Punch Force Calculator (punching and blanking tonnage)

Understanding the punch force calculator — concept and principle

This calculator estimates the force needed to punch or blank a hole in sheet metal from the cut perimeter, sheet thickness and shear strength. It gives the force in kN, metric tonnes-force and US tons-force, and for round punches the compressive stress on the punch. Toolmakers and press operators use it to choose a press.

The punch shears the metal around the rim, so force is perimeter × thickness × shear strength.

How it is calculated

F = L × t × τ × number of holes

Perimeter: round π d; rectangle 2(a + b); any shape as entered

Compressive stress on a round punch = F ÷ (π d² / 4)

t in mm, τ in MPa (N/mm²), so F comes out in newtons. 1 tonne-force = 9,806.65 N; 1 US ton-force = 8,896.443 N.

The principle

Punching cuts the sheet in shear along the edge of the punch. The area that must be sheared is the perimeter of the hole times the sheet thickness, and the force is that area times the material's shear strength. Several holes in one stroke add up. The punch itself carries the full force in compression, so a small punch in thick, strong sheet can be stressed more than the punch steel can bear.

Worked example

Default inputs: round hole 20 mm, t = 2 mm, τ = 300 MPa, one hole.

Perimeter = π × 20 = 62.83 mm.
Sheared area = 62.83 × 2 = 125.7 mm².
F = 125.7 × 300 = 37,700 N = 37.7 kN = 3.84 tonnes-force = 4.24 US tons-force.
Punch stress = 37,700 ÷ (π × 20²/4) = 120 MPa.

Each point is the calculator re-run with only “Diameter (round) or side a (rectangle) (mm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Diameter (round) or side a (rectangle) (mm)	Punch force (kN)
5	9.42
10	18.85
15	28.27
20	37.7
25	47.12
30	56.55
40	75.4

How to read the result

Choose a press rated comfortably above the calculated force, since dull tools and tight clearance raise the real force. Shearing angles on the punch lower the peak by cutting progressively. Check the punch stress for small holes, because it rises quickly as the punch diameter shrinks.

Common mistakes

Using tensile strength instead of shear strength.
Using the hole area instead of the perimeter.
Forgetting to multiply by the number of holes punched at once.

Keep in mind: Press selection must allow for tool condition, stripping force and safety margins. Follow press and tool manufacturers' data and machine safety rules.

Frequently asked questions

What shear strength should I use?

Use the sheet material's shear strength from its data sheet; for many steels it is lower than the tensile strength.

Why does a small punch break?

The same cutting force acts on a small cross-section, giving high compressive stress.

Does punching several holes add force?

Yes, if they are punched in the same stroke.

How can the peak force be reduced?

Use shear angles on the punch or stagger punches of different lengths.

Sources: Standard shearing force formula F = perimeter × thickness × shear strength; NIST Special Publication 811 for force unit conversions

Checked 9 Nov 2026. Shearing force F = perimeter × thickness × shear strength: standard formula. Unit factors 1 tonne-force = 9806.65 N, 1 US ton-force = 2000 lbf = 8896.443 N (standard values).

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