ConverterHub

Pulley Speed Calculator

Find the output speed of a belt-and-pulley drive.

How it is calculated

The belt travels at one speed around both pulleys, so rim speed is equal and rotation speed is inversely proportional to diameter: RPMdriven = RPMdrive × Ddrive / Ddriven. rpm is the speed of the drive (motor) pulley in revolutions per minute, drive is its diameter and driven is the diameter of the pulley being turned.

The diameters can be in any unit, mm or inches, as long as both use the same one, because only their ratio matters. The second result, the ratio, is shown as Ddrive / Ddriven : 1, so a value below 1 means the driven pulley turns slower. Both diameters must be above zero and the RPM cannot be negative. Belt slip is not modelled, so a real drive runs slightly slower than the figure shown.

Worked example

A motor turns at 1750 RPM with an 80 mm pulley, driving a 200 mm pulley. The diameter ratio is 80 / 200 = 0.4. Multiply the motor speed by it: 1750 × 0.4 = 700. The tool reports 700.00 RPM for the driven pulley and a ratio of 0.40 : 1. The larger driven pulley turns at 40 percent of the motor speed, which gives lower speed and, in exchange, more torque.

Try this example →

Common mistakes

  • Swapping the two diameters. The drive pulley is the one on the motor; putting its size in the driven field gives 1750 × 200 / 80 = 4375 RPM instead of 700.
  • Mixing units, such as one diameter in mm and the other in inches. Only the ratio matters, but both must use the same unit.
  • Reading the ratio as a reduction ratio. This tool shows drive diameter over driven diameter, so 0.40 : 1 means the output is slower, and the same drive is often quoted as 2.5 : 1 reduction.
  • Using the outside diameter of a grooved or V-belt pulley when the belt rides deeper in the groove. Use the diameter at which the belt actually runs.
  • Expecting an exact real-world speed. Slip and belt stretch lower the actual driven RPM a little.

About this calculator

In a belt-and-pulley drive, the belt moves at the same linear speed on both pulleys, which forces N₂ = N₁ × (D₁ / D₂) — the driven pulley's RPM equals the drive RPM times the ratio of the drive pulley's diameter to the driven pulley's diameter. A smaller driven pulley spins faster than the drive pulley; a larger one spins slower, trading speed for torque exactly the way gears do.

Mechanics and machine designers use this relationship constantly when sizing pulleys for a target output speed — automotive alternator and accessory drives, HVAC fan and blower belts, and workshop lathes and drill presses with step-cone pulleys all rely on this same diameter ratio to hit the RPM a motor or tool needs. Enter the drive RPM and both pulley diameters to find the driven pulley's speed.

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