What Size Rivet Do I Need?

There is no single right diameter — it depends on what you are joining and why. Three rules cover almost every case: match the rivet to the sheet thickness for general fabrication, use a classic empirical formula for boiler and structural plate, or size it directly from the load it must carry in shear. This calculator runs whichever you choose and rounds the answer up to the nearest stocked rivet.

The Sheet-Metal Rule and Unwin’s Formula

For thin sheet the everyday rule is a diameter about three times the thickness of the thinnest sheet — a 1 mm skin wants roughly a 3 mm rivet. For thicker boiler and structural plate, engineers long used Unwin’s formula, d = 6√t with both in millimetres, which gives the larger rivets those joints need. The two rules deliberately diverge because thin sheet and heavy plate fail in different ways.

Sizing by Load

When a joint carries a known force, size the rivet from shear stress instead: the diameter must give enough cross-sectional area to keep the shear stress under the allowable, across one or two shear planes. That is the most defensible approach for a designed connection — then confirm the chosen size with a full shear check.