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Easy way to measure motor torque

Motor torque (momentum) is one of the motor ratings which is used to indicate rotary motor force produced on its output shaft. Motor torque rating is usually captured at the point when the motor is stalled. The torque measurement unit is (Nm) in the metric system or (ft-lbs) in the US system. But the principle is the same torque is a multiplication of lever length (r) and force(F) applied to lever:

Ï„=r x F

So there is a simple experiment on how to measure motor torque quite accurately without any fancy measuring tools.


All you need is a motor of which torque will be measured, a nominal power supply of motor, thread long enough, a mass of known weight, ruler and a table or other lifted surface where the motor will be fixed.

Attach mass to one end of the string and another to motor shaft so that it could turn the windings. Fix the motor at the edge of the table so that string with mass would hang. Start the motor and led the rotating shaft to wind the thread while lifting the mass. In the beginning, the motor will turn very easily but later when the radius of winding will increase so the stopping force will increase also. And at last, the motor will stall. So this is the moment when you have to stop the motor and measure the radius of winding. Then take the formula and calculate the torque. For instance if mass m=0.1kg; radius of winding r=0.01m; acceleration due to gravity g=9.8m/s2.

τ=0.01 · 0.1 · 9.8=0.0098 ≈ 0.01(Nm)

if the motor will wind up all thread try to attach heavier mass. This technique will help to compare different motors or test motors torque and compare to documentation to see how reliable is this technique.

Robots Builder Guide 1992 – Fred G. Martin


  1. hi
    0.001 * 0.1 * 9.8 = 0.0098 that good
    but from where you got 0.01(NM)???

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