Torque is not a constant for a motor: it varies (a lot) with speed and other factors.  Let's consider simplest value: stalling torque -- which is the rotational force it generates while being prevented from turning.

Your calculations omit the efficiency of the motor, which obviously is one of the most important aspects of a motor.  You can easily see that the magnetic force generated depends on more than just geometry and how much electricity is going in; it also depends on, for example, how much energy is being converted to heat by resistance.

Consider measuring the torque in the most basic way from its definition: **torque is force (in newtons) x lever length (in metres)**.

You can do this by attaching small weights, determining the weight which just stops the motor turning.  Or equivalently, by pushing down on a small digital scales.

[![enter image description here][1]][1]  
_View along axis of motor_

Obviously, there are inaccuracies in any measurement, but this will be sufficient to distinguish, say, a motor with twice as many turns or the effect of varying the voltage on the motor or the strength of the magnets.


  [1]: https://i.sstatic.net/kLs52.png