Free tool
Pulley Speed Calculator
Calculate the theoretical driven-pulley RPM, speed ratio and belt linear speed for a two-pulley belt drive, with the no-slip formulas and limits shown.
- Free
- No signup
- Formula shown
- updated 2026-09
Calculate
Enter your values
Results update as you type. Everything is computed in the browser.
Use the pitch or datum diameter for the belt system, not an unrelated outside diameter.
Use measured shaft RPM where possible.
Use the same diameter convention as the driver pulley.
- Diameter ratio (driven:driver)
- 3
- Speed ratio (driven:driver)
- 0.3333
- Driven pulley speed (rpm)
- 400
- Belt linear speed (m/s)
- 6.2832
- Belt linear speed (ft/min)
- 1,236.8
Computed in your browser. Inputs are not sent to a server.
Method
The formula, and every assumption behind it
A number you cannot check is not usable in engineering or finance work. This is the whole calculation.
N₂ = N₁ × D₁ / D₂; v = π × D₁ × N₁ / 60D₁ is converted from millimetres to metres for belt speed. The diameter relationship is theoretical and assumes no belt slip. Use consistent pitch, datum or effective diameters for both pulleys.
Variables
| Symbol | Meaning | Unit |
|---|---|---|
| D₁ | Driver pulley effective diameter | mm |
| D₂ | Driven pulley effective diameter | mm |
| N₁ | Driver pulley rotational speed | rpm |
| N₂ | Theoretical driven pulley rotational speed | rpm |
| v | Theoretical belt linear speed | m/s |
Assumptions and limits
- The calculation covers one driver pulley and one driven pulley connected by a belt.
- The speed relationship assumes no belt slip. A friction belt drive can run slower at the driven pulley in practice.
- Both diameters use the same pitch, datum or effective-diameter convention; mixing outside and pitch diameters distorts the ratio.
- The tool does not size the belt, pulley, shaft, torque, power, wrap angle or tension.
How to use it
Five steps
- 01
Identify which pulley is the driver and measure or obtain its effective diameter.
- 02
Enter the driver pulley RPM, preferably from a measured operating speed.
- 03
Enter the driven pulley effective diameter using the same diameter convention.
- 04
Read the theoretical driven RPM and check whether the drive speeds up or slows down.
- 05
Use the belt-speed result only after checking the belt and pulley manufacturer limits for the application.
Worked example
Worked example — 100 mm driver at 1200 rpm, 300 mm driven pulley
The example shows the inputs and main steps so you can check the result.
Inputs
- Driver effective diameter: 100 mm
- Driver speed: 1200 rpm
- Driven effective diameter: 300 mm
| Step | Value |
|---|---|
| Diameter ratio | 300 / 100 = 3.000 driven:driver |
| Speed ratio | 100 / 300 = 0.3333 driven:driver |
| Driven pulley speed | 1200 × 100 / 300 = 400 rpm |
| Driver circumference | π × 0.100 = 0.3142 m |
| Belt linear speed | 0.3142 × 1200 / 60 = 6.283 m/s |
| Belt linear speed | 6.283 × 196.8504 = 1236.8 ft/min |
Units
Useful unit conversions
Shown so you can reconcile the output with the units in your own data.
| Metric | Equivalent |
|---|---|
| 1 m/s | 196.85 ft/min |
| 1 mm | 0.03937 in |
| 1 rpm | 1 rpm (no conversion) |
Questions
Common questions
- How do pulley diameters change output speed?
- Driven speed is inversely proportional to driven diameter. With no slip, doubling the driven-pulley diameter halves its RPM when driver diameter and driver RPM stay the same.
- Should I use outside diameter or pitch diameter?
- Use the diameter at the belt running line specified for the belt system. For V-belts this is commonly a datum or pitch diameter. Use the same convention for both pulleys.
- Does the calculator include belt slip?
- No. It reports the theoretical no-slip ratio. Friction belt drives can have slip, so measured driven RPM can be lower. Timing belts are positive drives but still require the correct pitch diameter or tooth count.
- Can this calculator select a belt or pulley?
- No. Selection also depends on power, torque, service factor, pulley limits, wrap angle, centre distance, belt profile and tension. Use the drive manufacturer’s design method for component selection.
- Why is belt speed the same at both pulleys?
- In the no-slip model, the connected belt has one linear speed. The pulleys rotate at different RPM so their circumferential speeds match that belt speed.
Understand the method
Check the calculation before you use the result
The guide explains which diameter and RPM belong in the formula, how a gearbox changes the input and when field measurement matters.
Keep exploring
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Belt Speed Calculator
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