Consider two equal and opposite forces F and F acting oppositely along parallel lines on two points A and B as shown below: 

Let and are their position vectors with respect to origin.




Magnitude of torque is given by: 

= r F sinq


Where q is the angle between and . 

rsinq is the perpendicular distance between the line of action of the two forces. Let it is denoted by d. 

Thus the magnitude of the torque of couple will be: 

= Fd


Where d is called the moment arm of the couple. 

Now the magnitude of the couple = Magnitude of any of the forces forming couple X moment arm of couple 