Punch Force Calculator: Punching Tonnage for Round, Square, Rectangular and Slotted Holes

How it works

F = perimeter × thickness × shear strength (× number of holes per stroke) · round hole: perimeter = π × d · obround: 2 × (L − W) + π × W · US tons = lbf ÷ 2,000 · metric tonnes = kN ÷ 9.807 · press capacity = F × factor

Punching (and blanking) works by shearing the metal around the edge of the punch, so the force needed is the area being sheared times the material’s shear strength. That area is the cut perimeter times the sheet thickness, which gives F = perimeter × thickness × shear strength. In US units, divide pounds-force by 2,000 for tons; a quick check for mild steel (about 50,000 psi in shear) is tons ≈ hole diameter × thickness × 80, both in inches. In metric, millimetres × millimetres × N/mm² gives newtons. Pick a round, square, rectangular or obround (slot) hole, or enter any cut perimeter, then the material. Shear strength is usually 60–80% of tensile strength for steels; stainless needs about 1.5 times the force of mild steel, while aluminum ranges from about 0.2 times (1100-O) to 0.6 times (6061-T6), so pick the alloy and temper when you know it. The calculator also gives a recommended press size with a margin, the stress on the punch face (high when the hole is small relative to the thickness, which is when punches break), and a table of forces for the same hole across common gauges and plate thicknesses. It covers punching and blanking presses, ironworkers and turret punches; it is unrelated to injection-molding clamp tonnage. Values are for a flat-faced punch with normal die clearance: check the press and tooling ratings before running a job.

Worked example

A 1 in round hole in 1/4 in mild steel: the perimeter is π × 1 = 3.1416 in, so the sheared area is 3.1416 × 0.25 = 0.7854 sq in. At 50,000 psi the force is 0.7854 × 50,000 = 39,270 lbf, or 19.63 US tons (174.7 kN, 17.81 metric tonnes). With a 1.3 capacity factor, use a press rated for at least 25.5 tons. The stress on the punch face is 50,000 psi, and the hole is 4 times the thickness, so the punch is not at risk.

Sources: Unipunch: Understanding the role of tonnage in punching parts (formula, 50,000 psi mild steel, 1 in × 1/4 in example) · MachineMfg: Punching force calculation (shear strengths and worked examples) · Omni Calculator: Punch force (F = P × S × t, 7075-T6 example) · FIRGELLI: Punching force calculator (shear strengths by alloy and temper, Mate cluster example)

Frequently asked questions