Sheave Ratio Calculator
Driven RPM from motor and sheave sizes.
Enter your values
Enter the motor rpm in rpm.
Enter the motor sheave diameter in in.
Enter the driven sheave diameter in in.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Driven RPM
- 972 rpm
- Sheave ratio
- 1.8:1
- Speed reduction
- 44.4%
Quick answer
Driven RPM equals motor RPM multiplied by the ratio of motor sheave diameter to driven sheave diameter.
RPM_driven = RPM_motor x D_motor / D_driven
At a glance
| What it does | Driven RPM from motor and sheave sizes. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Motor RPM, Motor sheave diameter, Driven sheave diameter |
| Main output | Driven RPM |
| Formula | RPM_driven = RPM_motor x D_motor / D_driven |
| Cost | Free — no sign-up, no download |
How to use the Sheave Ratio Calculator
- 1Enter the motor RPM and the diameters of the motor and driven sheaves.
- 2Read the resulting driven shaft (fan) RPM.
- 3Check against the manufacturer's fan curve for the desired operating point.
Inputs explained
- Motor RPM(rpm)
- Enter the motor rpm in rpm.
- Motor sheave diameter(in)
- Enter the motor sheave diameter in in.
- Driven sheave diameter(in)
- Enter the driven sheave diameter in in.
Worked example
Using the values the calculator loads with:
Inputs
- Motor RPM1750 rpm
- Motor sheave diameter5 in
- Driven sheave diameter9 in
Results
- Driven RPM972 rpm
- Sheave ratio1.8:1
- Speed reduction44.4%
Frequently asked questions
Which sheave is 'motor' vs 'driven'?
The motor sheave is on the motor shaft; the driven sheave is on the fan shaft.
Larger driven sheave means faster or slower fan?
A larger driven sheave relative to the motor sheave slows the fan down.
What do I need to enter into the Sheave Ratio Calculator?
Just 3 values: motor rpm, motor sheave diameter, driven sheave diameter. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Sheave Ratio Calculator work out the answer?
Driven RPM equals motor RPM multiplied by the ratio of motor sheave diameter to driven sheave diameter. It applies the formula RPM_driven = RPM_motor x D_motor / D_driven and shows the working so you can check each step by hand.
Is the Sheave Ratio Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.