How pulley diameter changes driven RPM

By FieldCalc operator · October 13, 2026

Identify driver and driven pulleys, use effective diameters, and calculate driven RPM or the diameter needed for a target speed.

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Identify the driver

The driver pulley supplies power, usually from the motor shaft. The driven pulley receives it through the belt. The smaller pulley is not necessarily the driver. Establish power direction before entering diameters.

Use the speed relationship

With no slip, driver RPM × driver effective diameter = driven RPM × driven effective diameter. Thus driven RPM = driver RPM × driver diameter ÷ driven diameter. Both diameters must use the same unit, such as mm or inches.

Compare diameter changes

At 1,440 RPM with a 150 mm driver and 300 mm driven pulley, output is 720 RPM. Increasing the driven diameter to 400 mm lowers it to 540 RPM. Increasing only the driver to 200 mm with the original 300 mm driven pulley raises output to 960 RPM.

Choose a target diameter

For 600 RPM output while retaining 1,440 RPM input and a 150 mm driver, driven effective diameter is 1,440 × 150 ÷ 600 = 360 mm. This is a theoretical value. Standard sizes, belt length, center distance, wrap angle and machine clearance require separate checks.

Use effective diameter, not just outside diameter

Use the pitch or effective diameter defined for the belt system. On V-belt sheaves, belt position in the groove can make outside diameter different from the calculation diameter. For timing pulleys with the same belt pitch, driven RPM = driver RPM × driver teeth ÷ driven teeth. A 24T driver and 48T driven pulley produce a 2:1 reduction.

Investigate measurement differences

Check actual motor RPM, pulley specifications and belt condition first. Friction belts can slip, and tension and wear matter. Do not attribute every discrepancy to slip; measurement conditions and instrument error also need attention.

Check limits before changing speed

Higher speed does not automatically increase motor power. Check speed-dependent loads such as fans and pumps. Verify permissible belt speed, minimum pulley diameter, shaft loads and guard clearance with manufacturer data. Stop and isolate the equipment before measuring or replacing pulleys.

A guide to preliminary calculations. Check manufacturer specifications and operating conditions before applying results.