How to use the wheel RPM and speed calculator
- Choose RPM → speed to find how fast your robot will go, or Speed → RPM to find which motor to buy for a target speed.
- Pick a wheel preset or type the wheel diameter in mm, cm or inches. Measure across the tyre, not the hub. The presets (65 mm BO wheel, 43 mm N20 wheel) are typical sizes — wheels vary between sellers.
- Enter the RPM of the wheel shaft (for a gear motor, the RPM printed on it is already the gearbox output) or the target speed.
- Set speed under load. Motor RPM ratings are usually measured with no load; on the floor, with the robot's weight and friction, a robot often runs at about 70–90 % of that. Leave 100 % for the pure maths.
- Optionally enter a distance to see how long the trip takes and how many times the wheel turns.
Formula
In one turn, a wheel rolls a distance equal to its circumference:
Distance per revolution C = π × d
Linear speed v (m/s) = π × d (m) × RPM ÷ 60
Required RPM = v × 60 ÷ (π × d)
Speed in km/h = v (m/s) × 3.6
Time to travel = distance ÷ v
Wheel turns = distance ÷ (π × d)
Speed table for common robot wheels
| Wheel | Distance per turn | 100 RPM | 150 RPM | 200 RPM | 300 RPM |
|---|---|---|---|---|---|
| 43 mm (N20 wheel) | 135 mm | 0.23 m/s | 0.34 m/s | 0.45 m/s | 0.68 m/s |
| 65 mm (BO wheel) | 204 mm | 0.34 m/s | 0.51 m/s | 0.68 m/s | 1.02 m/s |
| 100 mm | 314 mm | 0.52 m/s | 0.79 m/s | 1.05 m/s | 1.57 m/s |
Speeds are at the rated RPM with no slip; real robots are slower under load.
Common mistakes
- Using the radius instead of the diameter. The circumference is π × d (or 2 × π × r). Mixing them up halves or doubles the answer.
- Using the motor-shaft RPM. A bare DC motor spins at thousands of RPM; the wheel turns at the gearbox output speed. Use the gear ratio calculator to work it out.
- Forgetting to divide by 60. RPM is per minute; speed in m/s needs revolutions per second.
- Trusting no-load RPM. Weight, friction and a falling battery voltage all slow the robot. Measure: time the robot over 2 m and compare with the calculator.
- Ignoring wheel slip. On smooth tiles or dusty floors, wheels slip, especially when starting hard. Encoders on the wheel shaft count turns, not distance travelled.
Related tools and guides
Control the speed with the PWM duty cycle calculator, plan the battery with the battery life calculator, and check the stopping distance for your sensor with the ultrasonic distance calculator. Build the robot step by step with our L298N robot car guide, the line-following robot guide and the HC-SR04 obstacle-avoiding robot guide. Need motors and wheels? See our robotics kits.
