Pulley speed and size.

Worked example

A 100 mm motor pulley turning at 1,750 rpm drives a 250 mm pulley. Driven speed = 1,750 × 100 ÷ 250 = 700 rpm, a 2.5 : 1 reduction. Belt speed = π × 0.100 m × 1,750 = 549.8 m/min (about 1,804 ft/min). If the belt slips 2%, the driven pulley turns at 700 × 0.98 = 686 rpm (the 2% is an illustration only; real slip varies).

How it works

With no slip, the belt moves at the same surface speed on both pulleys, so d1 × n1 = d2 × n2: a pulley twice the diameter turns half as fast. Solve for any one of the four values. Belt speed is the pulley circumference times its rpm, v = π × d × n. Belt slip, which you can enter as a percentage, multiplies the driven speed by (1 − slip ÷ 100).

The speed ratio follows the diameters only approximately in practice: the angular-velocity ratio may not be exactly constant or equal to that of the pulley diameters, due to slip and stretch, and toothed belts largely avoid the problem. The V-belt range shown is a published rule for the optimal speed range and not a limit of any particular belt.

Sources and basis: Belt (mechanical) (Wikipedia). Checked 2026-10-08. Planning and learning aid only. It is not for structural, pressure-system or other safety-critical design, and it is not professional engineering advice. Have a qualified engineer check anything that carries people or could fail dangerously.

FAQ

How do you calculate driven pulley speed?

Driven rpm = driver rpm × driver diameter ÷ driven diameter. A 100 mm driver at 1,750 rpm turns a 250 mm pulley at 700 rpm.

How do I size a pulley for a target speed?

Driven diameter = driver diameter × driver rpm ÷ target rpm. Leave the driven diameter blank, enter the target speed in the driven rpm field and the calculator solves the diameter.

How is belt speed calculated?

Belt speed = π × pulley diameter × rpm. For 100 mm at 1,750 rpm that is 549.8 m/min. V-belts are listed as having an optimal range of roughly 300 to 2,130 m/min.

Does belt slip matter?

A little: the driven pulley runs slower than the diameter ratio predicts when the belt slips or stretches. Enter a slip percentage to see the effect; toothed belts largely avoid it.

Does this work for gears?

The same idea holds for gears using tooth counts instead of diameters. The Gear Ratio Calculator also gives output torque.