RPM Calculator

Find a driven pulley's or gear's RPM from its diameter and the driver's speed, using the standard no-slip belt-drive formula D1 × RPM1 = D2 × RPM2. Also returns the pulley ratio and belt (surface) speed.

Quick Facts

Pulley law
D1 × RPM1 = D2 × RPM2
In a no-slip belt drive, a larger driven pulley always turns slower than a smaller driver pulley.
Belt speed
Equal on both pulleys
Surface speed = π × diameter × RPM ÷ 12, in feet per minute.

Your Results

Calculated
Driven pulley RPM
-
Output speed (RPM2)
Pulley ratio
-
Driven : driver diameter
Belt speed
-
Surface speed, feet per minute
Drive type
-
Reducer, increaser, or direct

Ready

Enter both pulley diameters and the driver speed, then press Calculate.

About the RPM Calculator

This calculator finds the rotational speed of a driven pulley, gear, or wheel from the speed of its driver and the two diameters involved. It uses the standard belt-and-pulley (or meshed-gear) relationship taught in mechanical engineering and shop-math courses: for an ideal drive with no slip, the product of diameter and RPM is the same on both sides.

The formula

The governing relationship is:

D1 × RPM1 = D2 × RPM2

where D1 and RPM1 are the driver pulley's diameter and speed, and D2 and RPM2 are the driven pulley's diameter and speed. Solving for the unknown driven speed gives:

RPM2 = RPM1 × D1 ÷ D2

This says a bigger driven pulley turns proportionally slower than the driver, and a smaller driven pulley turns proportionally faster — the same inverse relationship that makes a small chainring on a bicycle spin the rear wheel faster than a large one at the same pedal cadence.

Belt (surface) speed

The calculator also reports belt speed — how fast the belt itself travels along the pulleys' rims. Because the belt is a single continuous loop, its surface speed is identical at both pulleys in an ideal drive:

Belt speed (ft/min) = π × D1 × RPM1 ÷ 12

using diameter in inches and dividing by 12 to convert inches per minute to feet per minute. This value matters for belt life and machine design — running a belt faster than its rated surface speed accelerates wear.

How to get the best results

  • Measure diameters at the same point — either both at the pulley's outer edge or both at the belt's pitch line — so the ratio is consistent.
  • Use the driver pulley's actual running speed (from a tachometer or the motor's nameplate RPM), not its rated no-load speed if the motor is under load.
  • Keep diameter and speed units consistent; this tool expects diameters in inches and speed in RPM.

Practical context

The formula assumes an ideal, no-slip belt or a direct gear mesh. Real flat and V-belts can slip roughly 1-3% under heavy load, so measured output speed may run slightly below the calculated value. For toothed timing belts and gear trains, slip is effectively zero and the formula is very accurate.

Frequently Asked Questions

What is the pulley RPM formula?
For a belt-and-pulley drive with no slip, the driver and driven pulleys obey D1 × RPM1 = D2 × RPM2, where D is pulley diameter and RPM is rotational speed. Solving for the driven pulley's speed gives RPM2 = RPM1 × D1 ÷ D2. A larger driven pulley always turns slower than the driver, and a smaller one turns faster.
How do I calculate belt speed?
Belt (surface) speed is the same at both pulleys in an ideal drive and equals π × diameter × RPM, converted to feet per minute by dividing by 12 when diameter is in inches: belt speed (ft/min) = π × D1 × RPM1 ÷ 12. This is also called the pulley's surface speed or peripheral speed.
Does this account for belt slip or efficiency losses?
No. The calculator assumes an ideal, no-slip belt drive, which is the standard textbook pulley relationship. Real belts can slip 1-3% under load, and toothed/timing belts or direct gear meshes behave closer to the ideal case than flat or V-belts.

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