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.

What is the Motor KV & Propeller Combination Calculator?

Enter motor KV, cell count in series, and propeller diameter and pitch to compute theoretical unloaded RPM, propeller tip speed, and an estimated pitch speed. For RC plane and multirotor builders selecting motors and props. It runs free in your browser on Gera Tools, with nothing uploaded.

Motor KV & Propeller Combination Calculator

Name: Motor KV & Propeller Combination Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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 sensible before you spin it up.

What KV actually means

KV (sometimes written Kv) is the motor’s RPM-per-volt constant measured at no load. A 2400 KV motor spins at 2400 RPM for every volt applied when the shaft is unloaded. It is a motor characteristic, not a measure of kilovolts. A high KV motor (1800–3500+) is designed for small, fast-spinning props. A low KV motor (900–1400) is designed for large, slower props with higher thrust per watt. The KV × voltage equation gives the theoretical upper limit; a loaded prop always pulls it down.

How it works

KV and pack voltage set the theoretical RPM; the prop dimensions give the speed figures:

voltage         = cells × 3.7 V (nominal LiPo)
RPM (no load)   = KV × voltage
tip speed       = π × diameter_m × (RPM / 60)   (m/s)
pitch speed     = pitch_m × (RPM / 60)           (m/s, no propeller slip)

Real loaded RPM is typically 70–85% of the theoretical figure because:

The propeller imposes aerodynamic torque, drawing current and loading the motor.
Under load, the battery voltage sags below nominal.
The motor’s internal resistance causes additional voltage drop.

Worked examples

FPV 5-inch freestyle quad — 2400 KV on 4S:

Pack voltage: 4 × 3.7 = 14.8 V
Theoretical RPM: 2400 × 14.8 = 35,520 RPM
On a 5×4.3 prop: tip speed ≈ π × 0.127 m × 592 r/s ≈ 236 m/s
Pitch speed (no slip): 0.109 m × 592 r/s ≈ 65 m/s (~234 km/h)

Fixed-wing sport plane — 900 KV on 3S:

Pack voltage: 3 × 3.7 = 11.1 V
Theoretical RPM: 900 × 11.1 = 9,990 RPM
On a 10×6 prop: tip speed ≈ π × 0.254 m × 166 r/s ≈ 133 m/s
Pitch speed: 0.152 m × 166 r/s ≈ 25 m/s (~90 km/h)

Tip speed and why it matters

As tip speed approaches the speed of sound (~340 m/s), propeller efficiency drops sharply and noise increases dramatically. For most multirotors, keeping tip speed below ~200–250 m/s is a practical efficiency target. Racing quads often push toward the upper end in exchange for top speed; efficiency-focused builds stay lower.

The KV-to-prop matching rule

The fundamental guideline:

High KV (1800 KV+): pair with small props (3–5 inch diameter, moderate pitch). Fast-spun small blades suit multirotors and racing applications.
Medium KV (1000–1800 KV): general-purpose range for sport aircraft and larger quads.
Low KV (below 1000 KV): pair with large, slow props (8 inch+). Efficient for aerial photography, long-range planes, and large multirotors where hover endurance matters.

Oversizing the prop on a high-KV motor draws far more current than the motor or ESC can handle, leading to overheating and failure. Always verify that the expected current draw (from actual measurements or manufacturer test data) is within the rated continuous current of both the motor and ESC.