The Tools page contains all of the essential reference information and guidelines a hobbyist needs to make decisions regarding powerplants, picking the right motor/battery/esc combination and calculating performance before you even leave the ground.

A word about safety

As a rule, recommendations made by calculators and guides provided by 2 Brothers Hobby NEVER supersede the manufacturers recommendations for your specific product. 2 Brothers Hobby’s calculators and formulas provide general guidelines but your specific equipment may deviate from our formulaic assumptions and following our recommendations could result in permanent equipment damage or personal injury.

Always consult your manufacturer specifications and recommendations for battery charger settings, battery charge rates, propeller sizing, or any other configuration regarding RC airplane or helicopter powerplants.

Please read the Terms of Use located in the footer of each page. Using the information provided by 2 Brothers is done so at your own risk.

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Li-Po Charge Rate Calculator

  1. Enter the Li-Po Battery Capacity in mAh.
  2. Enter the Battery Manufacturer's Maximum Charge Rate (in "C" rating)
  3. Press the Calculate! button.
  4. For different combinations, return to Step 1

Battery Specifications

  • Battery Capacity (in mAh) =
  • Battery Maximum Charge "C" Rating =

Charging Rates

  • *Fast Charge Rate in Amps =
  • **Medium Charge Rate in Amps =
  • *Li-Po fast charge rates should NEVER exceed the maximum charging specification for your battery. If you are unsure, call the manufacturer or place of purchase for guidance. Most newer Li-Pos are rated for higher than 1C charge rates but older Li-Pos are only safe at 1C charge rates. Always verify your batteries' maximum charging rate. Serious injury can occur from charging batteries at rates beyond their specifications. See the Batteries and Chargers sections for more information about battery charging.
  • ** Never trickle charge a Li-Po battery. Over-voltage from trickle charging will destroy your Li-Po and can explode the pack. Medium charge rates are calculated at .5 maximum C. Debate continues over the benefits of charging Li-Pos at medium rates but there is no data to support that it causes any harm provided you always use a charger made for Li-Po batteries with the correct charge termination methods. See the Batteries and Chargers sections for more information about battery charging.

Battery Charging Time Calculator

  1. Enter the Battery Capacity (in mAh)
  2. Enter the Battery Manufacturer's Maximum Charge Rate (in "C" rating)
  3. Press the Calculate! button
  4. For different combinations, return to Step 1

Battery Variables

  • Battery Capacity (in mAh) =
  • Max Charge Rate (in "C" rating) =

*Charging Times from 20% to Full

  • Fast Charge at "C" = Minutes
  • Slow Charge at 1/5 "C" = Hours
  • **Trickle Charge at 1/10 "C" = Hours
  • *Stated times include 5% charger efficiency loss. Your actual charge times will vary based on charging equipment, cell chemistry and remaining charge %.
  • ** Trickle charging is for NiCAD batteries only. Do NOT trickle charge Li-Po batteries! Follow your charger instructions carefully for your battery type. See the Batteries and Chargers sections for more information.

Battery Run-Time Calculator

  • Enter the mAh rating of the battery
  • Enter the motor's maximum amperage draw
  • Press the Calculate! button
  • For different combinations, return to Step 1

    • Power Variables

    • Battery mAh =
    • Motor Max Amps =

    *Run-Time Results

    • Minutes at Full Power =
    • Minutes at **Climbout Power =
    • Minutes at **Cruise Power =
  • *This calculator provides a rough guideline. Actual results will vary depending on prop specs, ambient temperature, ESC settings, motor efficiency, connector resistance, and transmitter throttle channel adjustments.
  • **Climbout and Cruise power is calculated as 75% and 50% of full power, respectively.

