KV to RPM Calculator

How this calculator works

Unloaded RPM = KV × voltage · voltage = cells × 3.7 V nominal

An 1800KV motor on 6S spins 1800 × 22.2 ≈ 40,000 RPM with no prop. Bolt a prop on and drag pulls it down to roughly 70-80% of that - the calculator shows both, plus the prop's tip speed for the diameter you enter. Full theory: KV explained.

Why you'd want this number

Matching KV to cells. The classic pairings - 6S/1700-1950KV, 4S/2400-2750KV for 5" - exist because they land on the same ~30k loaded RPM. Enter any combination and check whether it lands in a sane range instead of trusting folklore.

Prop stress. Prop makers rate their props for RPM ceilings. A 2550KV motor on 6S asks a 5" prop to survive ~42k unloaded RPM - beyond many props' comfort zone; that's a 4S motor by design.

Tip speed and noise. Past ~0.6 Mach at the tip (~740 km/h) props get loud and inefficient. Long, low-pitch props at low KV stay quiet - one reason cinematic builds run low-KV motors (prop pitch basics).

The nominal-voltage caveat

3.7 V/cell is the resting average. Fresh off the charger a 6S is 25.2 V (+13% RPM); sagging under load it drops well below nominal. Real flying RPM lives inside that spread, which the manufacturer bench tables on our motor pages capture precisely - they log actual RPM under load per throttle step (how to read them).

Motor pages embed this calculator pre-filled with the motor's own KV and its class prop diameter, so the numbers you see are that motor's numbers.