Belt-drive fan service

Fan Pulley RPM Calculator

Calculate driven fan speed from motor RPM and the measured pitch diameters of an open belt drive.

Created · Updated

Check fan RPM

Use pitch diameters from the sheave specification or measurement method—not outside diameter unless the manufacturer equates them.

Enter values and calculate.

Calculated fan speedRPM
Driven-to-driver ratio: 1
Approximate belt speedft/min (in) or m/s (mm)

How to use this calculator

  1. Lock out the equipment and identify motor nameplate speed.
  2. Enter motor and fan sheave pitch diameters in the same unit.
  3. Compare calculated fan RPM with the fan catalog and measured speed before any adjustment.

Formula

Fan RPM = Motor RPM × Motor sheave pitch diameter ÷ Fan sheave pitch diameter
Belt speed (ft/min) = π × driver diameter (in) × motor RPM ÷ 12

Variables

VariableMeaningUse limit
Motor RPMDriver shaft speedUse nameplate or measured loaded speed
Motor sheaveDriver pitch diameterNot automatically the outside diameter
Fan sheaveDriven pitch diameterUse the same diameter convention

Worked example

A 1,750 RPM motor with a 4-inch driver and 8-inch fan sheave produces 875 fan RPM before slip: 1,750 × 4 ÷ 8.

Result interpretation

The result is a kinematic speed check, not proof of acceptable airflow or motor loading. Actual RPM may differ with belt slip, wear, adjustable-sheave position, and measurement error. Confirm maximum allowable fan RPM and measure motor current after qualified adjustment.

Frequently asked questions

Which sheave diameter should I enter?

Use pitch diameter or the manufacturer’s effective diameter for the belt section. Outside diameter can produce a different result.

Does the calculator include belt slip?

No. It reports the theoretical speed ratio. Measure actual shaft speed when the difference matters.

Can the result prove the fan is moving the required CFM?

No. RPM alone does not establish airflow; use fan data and a measured operating point.

Related resources

Sources