Drone Motor Calculator

Estimate unloaded motor RPM, battery pack voltage, and theoretical propeller pitch and tip speed from your motor's KV rating, battery cell count, and propeller size.

Quick Facts

Core formula
Unloaded RPM = Motor KV x Battery Voltage
Pitch and tip speed use the standard conversion mph = (RPM x inches) / 1056.

Your Results

Calculated
Unloaded motor RPM
-
KV x battery voltage
Battery pack voltage
-
Cell count x per-cell voltage
Theoretical pitch speed
-
No-slip forward speed estimate
Propeller tip speed
-
Blade tip velocity at this RPM

Ready

Enter motor KV, battery configuration, and propeller size, then press Calculate.

About the Drone Motor Calculator

This calculator estimates the key speed figures for a brushless motor, battery, and propeller combination used in multirotor drones. Give it your motor's KV rating, your battery's cell count and per-cell voltage, and your propeller's pitch and diameter, and it works out the unloaded motor RPM, the battery pack voltage, and the theoretical pitch and tip speeds that follow from those numbers.

The core formula

A brushless motor's KV rating (RPM per volt) tells you how fast it spins per volt applied with no load attached. Multiplying KV by the battery's pack voltage gives the theoretical unloaded RPM:

RPM = Motor KV × Battery Voltage

Battery pack voltage itself is simply the number of cells in series (the "S" rating) multiplied by the voltage of each cell — 3.7V per cell is the commonly used nominal LiPo voltage, while a freshly charged cell reads closer to 4.2V.

Converting RPM to speed

Once you know the RPM, two more standard conversions describe how the propeller moves through the air. Theoretical pitch speed treats the propeller like a screw turning through solid material with zero slip:

Pitch speed (mph) = (RPM × Pitch in inches) / 1056

The constant 1056 converts inches traveled per minute into miles per hour (63,360 inches per mile, divided by 60 minutes per hour). The same conversion applies to the linear speed of the propeller's blade tip, using the blade's swept diameter instead of pitch:

Tip speed (mph) = (π × Diameter in inches × RPM) / 1056

How to get the best results

  • Use the KV value printed on the motor or listed in its datasheet — it is specific to that motor, not a universal constant.
  • Decide whether you want a nominal (3.7V/cell) or fully-charged (about 4.2V/cell) estimate, and be consistent about which one you're reading.
  • Double-check that you have not swapped propeller pitch and diameter — diameter is the full blade sweep, pitch is the theoretical distance per rotation.

Practical context

These are theoretical, unloaded figures. A propeller attached to the motor draws current, and that current causes voltage sag across the battery's internal resistance and the motor's winding resistance, so the real RPM under load is always somewhat lower than KV × Voltage. Likewise, real propellers "slip" through air rather than gripping it like a screw in wood, so actual forward airspeed is always somewhat below the calculated pitch speed. Use these numbers for comparing motor, battery, and prop combinations on equal footing — not as a guaranteed top speed.

Frequently Asked Questions

How do I find my motor's KV rating?
KV (RPM per volt) is printed on the motor's label or listed in its datasheet, such as 2400KV or 1700KV. It tells you the theoretical unloaded RPM the motor spins for every volt applied, so a 2400KV motor on 1 volt spins about 2400 RPM with no propeller and no load.
What is the difference between propeller pitch and diameter?
Diameter is the full tip-to-tip length the blades sweep through, in inches. Pitch is the theoretical forward distance the propeller would travel in one full rotation if it moved through a solid like a screw through wood, also in inches. A 5x4.3 propeller has a 5 inch diameter and a 4.3 inch pitch.
Why is my actual top speed lower than the calculated pitch speed?
Pitch speed is a theoretical figure that assumes zero propeller slip, meaning the prop moves through air exactly as if it were a screw in solid material. Real air is compressible and slips past the blades, and the motor's real RPM under load is also lower than the unloaded KV x voltage figure because current draw causes voltage sag, so actual airspeed is always somewhat below the calculated pitch speed.
Should I enter nominal or fully charged battery voltage?
Either is valid depending on what you want to see. Nominal LiPo voltage is 3.7V per cell and represents a typical mid-flight state, while a freshly charged pack reads about 4.2V per cell and gives the highest RPM the setup will ever produce. Using 3.7V per cell gives a realistic average estimate; using 4.2V per cell gives the ceiling value right after charging.

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