Motor RC Electric Calculator
Example Data Table
| Setup | Kv | Battery | Prop | Current | Use Case |
|---|---|---|---|---|---|
| Trainer | 920 | 4S 5000 mAh | 10 x 5 | 45 A | Balanced sport flight |
| Slow Flyer | 700 | 3S 2200 mAh | 11 x 4.7 | 24 A | High static pull |
| Speed Model | 1500 | 4S 3300 mAh | 7 x 6 | 55 A | Higher pitch speed |
| Quad Motor | 980 | 6S 5200 mAh | 10 x 4.5 | 22 A each | Multirotor lift check |
Formula Used
Battery voltage: cells x cell voltage.
No load RPM: Kv x battery voltage x throttle / gear ratio.
Loaded RPM: Kv x (battery voltage x throttle - current x motor resistance) / gear ratio.
Input power: voltage x current x number of motors.
Shaft power: total input power x efficiency.
Runtime: usable amp hours / total current x 60.
Pitch speed: loaded RPM x prop pitch / 1056.
Static thrust: Ct x air density x revolutions per second squared x prop diameter in meters to the fourth power.
Thrust ratio: total static thrust in grams / aircraft weight in grams.
How to Use This Calculator
Enter the motor Kv, battery cell count, and nominal cell voltage. Add the expected current draw per motor. Use wattmeter values when available.
Enter propeller diameter and pitch in inches. Add aircraft weight, motor count, ESC rating, pack capacity, and C rating.
Press the submit button. Review RPM, watts, runtime, thrust, ESC margin, battery margin, and tip speed. Change one value at a time to compare setups.
Understanding RC Electric Motor Planning
A strong RC electric system starts with balanced numbers. The motor, battery, propeller, ESC, and airframe all share the same electrical load. This calculator joins those values in one clear view. It helps you estimate speed, power, heat, runtime, and thrust before a flight.
Why Motor Kv Matters
Motor Kv tells how many revolutions a motor may turn per volt. Higher Kv usually gives more RPM. Lower Kv often suits larger propellers. The best choice depends on voltage, propeller size, and aircraft weight. A high Kv motor can overdraw current when the propeller is too large.
Battery And ESC Safety
Battery voltage sets the motor speed range. Current draw sets the stress level. The ESC must handle the current with extra headroom. A common planning margin is about twenty five percent. The battery also needs enough C rating to supply the total current. Weak packs sag, heat up, and reduce power.
Propeller Load And Thrust
Propeller diameter has a strong effect on load. Pitch affects pitch speed and current. Larger props can make more thrust, but they demand more torque. The thrust estimate here uses a coefficient method. It is useful for comparison. Real thrust changes with prop brand, blade count, air density, and test stand conditions.
Runtime And Flight Style
Runtime depends on usable battery capacity and total current. A gentle cruise may draw far less current than full throttle. This page uses entered current as a planning point. For better results, test current with a wattmeter. Then enter measured values from the same propeller and battery.
Good Setup Practice
Use this tool before buying parts. Compare several propellers and cell counts. Keep tip speed reasonable. Keep motor heat under control. Do not rely on one number only. Check RPM, amps, watts per kilogram, thrust ratio, ESC margin, and battery margin together. A balanced setup flies better, lasts longer, and is easier to tune.
Use conservative limits when data is uncertain. Small changes can shift current quickly. Record each test in a log. Note propeller size, cell count, temperature, and current. Safe records make future setups easier. They also help you spot worn packs or overloaded motors before damage happens during every flying season.
FAQs
What does motor Kv mean?
Kv means RPM per volt without propeller load. A 920 Kv motor may turn near 920 RPM for each volt. Real RPM is lower after prop load, resistance, and voltage sag.
Is the thrust estimate exact?
No. It is a planning estimate. Propeller shape, blade count, air density, motor torque, and test stand data can change real thrust. Use it for comparison first.
What ESC margin should I use?
A common planning target is at least 25% above measured current. Hard flying, poor airflow, or hot weather may need more margin.
Why is battery C rating important?
C rating estimates how much current a pack can supply. Low C margin can cause voltage sag, heat, weak power, and shorter battery life.
Should I enter full throttle current?
Use the current that matches your planning case. Full throttle current is useful for safety. Cruise current is better for runtime estimates.
What is pitch speed?
Pitch speed estimates forward speed from propeller pitch and RPM. It ignores slip, drag, and airframe limits, so real speed is usually lower.
What is a good thrust to weight ratio?
Trainers may fly with modest ratios. Sport aircraft often need more. 3D models may need a ratio above one. Use your aircraft goal.
Why should tip Mach be checked?
High propeller tip speed can increase noise and reduce efficiency. Keeping it reasonable helps smooth flight and protects the setup from overload.