RC Motor KV to RPM Calculator
Calculate RC motor RPM from KV rating and battery voltage, plus prop recommendations for planes, cars, and drones.
The KV rating of a brushless motor is the most important specification for matching motors to batteries and propellers in RC vehicles. KV stands for “RPM per Volt” (the K is from the SI prefix kilo, and V is voltage).
RPM Formula
RPM (no load) = KV × Battery Voltage
Under load (with a propeller or wheels), actual RPM is typically 70–85% of the no-load value due to back-EMF and mechanical resistance.
RPM (loaded) = KV × Voltage × Load Factor
| Application | Load Factor |
|---|---|
| RC Plane (efficient prop) | 0.80–0.85 |
| RC Plane (high pitch prop) | 0.70–0.80 |
| Multirotor/Drone | 0.75–0.85 |
| RC Car (on road) | 0.85–0.90 |
| RC Car (off road) | 0.75–0.85 |
LiPo Battery Voltage Reference
| Cell Count | Nominal Voltage | Full Charge |
|---|---|---|
| 2S | 7.4V | 8.4V |
| 3S | 11.1V | 12.6V |
| 4S | 14.8V | 16.8V |
| 6S | 22.2V | 25.2V |
KV Ranges by Application
| Application | Typical KV | Why |
|---|---|---|
| Large plane (slow flyer) | 400–900 | Large props, high torque |
| Sport plane | 900–1400 | Medium props, good speed |
| Racing plane/jet | 1400–2500 | Small props, very fast |
| FPV racing drone | 1700–2600 | 5" props, high thrust |
| Photography drone | 300–900 | Large props, stability |
| RC car (street) | 3000–5000+ | Small pinion, high RPM |
| RC car (crawler) | 1200–2000 | Low speed, high torque |
Worked Example
A 1400 KV motor on a 3S LiPo (11.1V nominal):
- No-load RPM = 1400 × 11.1 = 15,540 RPM
- Loaded RPM (sport plane) = 1400 × 11.1 × 0.82 = 12,743 RPM
- At full charge (12.6V): 1400 × 12.6 × 0.82 = 14,467 RPM
Prop Speed (tip speed)
Propeller tip speed should stay below Mach 0.7 (~540 mph / 870 km/h) to avoid efficiency loss and excessive noise:
Tip Speed (mph) = π × Prop Diameter (in) × RPM / (12 × 5280 / 60)
For a 10" prop at 12,743 RPM: Tip Speed = π × 10 × 12,743 / 1056 = 379 mph (safe, well under Mach 0.7)