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Pulley RPM Calculator

Enter driver speed and both pulley pitch diameters. An optional speed-loss percentage represents overall slip relative to the ideal drive.

Local calculation · inputs stay in your browser

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Results

    An input link contains the values you entered. Share only information you want to disclose.

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    Formula and method

    n₂ideal = n₁D₁/D₂; n₂ = n₂ideal × (1 − s/100); i = D₂/D₁; v = πD₁n₁/60000. D is in mm, n in rpm and v in m/s.

    Worked example

    1450 rpm, 100 mm driver, 200 mm driven and 2% speed loss: ideal 725 rpm, estimated 710.5 rpm, theoretical belt speed ≈ 7.592 m/s.

    Assumptions and limits

    • Use manufacturer pitch or effective diameters, rather than outside diameters. Timing belts may use tooth-count ratios for ideal speed; normally use 0% slip when correctly engaged.
    • Slip is the overall speed loss supplied by you and must be below 100%. Theoretical belt speed is the driver’s no-slip pitch-line speed; actual belt speed with slip is not determined.
    • This tool does not check belt capacity, wrap angle, tension, bearing load or maximum pulley speed. Use the manufacturer’s design manual for selection.

    Frequently asked questions

    What happens if the driver pulley gets larger?

    At the same driver RPM and driven pitch diameter, increasing driver pitch diameter raises ideal driven RPM and theoretical belt speed in proportion.

    What does a ratio below 1 mean?

    Here i = driver RPM / ideal driven RPM. A ratio below 1 speeds up the driven shaft; a ratio above 1 reduces its speed.

    References

    Pulley RPM Calculator practical guide and common questions

    What does Pulley RPM Calculator calculate?

    Enter driver speed and both pulley pitch diameters. An optional speed-loss percentage represents overall slip relative to the ideal drive. n₂ideal = n₁D₁/D₂; n₂ = n₂ideal × (1 − s/100); i = D₂/D₁; v = πD₁n₁/60000. D is in mm, n in rpm and v in m/s.

    When is this useful?

    1450 rpm, 100 mm driver, 200 mm driven and 2% speed loss: ideal 725 rpm, estimated 710.5 rpm, theoretical belt speed ≈ 7.592 m/s.

    How do I enter the inputs?

    Enter the inputs for Pulley RPM Calculator: Driver pulley speed, Driver pitch diameter, Driven pitch diameter, Overall speed loss (optional; blank = 0). Calculate, check the units and assumptions, then copy or download the results.

    How should I interpret the results?

    Use manufacturer pitch or effective diameters, rather than outside diameters. Timing belts may use tooth-count ratios for ideal speed; normally use 0% slip when correctly engaged. Slip is the overall speed loss supplied by you and must be below 100%. Theoretical belt speed is the driver’s no-slip pitch-line speed; actual belt speed with slip is not determined. This tool does not check belt capacity, wrap angle, tension, bearing load or maximum pulley speed. Use the manufacturer’s design manual for selection.

    Where are my inputs processed?

    Pulley RPM Calculator runs in this browser. Inputs are not uploaded to a calculation server; copying and CSV download require your action. The existing site analytics and advertising still load.