    • Convert Watts / Amps

    • Enter Voltage =
    • Enter 
    • Converts to =

    RC Brushless Motor Sizing Calculator

    1. Enter the model "All-Up" or flying weight in Ounces (16 oz per pound)
    2. Press the Calculate! button
    3. For different weights, return to Step 1

    Model Flying Weight (AUW)

    • In Ounces =

    Recommended Motor Size in Watts for:

    • Unlimited 3D Aerobatics =
    • High Performance and Sport =
    • Scale Passenger and Cargo =
    • Unlimited 3D Aerobatics (190+ Watts/lb)
    • High Performance and Sport (140+ Watts/lb)
    • Scale Passenger and Cargo (80+ Watts/lb)
    • Calculations are based on 1 Horsepower equal to 746 Watts of electrical power.
    • Always refer to your model recommendations for motor sizing. Different propeller/fan unit combinations will produce different performance levels. This motor sizing calculator provides a general reference.

    ESC Sizing Calculator

    1. Enter the motor maximum wattage
    2. Enter the battery voltage. (Each "S" = 3.7 Volts D.C.)
    3. For different wattage or volts, return to Step 1

    Powerplant Variables

    • Motor Maximum Wattage =
    • Battery Voltage (D.C.) =
    • For Lithium, each "S" indicates 3.7 volts. Example: A 6S battery would be 22.2 volts

    ESC Amperage Recommendations

    • Motor Max Amperage Draw =
    • Minimum ESC Amperage =
    • *Recommended ESC Amperage =
    • *Recommended amperage compensates for standard electrical allowances of 10% above circuit demand. Actual ESCs may not be available in the recommended amperage range but should always be rated at or greater than the minimum listed amperage.

    RC Airplane Propeller Pitch Speed Calculator

    1. Enter Motor RPM (See KV to RPM Calculator)
    2. Enter the Propeller Pitch (Second Number in Inches)
    3. Press the Calculate! button
    4. For different combinations, return to Step 1

    Powerplant Variables

    • Motor RPM =
    • Propeller Pitch (in Inches) =

    Speed Results (in MPH)

    • Calculated Pitch Speed =
    • *Real Pitch Speed (+ Prop Slip) =
    • *Real Model Speed (+ Prop Slip & Model Drag) =
    • *Propeller Slip calculated at 20%, Model Aerodynamic Drag calculated at 10%
    • Actual model and propeller performance will vary on your circumstances, including: Altitude, Model Design, Propeller Manufacture, Propeller Design and Weather Conditions.

    Brushless Motor Kv to RPM Converter

    1. Enter the Kv rating of your motor
    2. Enter your flight battery voltage
    3. Press the Calculate! button
    4. For different combinations, return to Step 1

    Powerplant Specifications

    • Motor Kv Rating =
    • Battery Voltage =

    Motor RPM Results

    • Motor *No-Load RPM =
    • * Motor No-Load RPMs are based on a bare motor where 1 volt produces 1 Revolution Per Minute (RPM). 1 RPM per volt is based on the inverse of 1 Kv of generation power produced per revolution of the motor.
    • Actual motor RPM will vary based on several factors including wiring resistance, ESC Pulse Width Modulation timing, battery current discharge rating, motor windings, propeller loading and motor efficiency. This calculator provides a common reference calculation when designing your powerplant or selecting your propeller.

    RC Propeller Tip Speed Calculator

    1. Enter the Motor RPM (See Kv to RPM Calculator)
    2. Enter the Propeller Diameter (in Inches)
    3. Press the Calculate! button
    4. For different combinations, return to Step 1

    Powerplant Variables

    • Motor RPM =
    • Propeller Diameter (in Inches) =

    Propeller Speed Results

    • Propeller Tip Speed (in MPH) =
    • *Percentage of MACH Speed =
    • **Propeller Thrust Status:
    • *MACH speed, or the Speed of Sound, is 1,125 ft/second at sea level in dry air at 200C (680F). Cooler air decreases MACH, warmer air increases MACH.
    • ** Propellers generally perform most efficiently with tip speeds up to about .75 MACH. Depending on the specific propeller airfoil, this efficiency range will vary. "Ripping" noises from the prop tips during high-speed runs indicate early loss of thrust efficiency. If you notice a lot of noise emanating from a balanced prop, verify the actual RPM with a tachometer and re-run your calculations above. If you're running over .75 MACH tip speeds and are experiencing noise and loss of performance, you may benefit from changing your propeller configuration.

