Brushless Motor KV, RPM & Voltage Calculator

How it works

No-load rpm = KV × pack voltage (cells × volts per cell); loaded rpm ≈ no-load × (1 − load drop 10–20%); KV needed = target rpm ÷ (1 − drop) ÷ volts; higher KV ⇒ lower torque constant (Kt ∝ 1/KV)

A brushless motor’s KV is its speed constant: how many rpm it spins per volt with no load. Multiply by the pack voltage — which itself swings from 4.2 to about 3.5 volts per cell across a flight — and you have the unloaded speed; a propeller drags that down 10–20%. KV is also the inverse of torque per amp, so a low-KV motor turns big props slowly and efficiently while a high-KV motor spins small props fast, and the same flying power can come from high KV on few cells or low KV on many: 2,400 KV on 4S and 1,600 KV on 6S give similar rpm, but the 6S setup draws fewer amps for the same watts, which is why the hobby has drifted to higher voltages. Check that no-load rpm stays under the motor’s and prop’s limits, since a prop that unloads in a dive lets the motor run up to it.

Worked example

A 2,400 KV motor on a nominal 4S pack (14.8 V) turns 35,520 rpm unloaded and about 31,260 rpm with a 5″ prop — 40,320 unloaded on a full 16.8 V pack, which is over a 35,000 rpm limit and worth noting. For 28,000 loaded rpm on 4S you would choose about 2,150 KV, or run the 2,400 KV motor on 3.6S — i.e. 3S if you accept lower rpm.

Frequently asked questions