RC Servo Torque Calculator
Calculate RC servo torque from control surface area, airspeed, and hinge moment.
Returns oz-in and kg-cm ratings needed for airplane, car, and boat builds.
Choosing the right servo torque for an RC model ensures reliable control surface movement under aerodynamic load. An undersized servo results in flutter, sluggish response, or complete control surface blowback at speed.
Hinge Moment Formula: The aerodynamic force on a control surface creates a hinge moment that the servo must overcome.
Hinge Moment (oz-in) = 0.5 × ρ × V² × S × C_h × Chord_fraction × Arm_length
For practical RC calculations, a simplified approach is used:
Simplified Torque Formula: Required Torque (oz-in) = Surface Area (in²) × Speed Factor × Deflection Factor × Safety Margin
Speed Factors:
| Aircraft Speed | Factor | Typical Models |
|---|---|---|
| Slow (under 30 mph) | 0.15 | Trainers, parkflyers |
| Moderate (30–60 mph) | 0.30 | Sport planes, scale |
| Fast (60–100 mph) | 0.50 | Pattern, EDF jets |
| Very fast (100+ mph) | 0.75 | Turbine jets, pylon racers |
Deflection Factor:
- 20° max deflection: ×0.8
- 30° max deflection: ×1.0
- 45° max deflection: ×1.3
Safety Margin: Always use 1.5× to 2.0× the calculated torque. Servos operating near their maximum torque stall, overheat, and strip gears.
Worked example, an aileron on a sport plane:
- Control surface: 12" span × 2.5" chord = 30 in²
- Speed: moderate (factor 0.30)
- Deflection: 30° (factor 1.0)
- Raw torque: 30 × 0.30 × 1.0 = 9.0 oz-in
- With 1.5× safety: 9.0 × 1.5 = 13.5 oz-in minimum
- Next standard rating up: 15 oz-in, which is what the calculator recommends
Fifteen is the minimum that clears the requirement, and plenty of builders would fit a 20 or 25 anyway. The extra costs a few grams and a few dollars, and it buys margin for a hinge that stiffens with age, a covering job that binds slightly, or the day you decide to increase throws. Sizing exactly to the calculated number leaves nothing for any of that.
Common Servo Sizes:
| Category | Torque Range | Weight | Use |
|---|---|---|---|
| Micro | 10–20 oz-in | 5–10 g | Parkflyers, micro quads |
| Mini | 20–50 oz-in | 10–20 g | Sport planes, small helis |
| Standard | 50–150 oz-in | 40–60 g | Scale, 3D, pattern planes |
| Large | 150–500 oz-in | 60–80 g | Large scale, turbine jets |
| Giant | 500+ oz-in | 80–150 g | Giant scale, 1/4 scale |
Digital vs. Analog: Digital servos provide faster response and higher holding torque at the cost of slightly more current draw. For any speed above “slow,” digital servos are strongly recommended.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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