    RC Prop Max RPM Calculator

    1. Enter Propeller Rotational Limit (See Propellers for more information)
    2. Enter the Propeller Diameter (in Inches)
    3. Press the Calculate! button
    4. For different combinations, return to Step 1

    Propeller Variables

    • *Propeller Class Rotational Limit (in RPM) =
    • Propeller Diameter (in Inches) =
    1. APC Glow Engine - 190,000
    2. APC Speed 400 Electric - 190,000
    3. APC Folding Electric - 145,000
    4. APC Slow Flyer - 65,000
    5. APC 8.8 Series 40 Racing - 225,000
    6. Master Airscrew Glass Filled Nylon - 165,000

    Propeller Safety Limit

    • Maximum Motor RPM =
    • * Your propeller manufacturer can provide this information. Each class of propeller has a rotational speed limit, often stated in "XX,XXX/Diameter" where "X"=Rotational RPM Limit. See the Powerplants section for more information about propeller classes and speeds. If your motor turns more than the maximum RPM calculated above, move to another class of propeller, i.e. Slow fly to Thin Electric.

    Gear Ratio Calculator

    Main Heli Rotor/Acro Gearbox

    • Motor RPM (S1) =
    • Pinion Driver Teeth (T1) =
    • Rotor/Prop Output Teeth (T2) =

    Tail Rotor (requires main rotor data input)

    • Tail Rotor Driver Teeth (T3) =
    • Tail Rotor Output Teeth (T4) =
    • Main Rotor/Prop RPM (S2)=
    • Tail Rotor RPM (S3) =
    Gear Ratio Calculator Diagram

    Wing Loading Calculator

    • Aircraft All Up Weight = oz
    • Total Wing Area = ft2
    • Wing Loading = oz/ft2

    Wing Area Calculator - Type 1 - Constant Chord Wing Shape

    • Wing Span in Inches =
    • Root Chord in Inches =
    • Each Wing Area = in2
    • Total Aircraft Wing Area =
      (includes both wings)ft2

    Wing Area Calculator - Type 2 - Tapered or Delta Wing Shape

    • Wing Span in Inches =
    • Root Chord in Inches =
    • Tip Chord in Inches =
    • Each Wing Area = in2
    • Total Aircraft Wing Area =
      (includes both wings)ft2

    Wing Area Calculator - Type 3 - Delta with Tip Chord of 0

    • Wing Span in Inches =
    • Root Chord in Inches =
    • Each Wing Area = in2
    • Total Aircraft Wing Area =
      (includes both wings)ft2
    Lengths
    1
    inch
    =
    25.4
    mm
    1
    inch
    =
    2.54
    cm
    1
    millimeter (mm)
    =
    0.03937
    inches
    1
    centemeter (cm)
    =
    0.3937
    inches
    1
    foot
    =
    0.3048
    meters
    1
    foot
    =
    30.48
    cm
    Power
    1
    horsepower
    =
    746
    watts
    Weights
    1
    pound (lb)
    =
    16
    ounces
    1
    ounce (oz)
    =
    28.3495231
    grams (g)
    1
    gram (g)
    =
    1000
    milligrams (mg)
    1
    kilogram (kg)
    =
    2.20462262
    pounds (lbs)
    Amps & Watts
    Amps
    =
    Watts
    Divided By
    Volts
    Watts
    =
    Amps
    Multiplied by
    Volts
    Ohms Law
    Voltage
    =
    Amps (or mhAh)
    Multiplied By
    Resistance (in ohms or Kilohms)
    Amps
    =
    Voltage
    Divided By
    Resistance (in ohms or Kilohms)
    Resistance (in ohms or Kilohms)
    =
    Voltage
    Divided By
    Amps (or mhAh)
    Wing Loading in oz/ft2
    Weight (in pounds)
    Mulitplied By
    2304
    Divided By
    Wing Area (in2)
    AWG Wire Gauge
    12V DC Amperage
    22
    5A
    20
    8A
    18
    10A
    16
    20A
    14
    40A
    12
    60A
    10
    100A
    8
    150A
    6
    190A