Blog · 2026-08-15 · Units
A motor listing gives you three numbers that people treat as three independent features. They're not — they're three views of one curve, and understanding how they trade against each other is what makes gearing, motor selection and spec sheets suddenly make sense.
"Is a 12 kg·cm motor better than a 200 RPM motor?" is a question with no answer, and the fact that it gets asked constantly points at a genuine gap. Torque and speed aren't features to compare — they're two ends of a trade-off that every motor makes continuously while it runs, and power is the quantity that ties them together. Once that relationship is clear, gearboxes stop being magic and spec sheets stop being intimidating.
Torque is rotational force — how hard the shaft twists. Measured in N·m, kg·cm, oz·in and several other dialects, all converting freely (the Torque Unit Converter handles the arithmetic, and Motor Torque Units Explained covers the notation). Torque alone determines whether a robot can start moving, hold a load, or climb.
Speed is how fast the shaft rotates, in RPM or radians per second. Speed alone determines how fast the robot travels once moving.
Power is the rate of doing work — the product of the two:
power (W) = torque (N·m) × angular speed (rad/s)
power (W) ≈ torque (N·m) × RPM ÷ 9.55
That equation is the whole relationship. A motor producing 0.5 N·m at 200 RPM delivers about 10.5 W. The same 10.5 W could equally be 0.05 N·m at 2000 RPM, or 5 N·m at 20 RPM. Power is what you're really buying; torque and speed are how it's packaged.
A brushed DC motor's behaviour on a fixed voltage is close to a straight line between two points. At stall the shaft isn't moving: torque is at maximum, speed is zero, and — since power is torque × speed — output power is zero, with every watt going into heat. At no load the shaft spins freely: speed is at maximum, torque is essentially zero, and output power is zero again. Between them, torque falls linearly as speed rises.
torque = stall torque × (1 − speed ÷ no-load speed)
peak output power occurs at half stall torque and half no-load speed
peak power = stall torque × no-load speed ÷ 4 (consistent units)
Peak power sits exactly in the middle — half the stall torque at half the free speed. That's a genuinely useful anchor: a motor quoted as 2 kg·cm stall and 300 RPM free delivers its maximum output around 1 kg·cm at 150 RPM. It's also where most of a robot's useful operating region lies, though peak efficiency occurs at a higher speed than peak power, typically around 70–85% of no-load speed — which is why a motor running lightly loaded is more efficient than one working near its power peak.
Here's where the relationship pays off. A gearbox multiplies torque and divides speed by the same ratio; power is essentially unchanged, minus losses. That's the entire mechanism, and it means gearing doesn't make a motor more powerful — it repackages the power you already have into a more useful shape.
output torque = input torque × ratio × efficiency
output speed = input speed ÷ ratio
A tiny motor spinning at 12,000 RPM with almost no torque is nearly useless directly, but behind a 100:1 gearbox it becomes 120 RPM with a hundred times the torque — which is exactly what a robot wheel wants. This is why almost every robot motor is a gearmotor, and why the meaningful question when buying is not "how much torque?" but "how much power, and at what ratio?" Working the ratio out for a specific robot is the job of the Gear Ratio Calculator.
It also explains something that confuses beginners: two motors with identical stall torque can behave completely differently. A high-power motor behind a low ratio and a low-power motor behind a high ratio can both stall at 5 kg·cm — but the first will hold that torque while moving quickly, and the second will crawl. Torque figures alone hide the difference; power reveals it.
| Spec | What it means | How to use it |
|---|---|---|
| Rated voltage | Design voltage for the quoted figures | All other specs scale roughly with actual voltage |
| No-load speed | Free-running RPM at rated voltage | Upper end of the curve; expect 70–85% loaded |
| No-load current | Current just to spin itself | Internal friction indicator; lower is better |
| Stall torque | Torque at zero speed | Defines the curve; design at ~1/3 of it |
| Stall current | Maximum current draw | Sizes your driver, fuse and wiring |
| Rated torque | Continuous safe operating torque | The number that actually matters |
| Rated / peak power | Output power, not electrical input | Compare motors honestly across ratios |
Two practical notes. Everything scales with voltage: run a 12 V motor at 6 V and no-load speed roughly halves, stall torque roughly halves, and peak power falls by about four times. And stall current is the specification your electronics care about most — it sets the motor driver rating, the fuse and the wire gauge, all of which the Power Budget Calculator derives.
Electrical input power is voltage × current; mechanical output is torque × speed. The gap is heat. A hobby gearmotor might peak around 50–70% efficient, and much worse away from its sweet spot — near stall, efficiency approaches zero because output power does. This matters for runtime: a robot working its motors near stall drains its battery far faster than the same robot cruising, which is why runtime estimates based on cruising current mislead badly on a robot that spends its life starting, stopping and climbing.
No — torque alone isn't power; you need speed as well. A 10 kg·cm motor at 60 RPM produces about 6 W; the same torque at 600 RPM produces 60 W. Any listing quoting torque without speed is telling you half the story.
Torque, almost always. Hobby robots move slowly and spend their effort starting, climbing and pushing — all torque problems. Insufficient speed is a mild disappointment; insufficient torque is a robot that doesn't move. Size for torque first via the Motor Sizing Calculator, then check the resulting speed is acceptable.
Broadly yes — the torque-speed relationship is similar — but BLDC motors are specified with a Kv rating (RPM per volt) and their behaviour depends heavily on the controller. They deliver much more power for their size and weight, which is why they dominate drones and increasingly appear in robot drivetrains.
Put the relationship to work on the question that opened this article. Motor A: 12 kg·cm stall, 60 RPM no-load. Motor B: 3 kg·cm stall, 200 RPM no-load. Which is better?
Convert and compute peak power for each. Motor A: 12 kg·cm is 1.18 N·m; peak power = 1.18 × (60 × 2π ÷ 60) ÷ 4 ≈ 1.85 W. Motor B: 3 kg·cm is 0.29 N·m; peak power = 0.29 × (200 × 2π ÷ 60) ÷ 4 ≈ 1.52 W. They're within about 20% of each other — because they are, most likely, the same motor behind different gear ratios. Neither is "stronger"; they're the same power packaged differently, and the right choice depends entirely on whether your robot needs to climb or to travel.
Now the useful follow-up: if Motor B is what you have and Motor A is what you need, a 3.3:1 reduction converts one into the other, minus gearbox losses. That's the practical payoff of thinking in power — it tells you when a gearbox will solve your problem and when it won't. If you need more power than either motor makes, no ratio will help, and the answer is a bigger motor or a higher voltage. Distinguishing those two situations correctly saves a great deal of money spent on the wrong fix.
One curve, three views, and a gearbox that slides you along it. Convert whatever units your spec sheet arrived in with the Torque Unit Converter — and if you'd rather trust a measurement than a listing, measuring torque at home takes about twenty minutes.