RC Airplane Prop & Motor Sizing Calculator
Use a target weight, power loading and pitch speed to estimate a prop diameter, then see the motor Kv, ESC current and battery capacity that go with that starting point. It is a reverse-design estimate for narrowing a component search, not a substitute for measuring the finished power system.
Calculated prop diameter (in)12.8
- Required input power (W)
- 500
- Required loaded RPM
- 7,260
- Nearest half-inch diameter (in)
- 13
- Estimated input power with rounded prop (W)
- 531.45
- Required motor Kv (rpm/V)
- 613
- Full-throttle current (A)
- 33.78
- Minimum ESC current rating (A)
- 40.54
- Minimum battery capacity (mAh)
- 3,378
- Required battery C rate
- 10
- Diameter-to-pitch ratio
- 1.6
- Prop proportion check
- Within the usual 1:1 to 2:1 prop diameter-to-pitch planning range.
Uses nominal 3.7 V per LiPo cell and empirical APC-family constants; verify the installed system with a wattmeter.
Battery C-rate is a theoretical minimum from capacity and full-throttle current, not evidence that a pack will avoid voltage sag or overheating.
How to use this calculator
- Enter the ready-to-fly weight, including the battery you expect to use.
- Choose watts per pound for the kind of flying you want the airplane to do.
- Set the LiPo cell count, desired pitch speed, propeller pitch and the closest listed prop family.
- Adjust efficiency and loaded-RPM assumptions, then set your flight time and battery-use assumptions.
- Compare the rounded prop, motor Kv, ESC rating and pack capacity with manufacturer limits before buying parts.
How the RC airplane power system is sized
The calculation starts with the power the battery must supply: Pin = (W ÷ 16) × q, where W is ready-to-fly weight in ounces and q is the selected watts per pound. It applies motor-and-ESC efficiency to find mechanical propeller power. A 5-pound airplane planned at 100 W/lb therefore needs 500 W from the battery and, at 80% efficiency, about 400 W at the propeller.
Pitch speed sets the loaded RPM: N = 1056 × v ÷ p. Here v is pitch speed in mph and p is pitch in inches. The tool then reverses the Boucher relation used in the referenced RC power-system procedure: Pprop = K × Df^4 × n^3 × pf. Diameter and pitch are converted to feet, and RPM is expressed in thousands. The constant K is 1.21 for APC Electric props and 1.11 for standard APC props.
Solving that relation for diameter gives Df = [Pprop ÷ (K × n^3 × pf)]^(1/4). The displayed half-inch size is convenient for shopping, but it is not a harmless rounding step: prop power changes with the fourth power of diameter. This calculator recomputes the input power for that rounded diameter so the difference is visible.
Motor, ESC and battery results
Nominal LiPo voltage is V = 3.7 × S. Required motor Kv is Kv = N ÷ (V × L), where L is the loaded-RPM factor. Full-throttle current is I = Pin ÷ V, and the ESC suggestion is Iesc = I × (1 + h). Battery capacity follows C = 1000 × [Iavg × (t ÷ 60) ÷ u], with average current taken as a fraction of full-throttle current and u as usable capacity.
Weight and watts per pound move the required power directly. Prop pitch and pitch speed move RPM directly, while diameter has a much steeper relationship to prop load. A higher-voltage pack lowers the current for the same input power, but it also changes the required Kv and requires matching voltage ratings throughout the system.
What this calculator leaves out
The constants are an approximate fit for the two listed propeller families, not every blade profile or blade count. The model uses nominal voltage and does not predict voltage sag, battery age, wiring loss, ESC timing, temperature, altitude, cooling, thrust, static-versus-in-flight loading, or airframe drag. Pitch speed is theoretical and exceeds actual airspeed because props slip.
Use the result to choose candidates, then check motor, ESC, battery and propeller limits. Secure the model and measure the assembled system with a wattmeter; an advertised battery C-rating does not by itself establish acceptable temperature or voltage sag.
Worked example
For an 80 oz airplane at 100 W/lb, required battery input power is (80 ÷ 16) × 100 = 500 W. With 80% motor-and-ESC efficiency, propeller power is 400 W. An 8-inch-pitch APC Electric prop at a 55 mph pitch-speed target needs 1056 × 55 ÷ 8 = 7,260 RPM.
Reversing the prop-power equation gives a diameter of about 12.8 inches, which rounds to 13 inches. The rounded prop predicts about 531.7 W input power rather than exactly 500 W, illustrating the fourth-power diameter effect. On 4S nominal voltage with an 80% loaded-RPM factor, the motor estimate is 613 rpm/V. Full-throttle current is 33.78 A, so 20% ESC headroom suggests 40.54 A. At 60% average current, eight minutes, and 80% usable capacity, the minimum planned pack is about 3,378 mAh with a theoretical 10C full-throttle requirement.
Common questions
How many watts per pound does an RC airplane need?
Common planning bands are about 50 to 90 W/lb for trainers, 90 to 150 W/lb for aerobatic models, and 150 to 200 W/lb for 3D flying. The useful number depends on wing loading, drag, propeller choice, desired climb and the way the airplane will be flown, so treat the band as a starting point rather than a performance guarantee.
How do propeller diameter and pitch affect motor load?
Diameter has a very strong effect because the Boucher relation varies with the fourth power of diameter, while RPM varies with the third power. Pitch helps set theoretical pitch speed at a given RPM. Changing either dimension can substantially change current, so a prop change requires a new wattmeter check.
What is loaded RPM, and why is it lower than Kv times voltage?
Kv times voltage describes the motor near no load. A propeller applies torque demand, and resistance in the motor, ESC, battery and wiring lowers the running RPM. The loaded-RPM factor is a planning estimate for that difference, not a motor-specific test result.
How much ESC headroom should an electric RC airplane have?
The right margin depends on the ESC cooling, installation, voltage, motor, propeller and manufacturer ratings. This calculator lets you choose the margin and uses it only to suggest a current rating. Confirm both continuous current and voltage ratings against the actual setup, because partial throttle can still involve full-current pulses.
Why must the finished setup still be checked with a wattmeter?
The installed system can differ from an empirical estimate because of the exact propeller, battery voltage under load, wiring, ESC settings, cooling and air conditions. A wattmeter measures the actual current and power before a motor, ESC or battery is overstressed. Test securely and follow the component manufacturers’ limits.