Pulley RPM Calculator
Find driven pulley RPM from diameters and drive speed.
Results update as you type.
How it works
In belt-and-pulley systems, the sizes of the pulleys determine how speed transfers between them, and this calculator finds the driven pulley's RPM. You enter the drive pulley's RPM and the diameters of both the drive and driven pulleys, and it returns the driven pulley's speed and the ratio. The principle is that the belt travels at the same linear speed around both pulleys, so a pulley's rotational speed is inversely proportional to its diameter. The driven RPM equals the drive RPM times the ratio of drive diameter to driven diameter. If the driven pulley is smaller than the drive pulley, it spins faster (a speed increase); if larger, it spins slower (a speed reduction). This is the basis of countless mechanical systems, from workshop machinery and HVAC blowers to engines and conveyor drives, where pulley sizing sets the operating speed of driven components. The speed ratio also relates inversely to torque: a driven pulley spinning faster has proportionally less torque, and one spinning slower has more, so pulley ratios trade speed for torque just like gears. This calculator is handy for anyone designing or adjusting a belt drive to hit a target speed. Use consistent diameter units, and note that diameters can be measured at the belt's pitch line for best accuracy. Enter the drive RPM and both diameters to find the driven RPM.
Driven RPM = drive RPM × (drive diameter / driven diameter)
In a belt drive, the belt speed is the same on both pulleys, so their surface speeds match. The driven pulley's RPM equals the drive RPM multiplied by the ratio of the drive diameter to the driven diameter. A smaller driven pulley spins faster; a larger one spins slower.
Source:Mechanical drives
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter drive pulley rpm, drive pulley diameter and driven pulley diameter.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Driven pulley RPM updates instantly as you type — no button to press.
Frequently asked questions
How do you calculate driven pulley RPM?
Multiply the drive pulley's RPM by the ratio of the drive diameter to the driven diameter. For a 1000 RPM drive with a 100 mm drive pulley and a 50 mm driven pulley, the driven speed is 1000 times 100 divided by 50, which is 2000 RPM. The smaller driven pulley spins faster.
Why is pulley speed inversely proportional to diameter?
The belt moves at the same linear speed around both pulleys, so each pulley's rotational speed depends on its circumference, which is proportional to diameter. A smaller pulley must turn more times to keep up with the same belt speed, so it spins faster, while a larger pulley spins slower. Hence speed is inversely proportional to diameter.
How do pulleys trade speed for torque?
Just like gears, pulley ratios trade speed for torque. A driven pulley that spins faster than the drive has proportionally lower torque, while one that spins slower has higher torque. So a speed-reduction setup (large driven pulley) multiplies torque, and a speed-increase setup (small driven pulley) reduces it, ignoring losses.
Does this account for belt slip or losses?
No. This calculation assumes an ideal belt with no slip, giving the theoretical driven speed. Real belt drives can have slight slip, especially under heavy load or with worn or loose belts, which slightly reduces the actual driven speed. For most purposes the calculated value is a very good estimate.
Should I measure pulley diameter at the belt line?
For best accuracy, use the pitch diameter, where the belt effectively contacts the pulley, rather than the outer edge, especially for V-belts and toothed belts. The ratio still uses drive over driven diameter. Using outer diameters gives a close approximation, but pitch diameters give the most precise speed calculation for a belt drive.