RPM Calculator
Enter the driver's RPM and the diameters of both pulleys or gears to find the driven speed.
How pulley and gear speed ratios work
In a belt-and-pulley or gear system, speed and diameter (or tooth count) are inversely proportional: a larger driven pulley spins slower than the driver, and a smaller driven pulley spins faster, following the formula RPM₂ = RPM₁ × (D₁/D₂).
This relationship holds because the linear speed at the edge of each pulley must match (they're connected by the same belt), so a bigger circumference covers more distance per rotation, meaning fewer rotations are needed to keep pace with the smaller pulley.
Frequently asked questions
Does this work for gears as well as pulleys?
Yes — the same inverse relationship applies to meshed gears, though gear ratios are more commonly expressed using tooth count instead of diameter. You can substitute tooth counts directly in place of diameters since they're proportional for gears of the same pitch.
Why does a smaller driven pulley spin faster?
The belt moves at the same linear speed around both pulleys. A smaller pulley has less circumference, so it needs more rotations per minute to keep up with the same belt speed — resulting in a higher RPM.
What's a typical use case for this calculation?
This is commonly used for sizing motor-to-spindle drives on machine tools, designing conveyor systems, calculating alternator or compressor pulley speeds in automotive applications, and general mechanical power transmission design.