) for mild steel, about 75 ksi (517 MPa) for annealed 304 stainless, 20 ksi for 5052-H32 and 30 ksi for 6061-T6 aluminium from Aluminum Association data, and around 35 ksi for half-hard brass. When only the tensile strength is known, shear strength is often taken as about 0.8 times it for steels, the default ratio here.
The old ironworker shortcut, tons = diameter x thickness x 80 for mild steel in inches, is the same formula with π x 50 ksi / 2,000 lb per ton = 78.5 rounded up. Some charts use 30 tons per square inch rather than 25 to include an allowance for friction and dull tooling. Either way, the force is proportional to perimeter, so a 2 in hole needs twice the force of a 1 in hole, not four times.
The formula is for sharp tools at proper clearance, typically 5 to 10% of thickness per side for mild steel. Worn punches and tight clearance push the force up; a shear angle ground on the punch face spreads the cut over part of the stroke and lowers the peak. Very small holes are limited by the punch itself: a punch thinner than the sheet tends to buckle or snap, which is why the common minimum is a hole at least as large as the thickness in mild steel and about twice the thickness in stainless.
Common use cases
Ironworker and punch press capacity
Check that a hole size and thickness are inside the machine’s punching tonnage.
Turret punch tooling
Estimate the force for a station before choosing standard or thick-turret tooling.
Blanking dies
Size the press for a blanking or piercing die from the total cut perimeter.
Hydraulic punch kits
See whether a portable hydraulic punch can make a given hole in field steel.
Tips and limitations
- Use the full perimeter that cuts in one stroke. For progressive dies, add up all stations that cut at the same time.
- Allow a margin of 20% or more over the calculated force for dull tooling, tight clearance and material variation.
- Stripping the punch out of the sheet takes additional force, often quoted at up to 10 to 20% of the punching force.
- Hardened, high-strength or work-hardened sheet can have far higher shear strength than the typical values; use the mill certificate.
- A shear angle on the punch or stepping the punch lengths reduces the peak force on the press.
Frequently asked questions
What is the formula for punching force?
Force = cut perimeter x sheet thickness x ultimate shear strength. In metric, mm x mm x MPa gives newtons. In inch units, in x in x psi gives pounds-force; divide by 2,000 for US tons.
How many tons does it take to punch a 1 inch hole in 1/4 inch steel?
About 19.6 US tons with mild steel at 50 ksi shear strength, or 20 tons by the shortcut diameter x thickness x 80.
What shear strength should I use for mild steel?
50 ksi (345 MPa) is the usual value in punching charts. Some use 60 ksi or 30 tons per square inch to build in an allowance for friction and wear.
How is shear strength related to tensile strength?
For steels the ultimate shear strength is roughly 0.75 to 0.8 of the tensile strength. Aluminium alloys are closer to 0.6 to 0.67 of tensile, so use published shear values for them.
What is the smallest hole I can punch?
As a rule, the hole diameter should be at least equal to the sheet thickness in mild steel and about twice the thickness in stainless. Smaller holes need special tooling or drilling.
How do I calculate blanking force for an irregular shape?
Measure the total length of the cut edge, from CAD or by adding up straight and arc segments, and enter it as a custom perimeter.
Is there an in-depth guide to the Punching Force Calculator?
Yes. “Punching Force: Perimeter, Thickness and Shear Strength” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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