Blog · 2026-07-05 · Motors
Half the pain of motor shopping is that one seller quotes kg·cm, another oz·in, a third mN·m — and none of them say whether it's stall or rated. This article ends the confusion permanently.
Torque is the simplest concept in a motor datasheet and somehow the most mangled by the marketplace. Once you understand what torque physically is, why four different units exist for it, and the crucial difference between stall and rated figures, you can read any motor listing in any unit system and know exactly what you're buying. Let's build that understanding from the ground up.
Torque is rotational force: a force applied at a distance from an axis. Push a wrench with 10 newtons of force at 0.2 metres from the bolt, and you apply 10 × 0.2 = 2 newton-metres of torque. The same applies in reverse at a robot wheel: a motor producing 2 N·m of torque through a wheel of 0.1 m radius pushes the ground with 2 ÷ 0.1 = 20 N of force. That inverse relationship is why wheel size matters so much — bigger wheels mean less pushing force from the same motor, and it's the single most common reason a robot that worked on paper crawls on carpet.
Torque = Force × Distance ⇄ Force at wheel = Torque ÷ Wheel radius
Newton-metres (N·m) is the SI unit and the one physics formulas produce. Small motors are often quoted in millinewton-metres (mN·m): 1 N·m = 1000 mN·m.
Kilogram-centimetres (kg·cm) dominates hobby listings, especially servos, because it's intuitive: a 10 kg·cm servo can hold a 10 kg weight on a 1 cm arm — or 1 kg on a 10 cm arm. Strictly it's kilogram-force centimetres, so converting to SI multiplies by g: 1 kg·cm = 0.0981 N·m.
Ounce-inches (oz·in) appears on American motors and stepper datasheets. 1 oz·in = 0.00706 N·m.
| From \ To | N·m | kg·cm | oz·in |
|---|---|---|---|
| 1 N·m | 1 | 10.20 | 141.6 |
| 1 kg·cm | 0.0981 | 1 | 13.89 |
| 1 oz·in | 0.00706 | 0.0720 | 1 |
Three conversions worth memorizing: 1 N·m ≈ 10.2 kg·cm ≈ 141.6 oz·in. Our Motor Sizing Calculator and Servo Torque Calculator display results in all three simultaneously, so you can compare listings without a spreadsheet.
This is the paragraph that saves you money. A DC motor's torque is highest when its shaft isn't moving at all — that maximum is stall torque, and it comes paired with stall current, the maximum the motor can draw. At stall, all electrical power becomes heat; a motor can survive it for seconds, not minutes. Rated (continuous) torque is what the motor delivers indefinitely without overheating — typically 25–50% of stall for small brushed motors.
Marketplaces overwhelmingly advertise the stall figure because it's bigger. A "10 kg·cm motor" that means 10 kg·cm stall gives perhaps 3–4 kg·cm continuously — and remember that at stall the motor produces zero speed and zero useful mechanical power. So the sizing rule is:
For a brushed DC motor at fixed voltage, torque and speed trade linearly: maximum speed at zero torque (no-load), maximum torque at zero speed (stall), a straight line between. Two consequences matter for builders. First, a loaded motor always runs slower than its listed RPM — plan on 75–85% of no-load speed at rated load, which is why our Gear Ratio Calculator suggests using 80%. Second, mechanical power peaks at half of stall torque, but maximum efficiency sits much closer to the no-load end, around 10–30% of stall. A motor sized so its working point is a small fraction of stall runs cool, efficient and long; one working near stall runs hot, slow and briefly.
Speed at load ≈ No-load speed × (1 − Load torque ÷ Stall torque)
Mechanical power (W) = Torque (N·m) × RPM × 2π ÷ 60
That power formula is your universal cross-check. A ₹300 motor claiming "20 kg·cm at 500 RPM" would deliver 0.0981 × 20 × 500 × 2π ÷ 60 ≈ 103 W of mechanical output — from a motor that visibly couldn't dissipate 20 W. The claim refutes itself. Run this check on any suspiciously cheap listing.
A gearbox multiplies torque by its ratio and divides speed by the same ratio, minus friction losses. A 3000 RPM motor producing 0.5 kg·cm behind a 30:1 gearbox at 90% efficiency becomes a 100 RPM output producing 0.5 × 30 × 0.9 = 13.5 kg·cm. This is why gearmotor listings show such impressive torque from tiny motors — and why the same motor is sold in a dozen RPM variants: same motor, different gearbox. The number to check is the same either way: rated torque at the output shaft, at the RPM you need. The full theory is in Gear Ratios Explained.
Take a typical 37 mm gearmotor listing: "12 V, 100 RPM, rated torque 8 kg·cm, stall torque 32 kg·cm, rated current 0.8 A, stall current 5.5 A, gear ratio 1:56."
12 V — nominal voltage; a 3S LiPo (11.1 V) runs it slightly slow, which is fine. 100 RPM — no-load output speed; expect ~80 RPM working. Rated 8 kg·cm — your sizing number: this motor suits a requirement up to about 8 kg·cm continuous. Stall 32 kg·cm — a healthy 4× rated; good headroom for bumps and starts. Stall current 5.5 A — your driver must handle this (an L298N's 2 A channels won't; a BTS7960 will), and two of these motors stalling together demand 11 A from the battery — check that against your pack's C-rating with the Battery Runtime Calculator. 1:56 — the internal motor spins at 5600 RPM; useful only if you plan to re-gear.
Don't trust a suspicious listing? Measure. Fix the motor, attach a horn or pulley of known radius, hang a load (a water bottle on a string over the pulley works), and increase weight until the powered motor can no longer lift it. Torque in kg·cm = weight in kg × radius in cm. Do it quickly — this is a stall test and the motor is heating the whole time. For rated torque, find the load the motor lifts continuously for several minutes while staying merely warm, not hot, to the touch. A luggage scale pulling on a lever arm gives the same result with less rigging.
Not directly — a motor loafing far below its rating just runs cool. You pay in weight, size, cost and current draw, and heavily geared high-torque motors are slow. Oversize by 1.5–2×, not 10×.
Torque is proportional to current, and current to voltage. A LiPo sagging from 12.6 V to 10.5 V takes roughly 17% of your stall torque with it. Size with sag in mind — it's part of why safety factors exist.
Steppers invert the picture: they quote holding torque (torque while energized and stationary), and torque falls as speed rises. A stepper spec without a torque-vs-speed curve is half a spec.
With units, curves and the stall/rated distinction in hand, motor listings become transparent. Put the knowledge to work in the Motor Sizing Calculator, or step back to the full selection process in How to Choose a Motor for Your Robot.
Close the loop with the conversion that started this article. Your gearmotor delivers 8 kg·cm rated at the shaft; on a 90 mm wheel, how hard does the robot push? Convert to SI: 8 × 0.0981 = 0.785 N·m. Divide by wheel radius: 0.785 ÷ 0.045 = 17.4 N per wheel — two wheels give ~35 N, enough to accelerate a 4 kg robot at nearly 9 m/s²... except traction intervenes: rubber on wood grips at roughly μ ≈ 0.7, capping usable force at 0.7 × 4 × 9.81 ≈ 27 N. The motors out-muscle the tyres, meaning hard starts spin wheels — and meaning a slightly cheaper motor would have been equally fast in practice. That three-line calculation, run before purchase, is the entire value proposition of understanding torque units: it converts marketing numbers into predicted behavior, and predicted behavior into money unspent. The Motor Sizing Calculator runs the forward version of this chain automatically; being able to run it backwards by hand is what lets you audit any robot, listing or forum claim in your head.