Efficiency is mechanical power out divided by electrical power in. Enter the working point of your motor, not its no-load or stall figure.
Use values measured at one working point under load. Efficiency changes with load.
How it is calculated
Electrical power = voltage × current. Mechanical power = torque × angular speed, where angular speed in rad/s = RPM × 2π ÷ 60. Efficiency = mechanical power ÷ electrical power. 1 N·m is about 10.197 kg·cm.
A worked example
At 6 V and 0.8 A the motor takes 4.8 W. At 200 RPM and 0.1 N·m it delivers 0.1 × 200 × 2π ÷ 60 = 2.09 W. Efficiency is 2.09 ÷ 4.8 = 43.6%. The torque is about 1.02 kg·cm.
Common mistakes
- Using stall current with no-load speed. They never happen together.
- Ignoring that the gearbox adds losses.
- Mixing N·m and kg·cm.
Related
gear train calculator, wheel speed calculator, battery runtime calculator.
Last reviewed: November 2026. The calculator runs in your browser and sends nothing anywhere. Results are ideal-case figures; add margin before you order parts.
How to read the three outputs
The calculator gives electrical power in, mechanical power out and efficiency. The difference between the first two is lost as heat in the windings, brushes and bearings. A motor that shows low efficiency at your working point is either badly matched to the load or running far from the point it was designed for.
A second worked example
A 12 V motor draws 1.5 A while turning at 300 RPM and delivering 0.4 N·m.
- Electrical power in = 12 × 1.5 = 18 W.
- Angular speed = 300 × 2π ÷ 60 = 31.42 rad/s.
- Mechanical power out = 0.4 × 31.42 = 12.57 W.
- Efficiency = 12.57 ÷ 18 = 69.8 percent.
- Heat lost = 18 − 12.57 = 5.43 W.
- Torque in kg·cm = 0.4 × 10.197 = 4.08 kg·cm.
How to get the input values
Measure current with a multimeter in series or a current-sensing module while the motor drives its real load. Measure speed with a tachometer, an encoder or by counting turns over a known time. Torque is the hardest value. You can estimate it from a known weight on a lever arm (torque = force × distance, where force in newtons is mass in kg × 9.81), or take it from the datasheet curve at that speed and current.
Assumptions and limits
- Efficiency is not constant. For a brushed DC motor it typically peaks at a modest load, not at stall and not at no load. Datasheet curves show this.
- If the motor has a gearbox, decide whether you enter the motor shaft values or the output shaft values, and be consistent. Using output torque with motor-side current mixes gearbox losses into the figure.
- A cheap hobby motor may have a datasheet that lists only no-load and stall values. In that case measure your own working point.
Common mistakes
- Reading current from the supply that also powers the controller and sensors.
- Using a torque from the stall specification at a speed that is not stalled.
- Quoting efficiency from one reading as if it applied at every load.