Example Data Table
| Setup |
Voltage |
Kv |
Current |
Propeller |
Use Case |
| Trainer |
11.1 V |
1000 |
22 A |
9 x 5 |
Slow sport flying |
| Sport |
14.8 V |
1200 |
28 A |
10 x 5 |
Balanced power |
| Fast |
22.2 V |
900 |
46 A |
11 x 7 |
High pitch speed |
Formula Used
Loaded voltage: Loaded voltage = Pack voltage - Current × Winding resistance.
Loaded RPM: RPM = Kv × Loaded voltage × Throttle ratio × RPM efficiency.
Input power: Input power = Loaded voltage × Current.
Shaft power: Shaft power = Input power × Motor efficiency × Drive efficiency.
Torque: Torque = Shaft power × 60 ÷ 2π ÷ RPM.
Runtime: Runtime = Capacity Ah × Usable capacity ÷ Current × 60.
Pitch speed: Pitch speed = RPM × Pitch ÷ 1056 × Slip correction.
Static thrust: Thrust = Air density × Rev/s² × Diameter⁴ × Ct.
How To Use This Calculator
Enter the battery voltage under load. Add measured current from a wattmeter. Enter motor Kv, throttle, efficiency, and winding resistance. Add propeller diameter and pitch. Enter battery capacity, model weight, motor weight, and cooling values. Press the submit button. The result appears above the form and below the header.
RC Electric Motor Guide
What This Calculator Does
An RC electric motor looks simple. The final setup is not simple. Battery voltage, current, motor Kv, propeller size, and efficiency all pull on each other. This calculator brings those values into one view. It estimates loaded RPM, electrical input, shaft power, torque, heat, pitch speed, static thrust, runtime, ESC headroom, and battery C demand.
Why Motor Matching Matters
A motor can spin fast with no propeller. It may overheat when a large propeller is fitted. A battery can show good voltage at rest. It can sag under high current. A propeller can make strong static thrust. It can still waste power at speed. These checks help you compare setups before a flight, boat run, or bench test.
Inputs You Should Know
Use measured current when possible. A wattmeter gives the best value. Use pack voltage under load for the most accurate result. If you only know nominal voltage, add winding resistance so the calculator can estimate voltage sag. Use realistic motor and drive efficiency values. Small outrunners may run near seventy to eighty five percent efficiency. Gearboxes, shafts, belts, and couplers add more losses.
Reading The Results
Loaded RPM shows the expected motor speed after throttle and voltage loss. Shaft power shows power left after losses. Torque shows how hard the motor turns the propeller or drivetrain. Heat loss shows power that becomes heat inside the system. Estimated motor temperature uses thermal resistance, so treat it as a warning value. Static thrust is approximate because propeller design changes the thrust coefficient.
Practical Setup Advice
Leave current margin for the ESC. A twenty five percent margin is a common starting point. Keep battery C demand below the pack rating. Check motor temperature after short runs. Reduce propeller diameter or pitch if heat rises fast. Increase cooling before adding more throttle. For aircraft, compare power loading with model weight. Sport models often need more power loading than slow trainers. Racing models need much more.
Best Use Cases
Use it during propeller selection, ESC sizing, battery planning, and cooling checks. It also helps compare two motors with different Kv values. The numbers are estimates, not lab certification. Always confirm them with safe bench testing and a wattmeter first.
FAQs
What is motor Kv?
Kv means revolutions per minute per volt. A 1000 Kv motor can spin near 1000 rpm per volt without load. Real loaded RPM is lower because current, propeller drag, heat, and voltage sag reduce speed.
Should I use nominal voltage or loaded voltage?
Loaded voltage is better. Nominal voltage is only a rating. A battery drops voltage under current. Use a wattmeter reading when available. That gives a safer and more realistic motor estimate.
Why is static thrust only approximate?
Static thrust depends on propeller shape, blade count, air density, and test stand quality. The calculator uses a thrust coefficient. Change Ct when you have manufacturer data or measured thrust data.
How much ESC margin should I keep?
A twenty five percent current margin is a common starting point. More margin helps with hot weather, airflow limits, burst throttle, and propeller changes. Always follow ESC maker limits.
What causes motor overheating?
High current, oversized propellers, poor airflow, low efficiency, and long full throttle runs create heat. If the estimated heat loss is high, reduce prop size, improve cooling, or choose a larger motor.
What is power loading?
Power loading compares shaft power with model weight. Higher values mean stronger acceleration and climb. Trainers need less power loading. Sport, aerobatic, and racing models usually need more.
Can this calculator replace bench testing?
No. It is a planning tool. Always confirm current, voltage, RPM, thrust, and temperature with safe bench tests. Stop testing if the motor, ESC, wires, or battery become too hot.
Why include motor weight?
Motor weight helps estimate stress through watts per gram. A small motor can produce high power briefly. It may still overheat during longer runs if the load is too high.