The clamping force on a bolt is defined as the compressive force a fastener exerts on a joint. This is can be dependent on a number of factors, but is largely a function of the preload force, the force exerted in compressing the bolt, and the tension force, the force exerted in pulling the bolt apart. Calculating a bolt's clamping force can be accomplished with knowledge of both of these variables.
Things You'll Need
 Pen or pencil
 Paper
 Calculator
 Force transducer (if force values are not already known)

Determine the preload force on the bolt. This force is caused by rotating the bolt, which stores force, much like a spring. This can be experimentally determined using a force transducer (if the value is not already known). As an example, assume the preload force is 2 lbs.

Determine the tension force on the bolt. This can also be measured using a force transducer (again, if the value is not already known). Assume the tension force is 3 lbs.

Subtract the preload force from the tension force. In our example, the tension force (2 lbs.) subtracted from the preload force (3 lbs.) gives 1 lbs. This is the bolt's clamping force.
References
 Photo Credit rusty bolt image by Thor Jorgen Udvang from Fotolia.com