# RPM Calculator

Calculate RPM from speed and wheel diameter, or find speed from RPM. Works for tires, gears, pulleys, and machining spindles.

## What this calculates

Need to know how fast a wheel, gear, or spindle is turning? Enter any two of RPM, speed, and diameter to calculate the third. Works for vehicle tires, CNC spindles, pulleys, conveyor rollers, and anything else that spins.

## Inputs

- **Solve For** — options: RPM (from speed & diameter), Speed (from RPM & diameter), Diameter (from RPM & speed)
- **Speed** — min 0
- **Speed Unit** — options: mph, km/h, ft/min (surface speed), m/min (surface speed)
- **Diameter** — min 0.1 — Wheel, tire, gear, or pulley diameter
- **Diameter Unit** — options: Inches, Millimeters, Feet
- **RPM** — min 0 — Used when solving for speed or diameter

## Outputs

- **Result** — formatted as text
- **Circumference** — formatted as text — Distance traveled per revolution
- **Surface Speed** — formatted as text
- **Surface Speed (Metric)** — formatted as text

## Details

The core formula:

RPM = Speed / Circumference, where Circumference = PI x Diameter

Once you know two of the three values (RPM, speed, diameter), you can solve for the third.

Common applications:

  - Vehicle tires -- find engine RPM at highway speed for a given tire size

  - Machining -- calculate spindle RPM for a cutter diameter and recommended surface speed

  - Bicycles -- determine wheel RPM from riding speed and wheel size

  - Conveyor systems -- calculate roller RPM for a target belt speed

  - Pulleys and gears -- find output RPM from input RPM and diameter ratio

Surface speed (shown in ft/min and m/min) is critical in machining. Material data sheets specify recommended surface speeds for cutting tools, and you convert that to RPM based on your tool diameter.

## Frequently Asked Questions

**Q: How do I calculate RPM for a car tire?**

A: Enter your vehicle speed (e.g. 60 mph), select the tire diameter (a 225/45R17 tire is about 25 inches in diameter), and solve for RPM. At 60 mph with a 25-inch tire, you get roughly 808 RPM at the wheel. Your engine RPM depends on the gear ratio.

**Q: What is surface speed and why does it matter for machining?**

A: Surface speed (also called SFM or surface feet per minute) is how fast the outer edge of a rotating tool moves. Cutting tool manufacturers recommend specific surface speeds for each material. Too slow wastes time, too fast burns the tool. You convert surface speed to spindle RPM based on the tool diameter.

**Q: How does gear ratio affect RPM?**

A: When two gears mesh, the RPM ratio is inversely proportional to the diameter (or tooth count) ratio. A small gear driving a large gear reduces RPM but increases torque. Multiply the input RPM by the ratio of input diameter to output diameter to find the output RPM.

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Source: https://vastcalc.com/calculators/everyday/rpm
Category: Everyday Life
Last updated: 2026-04-08