How is RPM calculated from KV?

KV is RPM per volt with no load. Theoretical RPM equals KV times pack voltage, where voltage is the cell count times the nominal 3.7 volts per cell. A 2400 KV motor on a 4S pack (14.8 V) spins about 35,500 RPM unloaded.

Why is real RPM lower than this number?

KV-based RPM is the no-load figure. A propeller loads the motor and pulls voltage down under current, so actual in-flight RPM is typically 70 to 85 percent of the theoretical value. Use this number for comparison, not as an exact spec.

What is propeller tip speed and why does it matter?

Tip speed is how fast the blade tips travel and equals pi times diameter times RPM. Very high tip speeds approach the speed of sound, becoming inefficient and noisy. Keeping tip speed reasonable is a key constraint when pairing high KV with large props.

What does pitch speed estimate?

Pitch speed is the theoretical forward speed if the prop screwed through air with no slip, equal to pitch times RPM. Real airspeed is lower due to slip, but pitch speed gives a useful upper bound for plane top speed.

How do I avoid burning out a motor with the wrong prop?

Too much prop (large diameter or pitch) on a high-KV motor draws excessive current and overheats the motor and ESC. Lower KV motors swing bigger props efficiently; high KV pairs with small props. Always check current draw against your ESC and battery limits.

Motor KV & Propeller Combination Calculator

Choosing a motor and propeller together is the heart of any RC power system. This calculator turns a motor’s KV, your pack voltage, and the prop’s diameter and pitch into the numbers that matter — theoretical RPM, blade tip speed, and a pitch-speed estimate — so you can judge whether a combination is sane before you spin it up.

How it works

KV is RPM per volt, so unloaded RPM scales with pack voltage, and the prop geometry gives the speed figures:

voltage     = cells × 3.7 V (nominal)
RPM (no load) = KV × voltage
tip speed   = π × diameter(in→m) × RPM / 60   (m/s)
pitch speed = pitch(in→m) × RPM / 60          (m/s, no slip)

Real loaded RPM runs roughly 70–85% of the theoretical figure because the prop draws current and sags the voltage.

Example and tips

A 2400 KV motor on 4S (14.8 V) turns about 2400 × 14.8 = 35,520 RPM unloaded. On a 5×4.3 prop that is a tip speed near 240 m/s and a pitch speed around 130 m/s (no slip) — typical for a fast 5-inch FPV quad. Pair high KV with small props and low KV with large props: oversizing the prop on a fast motor is the classic way to cook a motor or ESC, so always cross-check current draw against your component limits.