KV to RPM Calculator

What the KV number on a motor label means

KV is not a power rating and it is not a quality score. It is a slope: the RPM the motor turns per volt when nothing is attached to the shaft. Hobby brushless motors are labeled this way, while industrial motors follow NEMA motor standards and quote nameplate horsepower instead. A 1900 KV motor on 1 volt settles near 1900 RPM. The same motor on 22.2 volts settles near 42,180 RPM.

The number comes from the winding. A few turns of thick wire give a high KV. Many turns of thin wire give a low KV.

Two motors of the same size can be sold as 900 KV and 2400 KV because they suit different props: the low KV motor swings a large prop on a high cell count, and the high KV motor swings a small prop or drives a gearbox.

Because KV is measured with no load, treat it as a ceiling. The moment a prop bites the air, the motor slows down. That gap is why this calculator asks for a load figure instead of treating KV times volts as the answer.

The four lines behind every answer

Every output on this page comes from four short steps, run in order.

1. No-load RPM = KV times pack volts.
2. Loaded RPM = no-load RPM times the load factor.
3. Output shaft RPM = loaded RPM divided by the gear ratio.
4. ESC switching frequency = loaded RPM divided by 60, times pole pairs.

The load factor is the share of free speed the motor keeps under load, written as a decimal. A motor that holds 85 percent of its no-load speed at full throttle uses 0.85. This is a speed ratio, not the electrical efficiency on a datasheet, which is a different and usually larger number.

Kt is a separate conversion. Kt = 9.5493 divided by KV. The constant 9.5493 is 60 divided by 2 pi, the radian conversion described in the NIST guide to the International System of Units, and it is what makes Kt come out in newton meters per amp.

Pole pairs are half the pole count. A 14 pole motor has 7 pairs. The ESC cares about pairs, because each pair passing a stator tooth is one electrical cycle.

Units are where most wrong answers come from: pack volts, not cell volts, and poles, not pairs.

Why a prop never sees KV times volts

A propeller is a brake that gets stronger as it spins faster. The torque it demands climbs with RPM, while the motor's push falls as it nears free speed. The two curves meet below the no-load figure, and where they meet is your real operating point.

Battery sag, ESC timing, bearing drag and winding heat all push that meeting point lower. A cool motor on a fresh pack sits near the top of the range below. A hot motor on a tired pack sits near the bottom.

A watt meter and a tachometer beat any estimate. Without them, start one prop size small and go up until the motor lands warm rather than hot.

The 85 percent figure describes full throttle. At half stick the ESC chops voltage, so RPM falls faster than the stick.

Voltage: nominal, full charge, and sag

A lithium polymer cell is 3.7 V nominal, 4.2 V at full charge, and about 3.5 V when it is time to land. A 6S pack is sold as 22.2 V, charges to 25.2 V, and can drop to 21 V under a hard pull.

The same 1900 KV motor gives 42,180 RPM at nominal voltage, 47,880 RPM at full charge, and about 39,900 RPM at 21 V. That spread of roughly 8,000 RPM is normal. It is also why two people with the same motor and prop can report numbers that disagree.

Plan with nominal voltage. Check the worst case with full charge voltage, because that is the moment when the prop turns fastest, the current peaks, and an undersized ESC lets out its smoke.

Sag depends on the C rating and the pack's age: a tired 6S pack dips further than a new one under the same load.

KV and Kt are the same fact, upside down

Kt is the torque constant, in newton meters per amp. A low KV motor turns slowly per volt and makes more torque per amp. The two numbers are tied together: KV times Kt equals 9.5493 when KV is in RPM per volt and Kt is in newton meters per amp.

This is the trade every build faces. A 300 KV motor swings a 24 inch prop at low RPM without huge current, because each amp buys more torque. A 3000 KV motor needs a small prop, a low gear ratio, or both.

Torque at a given current is Kt times amps. The 1900 KV motor at 30 A makes 0.005026 times 30, which is 0.151 N·m. In pound inches that is 0.151 times 8.851, or 1.34 lb-in.

KV says nothing about current handling. That comes from stator size, wire gauge and cooling, and two 1900 KV motors can differ by 30 A of safe continuous draw.

Gearing: RPM down, torque up

A gearbox is a trade: RPM out, torque in. A 4.5:1 box turns 27,232 motor RPM into 6,052 RPM at the shaft, and multiplies torque by 4.5 on the way through.

Gearboxes are not free. A planetary set costs 3 to 7 percent of the power in friction, so the shaft sees 4.5 times the motor torque minus that loss: about 0.47 N·m before loss and 0.44 N·m after 5 percent.

The ratio is written as motor turns to output turns. A 4.5:1 box means the motor spins 4.5 times for one turn of the output. If a listing shows 0.22:1, divide by 0.22, which is the same as multiplying by 4.55.

Direct drive is a ratio of 1. Belt drives work the same way: a 20 tooth motor pulley driving a 60 tooth wheel pulley is 3:1.

Shaft RPM becomes ground speed through the tire. A 1:10 truck with 3.5 inch tires at 6,052 shaft RPM covers about 63 mph, which is why crawlers use a lower KV or a second reduction.

Pole count and the frequency inside the ESC

A brushless motor has no brushes. The ESC replaces them by switching current between three wires, and it has to switch in step with the magnets. Each time one north and one south pole pass a stator tooth, that is one electrical cycle.

Pole count is always even, because magnets come in pairs. A 14 pole motor has 7 pole pairs, so one turn produces 7 electrical cycles. At 35,853 RPM the motor completes 597.6 turns per second, and the ESC switches 597.6 times 7, which is 4,183 times per second, or 4.18 kHz.

Most hobby ESCs are happy between 4 and 8 kHz. Above 12 kHz some older units lose sync at full throttle and stutter. A lower KV motor or a higher gear ratio pulls the frequency down.

High pole counts are popular on multirotors because they make smooth torque at low RPM, at the cost of a higher switching frequency.

Two builds, worked from the label to the shaft

Build one, a 6S sport plane. Motor 1900 KV, pack 6S at 22.2 V, prop 10x7, load 85 percent, direct drive, 14 poles. No-load RPM is 1900 times 22.2, which is 42,180. Loaded RPM is 42,180 times 0.85, which is 35,853, and the shaft turns the same because the ratio is 1. Kt is 9.5493 divided by 1900, or 0.005026 N·m per amp, so 30 A gives 0.151 N·m. Switching is 35,853 divided by 60, times 7 pairs, or 4,183 Hz.

Build two, a 4S crawler with a gearbox. Motor 2300 KV, pack 4S at 14.8 V, gearbox 4.5:1, 18 poles, load 80 percent. No-load is 2300 times 14.8, or 34,040 RPM. Loaded is 34,040 times 0.8, or 27,232 RPM. Shaft speed is 27,232 divided by 4.5, or 6,052 RPM. Kt is 9.5493 divided by 2300, or 0.004152 N·m per amp, so 25 A gives 0.104 N·m at the motor and about 0.44 N·m at the output after 5 percent gear loss. Switching is 27,232 divided by 60, times 9 pairs, or 4,085 Hz.

A quick check. Multiply shaft RPM by the gear ratio and you should land back on loaded motor RPM. Multiply Kt by KV and you should get 9.5493.

What this does not model. Current limit, timing advance and heat all bend the real curve. A motor pushed past its rating loses RPM faster than the load factor predicts. Treat the output as a planning number and confirm it with a watt meter.