Work out the force needed to punch a hole in sheet or plate — round, square, rectangular, slotted or any custom perimeter — in kN and tonnes, with the stripping force, the minimum punch size for the thickness, and the reduction you get from shear-ground tooling. Formula shown, no sign-up.
One hole at a time. For a multi-tool hit, add the perimeters together and use the total.
τ is shear strength. Punching formulas in tooling catalogues take it as roughly 80% of tensile strength, which is what the material list above uses — the value shown in the results is the one being applied. Shear-ground punches do not reduce the work done, only the peak force, by spreading the cut over more of the stroke.
| Result | Value |
|---|---|
| Perimeter cut | |
| Shear strength used | |
| Force (flat punch) | |
| Force (as ground) | |
| Stripping force | |
| Machine load | |
| Minimum punch size |
Guidance only. Check your turret, tooling and machine ratings before punching. Full disclaimer.
Bending the same part? Press brake tonnage calculator →
Rm 410 N/mm², τ 328 N/mm², flat-face punch. Shown as kN / tonnes. Generated by the same code as the calculator above.
| Thickness | ø10 mm | ø20 mm | ø30 mm | Min. punch ø |
|---|---|---|---|---|
| 1 mm | 10 / 1.1 | 21 / 2.1 | 31 / 3.2 | 1.0 mm |
| 1.5 mm | 15 / 1.6 | 31 / 3.2 | 46 / 4.7 | 1.5 mm |
| 2 mm | 21 / 2.1 | 41 / 4.2 | 62 / 6.3 | 2.0 mm |
| 3 mm | 31 / 3.2 | 62 / 6.3 | 93 / 9.5 | 3.0 mm |
| 4 mm | 41 / 4.2 | 82 / 8.4 | 124 / 12.6 | 4.0 mm |
| 5 mm | 52 / 5.3 | 103 / 10.5 | 155 / 15.8 | 5.0 mm |
| 6 mm | 62 / 6.3 | 124 / 12.6 | 185 / 18.9 | 6.0 mm |
| 8 mm | 82 / 8.4 | 165 / 16.8 | 247 / 25.2 | 8.0 mm |
| 10 mm | 103 / 10.5 | 206 / 21.0 | 309 / 31.5 | 10.0 mm |
| 12 mm | 124 / 12.6 | 247 / 25.2 | 371 / 37.8 | 12.0 mm |
Read across: a ø20 mm hole in 3 mm mild steel needs about 6.3 tonnes; the same hole in 10 mm plate needs about 21 tonnes. Force scales linearly with thickness here — unlike bending, where it scales with the square.
Punching shears a strip of material whose area is the cut perimeter multiplied by the thickness. Multiply that area by the shear strength and you have the force. This is why a long thin slot can cost more tonnage than a large round hole: the slot has more perimeter for less area removed. Adding a second identical hole in the same hit doubles the force.
As a working rule the punch diameter should not be smaller than the material thickness in mild steel — and for higher-strength or abrasion-resistant plate, allow 1.5 × t or more. Below that the punch is being asked to carry a compressive load close to its own buckling limit, and it snaps rather than cuts. If the drawing needs a smaller hole than that, it is a drilled or laser-cut hole, not a punched one.
Total die clearance normally runs about 10–20% of thickness for mild steel (tighter for thin material, wider for thick or hard plate). Too little clearance and you get secondary shear, heavy burr and rapid punch wear; too much and the hole rolls over and the slug jams. Clearance barely moves the tonnage figure — but it decides whether the hole is any good and how long the tooling lasts.
After the cut, the sheet grips the punch and has to be pushed off it. That stripping force is typically 5–10% of the punch force, higher on stainless and on thick material, and it is what the stripper spring or die holder has to provide. Undersized stripping is a common cause of galling and of punches pulling out of their holders.
Force in kN equals the cut perimeter in mm, times the thickness in mm, times the shear strength in N/mm², divided by 1000. Divide by 9.81 for tonnes. Shear strength is roughly 80% of tensile strength, so mild steel is about 328 N/mm².
About ø6 mm as a rule of thumb — punch diameter no less than material thickness in mild steel, and more like 9 mm if the plate is high-strength. Smaller than that and the punch buckles.
Yes, roughly 45–50% more than mild steel of the same thickness, because it follows tensile strength directly. Stainless also work-hardens at the cut edge and grips the punch harder, so allow more stripping force and expect faster tool wear.
Because force follows perimeter. A 40 × 10 mm slot has about 91 mm of perimeter against 63 mm for a ø20 mm hole, so it needs about 45% more force even though it removes less material.
Please read this before using any figure from this page in production.
Every output here is an indicative calculation produced from the values you enter. Real results vary with springback, grain direction, tooling condition and geometry, friction and lubrication, machine calibration and load distribution, temperature, and batch-to-batch variation in material properties. Actual tensile strength commonly differs from the nominal figures listed. Treat these numbers as a starting point to be proven on a test piece — never as a verified value for a production run, a load calculation, or a safety-critical component.
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