Pulley Ratio Calculator
Enter the driver and driven pitch diameters and input speed to estimate the driven pulley speed and belt speed.
Ratio:
Output speed:
Belt speed:
Assumes no belt slip. Use pitch diameter rather than outside diameter when available. Check torque changes and shaft, key, and coupling capacity before operation.
A QUICK WALKTHROUGH
How to use this tool
- Enter positive driver and driven pitch diameters in mm.
- Enter the input speed in RPM.
- Calculate the ratio, output RPM, and belt speed.
Formulas
Ratio = driven pitch diameter ÷ driver pitch diameter. Output RPM = input RPM ÷ ratio. Belt speed = π × driver pitch diameter (mm) × input RPM ÷ 60,000.
Use pitch diameter
Use the belt pitch diameter, rather than outside diameter, for toothed or V-belt pulleys. Results assume no belt slip.
Torque and mechanical limits
As speed changes, torque changes inversely in an ideal transmission. Check shafts, keys, couplings, belt capacity, and guards because the changed torque or load may overload them.
GOOD TO KNOW
Common questions
What does a 2:1 pulley ratio mean?
The driven pulley has twice the pitch diameter of the driver, so its ideal speed is half the input speed.
Should I use outside diameter?
Use pitch diameter when available. Outside diameter can give inaccurate results, especially for toothed or V-belt pulleys.
Does this account for belt slip?
No. The calculation assumes no slip; real output speed can be lower.