RC Servo Guide: Sizes, Torque, Speed and How to Choose
An RC servo is a small motor with gears and a position sensor that turns its output arm to the angle the receiver commands, usually with a PWM pulse of about 1.0–2.0 ms (1.5 ms is centre). Servos are chosen by size class (nano, micro, mini, standard, large scale), torque (kg·cm or oz·in), speed (seconds per 60°), operating voltage (standard or HV) and gear type. This guide explains each spec and gives typical choices for cars, planes, helicopters and boats.
By Kyle HilsonUpdated: October 20264 min read
How does a servo work?
Inside a servo are a DC motor (brushed, coreless or brushless), a gear train, a potentiometer or magnetic encoder that measures the output position, and a control board. The board compares the commanded position (from the pulse width) with the actual position and drives the motor until they match. That’s why a servo holds its position under load and buzzes slightly when it does.
| Signal | Pulse width | Result |
|---|---|---|
| Full one side | about 1.0 ms | about −45° (varies by servo and radio) |
| Centre | about 1.5 ms | Neutral |
| Full other side | about 2.0 ms | about +45° |
Analogue servos expect pulses about 50 times a second; digital servos update their motor much more often internally, which gives more holding power and precision.
Servo size classes
| Class | Weight | Typical torque | Used in |
|---|---|---|---|
| Nano / sub-micro | 2–5 g | under 1 kg·cm | Indoor planes, 1/24–1/18 cars, micro helis |
| Micro | 8–12 g | 1.5–3 kg·cm | Park flyers, foamboard planes |
| Mini | 20–30 g | 3–8 kg·cm | 450–550 helis, sport planes, small cars |
| Standard | 40–80 g | 5–35+ kg·cm | 1/10–1/8 cars, larger planes, boats |
| Large scale (1/5) | 150 g+ | 30–60+ kg·cm | 1/5 cars, giant planes, big boats |
Standard servos share roughly the same mounting footprint (about 40 × 20 mm), which makes swapping easy.
Reading the specs
- Torque in kg·cm (or oz·in; 1 kg·cm ≈ 13.9 oz·in): force at 1 cm from the centre. Longer servo arms give more travel but less force.
- Speed in s/60°: lower is faster. 0.10 s/60° is fast; 0.20 s/60° is slow.
- Voltage: standard servos run at 4.8–6.0 V; HV servos at up to 7.4–8.4 V and are faster and stronger at higher voltage. Check your BEC or receiver pack.
- Gears: plastic (light, cheap), metal (strong; for cars and big planes), titanium/steel (high end).
- Bearings: ball bearings on the output shaft reduce slop.
- Waterproof rating: needed for cars driven in wet conditions and for boats.
- Brushless/coreless motor: faster, smoother and longer-lasting than cheap brushed motors.
Which servo for which model?
| Model | Function | Recommendation |
|---|---|---|
| 1/10 basher | Steering | Waterproof standard, metal gear, 10–20 kg·cm |
| 1/10 crawler | Steering | High torque (25–35 kg·cm), speed less important |
| 1/5 car (e.g. King Motor Baja 5B) | Steering, throttle | Large-scale servos, high torque |
| Park flyer plane | Surfaces | 9 g micro servos |
| 3D foamy | Surfaces | Fast digital micro servos |
| Giant-scale plane | Surfaces | High-torque HV digital servos, often two per surface |
| 450–700 heli | Cyclic, tail | Fast digital heli servos; dedicated fast tail servo |
| RC boat | Rudder | Waterproof standard servo; strong servo for fast boats |
Connectors, wiring and setup
- Three wires: signal (white/yellow/orange), positive (red, middle), negative (black/brown).
- Futaba- and JR-style plugs fit most receivers; Futaba plugs have a key tab that sometimes needs trimming.
- Centre the servo electronically (with the radio at neutral) before fitting the arm.
- Set end points so the servo doesn’t push against a mechanical stop; a stalled servo draws high current and overheats.
- Big or many servos may need a separate BEC or power box; voltage drops cause brownouts.
FAQ
Analogue or digital servo?
Digital servos are more precise and hold position better but draw more current. Analogue is fine for simple planes and toys.
What does HV mean on a servo?
High voltage: rated for up to 7.4–8.4 V, typically for 2S LiPo or HV BECs. Running HV servos at lower voltage works but gives less speed and torque.
How much servo torque do I need?
Use the model’s recommendation; for cars, bigger tyres and heavier models need more. When in doubt, choose a step up.
Why does my servo buzz?
It’s holding position under load or pushing against an end stop. Reduce end points if it buzzes at full travel.
More on this topic: the complete RC Basics guide covers the basics behind this article.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
