Combine it with pack voltage, wheel diameter and any gearing, and you get the theoretical and realistic wheel speed — plus the torque available at a given power. Essential for choosing a winding or checking whether a wheel swap will ruin your gearing.
How do you calculate motor RPM from KV?
No-load RPM is KV multiplied by pack voltage, so a 15 KV hub motor on a 60 V pack spins 900 RPM unloaded. Under load expect 80–90% of that. Wheel speed is RPM times wheel circumference: 900 RPM on a 10-inch wheel is about 43 km/h unloaded.
What KV actually measures
KV is revolutions per minute per volt, unloaded. A 100 KV motor on a 60 V pack spins 6000 RPM with nothing attached. Put a wheel on the ground and you will see roughly 80–90% of that at full throttle, because generating torque requires current, and current through the winding resistance costs you speed.
KV and torque trade off directly. Halving KV doubles torque per amp at the same voltage, which is why low-KV motors climb well and high-KV motors run fast. Rewinding a motor to a lower KV does not create power — it just moves the same power to a different point on the speed-torque line.
Wheel size changes everything downstream
Wheel speed is RPM × circumference, and circumference is π × diameter. A 10-inch wheel travels 0.798 m per revolution, an 8.5-inch wheel 0.678 m. Fitting smaller wheels to the same hub motor cuts top speed by that ratio — about 15% — while raising force at the ground, and so acceleration and hill climbing, by roughly the same factor.
For geared or belt-drive builds, divide motor RPM by the gear ratio before working out wheel speed. A 5:1 reduction on a 3000 RPM motor gives 600 RPM at the wheel; with a 10-inch wheel that is 28.7 km/h. Gearing is how mid-drive builds get both torque and a sensible motor operating range.
| Wheel | Circumference | At 500 rpm | At 900 rpm | Typical on |
|---|---|---|---|---|
| 6.5″ | 519 mm | 15.6 km/h | 28.0 km/h | Hoverboards, mini scooters |
| 8″ | 638 mm | 19.1 km/h | 34.5 km/h | Entry commuters |
| 8.5″ | 678 mm | 20.3 km/h | 36.6 km/h | Xiaomi-class scooters |
| 10″ | 798 mm | 23.9 km/h | 43.1 km/h | Mid and long-range scooters |
| 11″ | 878 mm | 26.3 km/h | 47.4 km/h | Performance scooters |
| 12″ | 957 mm | 28.7 km/h | 51.7 km/h | Seated scooters, mopeds |
Torque, current and the numbers that break things
Torque in newton-metres is 9.5488 × power in watts ÷ RPM. That relationship is why low-speed, high-power operation is so hard on hardware: at 200 RPM, 1000 W means 47.7 Nm at the wheel and a great deal of current through the phase wires, with almost no airflow over the motor.
Two practical checks before ordering parts. Confirm the controller can supply the phase current the winding needs for your target torque, and confirm the motor's thermal limit at your typical climbing speed rather than at top speed. Most hub motor failures are thermal, and they happen slowly at low RPM, not dramatically at high RPM.
How the calculator works
No-load RPM is KV multiplied by pack voltage. The load factor then takes off the speed the motor loses while producing torque, and any gear ratio divides it down to wheel RPM. Wheel speed is wheel RPM times the wheel’s circumference. Torque comes from your motor power at that loaded RPM, multiplied by the gear ratio at the wheel, and dividing by wheel radius gives the pushing force at the ground.
The formula
τ (Nm) = 9.5488 × P ÷ RPM // torque from power and speed
Fground = τwheel ÷ rwheel
Related calculators
KV sets the RPM, but the road decides the rest. The power-to-speed calculator turns a winding into a real top speed, the hill climb calculator shows its torque under load, and the pack calculator checks the battery can supply what it draws.
Questions riders actually ask
How do I find my motor's KV if it is unmarked?
Spin it with a drill at a known RPM and measure the AC voltage between any two phase wires. A multimeter reads RMS, so multiply the reading by about 1.41 to get peak voltage, then divide RPM by that peak figure to get KV — you are really measuring its inverse, the back-EMF constant. It is a five-minute bench test and far more reliable than guessing from listings.
Why is my actual speed lower than the KV calculation?
The no-load figure ignores load entirely. Under real load expect 80–90% of no-load RPM, less on a hill, and less again as the pack voltage sags. Add tyre wear and firmware speed caps and a 15–20% shortfall against the theoretical figure is completely normal.
Does a higher voltage pack make the motor faster?
Yes, proportionally: RPM equals KV × volts, so moving a 100 KV motor from 48 V to 60 V raises no-load speed by 25%. The controller must be rated for the higher voltage, and the motor must handle the extra RPM and heat — this is the most common way people cook a hub motor.
How does wheel size affect torque at the ground?
Inversely. Ground force is wheel torque divided by wheel radius, so a smaller wheel multiplies your effective pulling force while cutting top speed by the same ratio. Going from 10-inch to 8-inch wheels is roughly a 25% torque gain and a 20% speed loss.
What is a typical KV for a scooter hub motor?
Direct-drive scooter hubs are usually in the 10–20 KV range because they turn the wheel directly and need low RPM: 15 KV on 60 V gives 900 RPM unloaded — about 43 km/h on a 10-inch wheel, or nearer 37 km/h under real load. The high KV figures you see quoted — hundreds or thousands — belong to geared RC-style motors, not hub drives.
Sources
Primary and reference sources for the physics, standards and figures used on this page.
- Motor constants (KV and Kt)Wikipedia