RPM Calculator
What the RPM calculator does
The RPM calculator converts a linear speed into revolutions per minute. Enter the distance covered in one revolution in metres and the speed in metres per minute, press Calculate, and the tool returns how many turns per minute the wheel, shaft, pulley or drum must make. It is the basic link between how fast something moves and how fast it spins.
The formula
RPM = speed (m/min) ÷ distance per revolution (m)
The distance per revolution is the circumference: π × diameter, or 2π × radius. For a wheel of 0.65 m diameter the circumference is 2.042 m, so at 1,500 m/min (90 km/h) the wheel turns 1,500 ÷ 2.042 = 735 RPM.
Converting speed into metres per minute
| Given | Multiply by | Example |
|---|---|---|
| km/h | 16.667 | 60 km/h = 1,000 m/min |
| mph | 26.822 | 30 mph = 805 m/min |
| m/s | 60 | 5 m/s = 300 m/min |
| ft/min | 0.3048 | 1,000 ft/min = 305 m/min |
Worked examples
Car wheel. A 205/55 R16 tyre has an overall diameter of about 0.632 m, so one revolution is 1.985 m. At 100 km/h (1,667 m/min) the wheel spins at 1,667 ÷ 1.985 = 840 RPM. The tire diameter to RPM calculator starts from the tyre size code directly.
Bicycle. A 700c road wheel is 2.1 m around. At 30 km/h (500 m/min) it turns at 238 RPM. With a 50-tooth chainring and 14-tooth sprocket the gear ratio is 3.57, so the rider’s cadence is 238 ÷ 3.57 = 67 RPM.
Conveyor drum. A belt runs at 1.2 m/s (72 m/min) over a 0.4 m diameter drum (circumference 1.257 m). Drum speed = 72 ÷ 1.257 = 57 RPM.
Lathe surface speed. Cutting mild steel with a carbide tool at 150 m/min surface speed on a 50 mm bar (circumference 0.157 m): spindle speed = 150 ÷ 0.157 = 955 RPM.
From wheel RPM to engine RPM
In a vehicle the engine turns faster than the wheels by the product of the gearbox ratio and the final drive ratio. Wheel speed of 840 RPM in a gear with an overall ratio of 2.8 means the engine is at 840 × 2.8 = 2,352 RPM. The speed vs RPM calculator and gear ratio calculator handle that chain, and the RPM drop after gear shift calculator shows what happens at each upshift.
Angular speed in other units
RPM converts to radians per second by multiplying by 2π/60 = 0.1047, and to hertz (revolutions per second) by dividing by 60. 955 RPM is 100 rad/s or 15.9 Hz. Engineers use rad/s in power calculations: power (W) = torque (N·m) × angular speed (rad/s). The torque to horsepower at RPM calculator applies that relation.
Common mistakes
- Entering diameter instead of circumference. The first field wants the distance travelled per turn, which is π times the diameter.
- Mixing units. Circumference in millimetres with speed in metres per minute gives an answer 1,000 times too small.
- Using speed in km/h directly. Convert to m/min first (multiply by 16.667).
- Ignoring tyre deflection. A loaded tyre’s rolling circumference is 2 to 3% less than its unloaded geometry suggests.
Related tools
The tire size calculator gives the diameter for any tyre code. The engine displacement and compression ratio calculators cover engine geometry, and the speed calculator finds the linear speed you need as input here.
External references
Frequently asked questions
How do I find the distance per revolution?
Measure the diameter and multiply by π (3.1416). A 0.5 m wheel travels 1.571 m per turn. For tyres, the sidewall code gives the diameter via the tire size calculator.
Can I use this for a motor driving a pulley?
Yes. Use the belt speed and the pulley’s circumference. The motor shaft RPM equals the pulley RPM if they are directly coupled.
What is the relationship between RPM and frequency?
Frequency in hertz is RPM ÷ 60. A 3,000 RPM motor turns at 50 Hz, which is why two-pole induction motors on 50 Hz mains run just under 3,000 RPM.
How do I go from RPM back to speed?
Multiply RPM by the circumference to get metres per minute, then divide by 16.667 for km/h.
Why is my calculated wheel RPM different from the vehicle’s sensor?
Sensors read the hub, which matches the wheel, but tyre wear, pressure and load change the rolling circumference by a few percent.