Calculate BLDC motor speed from KV rating and supply voltage, or estimate loaded RPM using a load factor for no-load and under-load results.
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BLDC Speed Formula
The BLDC speed calculator estimates motor speed from the motor KV rating and supply voltage. KV means revolutions per minute per volt, so a higher KV motor spins faster at the same voltage.
No-load speed:
Speed under load:
Angular speed conversion:
Rotations per second conversion:
- RPM = motor speed in revolutions per minute
- KV = motor KV rating in RPM per volt
- V = supply voltage in volts
- L = load factor as a percent of no-load speed
- rad/s = angular speed in radians per second
- Hz = rotations per second
- pi = approximately 3.14159
In no-load mode, the calculator multiplies KV by voltage to estimate the theoretical free-running speed. In under-load mode, it first calculates no-load speed, then multiplies by the load factor. A load factor of 80 means the motor is assumed to run at 80% of its no-load RPM. The result also includes rad/s and Hz conversions for comparison with other speed units.
Typical BLDC KV and Load Factor Ranges
These values are rough reference ranges. Actual speed depends on the motor, ESC settings, battery sag, propeller or wheel load, gearing, and current limits.
| Application | Typical KV | Common voltage range | Speed note |
|---|---|---|---|
| E-bike hub motor | 50 to 120 KV | 36 to 52 V | Lower motor RPM, often direct-drive or geared at the wheel |
| RC plane outrunner | 200 to 1200 KV | 3S to 6S LiPo | Chosen to match propeller size and thrust needs |
| 5 inch quadcopter | 1400 to 2600 KV | 4S to 6S LiPo | High RPM with significant drop under propeller load |
| Micro drone | 4000 to 12000 KV | 1S to 3S LiPo | Very high RPM with small propellers |
| Load factor | Meaning | When to use it |
|---|---|---|
| 95% | Very light load | Bench testing with little resistance |
| 85% | Moderate load | Typical running estimate for a reasonably matched setup |
| 75% | Heavy load | Large propeller, high wheel load, or strong voltage sag |
| 60% | Near-stall or overloaded | Use only as a rough warning-level estimate |
Example BLDC Speed Calculations
Example 1: No-load speed
You have a 1400 KV motor on a 4S LiPo battery listed as 14.8 V nominal.
The estimated no-load speed is 20,720 RPM.
Example 2: Speed under load
You have a 2300 KV motor on 11.1 V and estimate it will run at 80% of no-load speed under load.
The estimated loaded speed is 20,424 RPM.
FAQ
Is BLDC KV the same as motor speed?
No. KV is not the speed by itself. KV tells you how many RPM the motor would ideally produce per volt with no load. To estimate speed, multiply KV by the voltage. For example, a 1000 KV motor at 12 V has an estimated no-load speed of 12,000 RPM.
Why is the real motor speed lower than the no-load calculation?
The no-load formula is an ideal estimate. Real speed is usually lower because the motor is doing work. Propellers, wheels, pumps, belts, gearing, bearing friction, ESC limits, battery voltage sag, and current limits all reduce RPM. Use the under-load mode when you want a more realistic estimate.
Should I use nominal battery voltage or fully charged voltage?
Use the voltage that matches the condition you want to estimate. For a typical average-speed estimate, use nominal voltage, such as 14.8 V for a 4S LiPo. For maximum possible no-load speed on a fresh battery, use fully charged voltage, such as 16.8 V for a 4S LiPo.