How To Calculate The Torque Used In A Wrench ?

Torque = force x distance.

So, if you apply 100 lbs on the end of a 1 foot long wrench you have (100 x 1) 100 ft-lbs.

Of course if you wanted to do this differently: The wrench is also 12 inches long, then you would have (100 x 12) 1200 in-lbs.

To give you some perspective a typical lug nut on a wheel could need around 100 ft-lbs and a typical spark plug could need only 5 ft-lbs

Just make sure that the force you apply is applied perpendicular to the end of the wrench!