1. Measure the weight of the object, in pounds, using a scale. For example, you might have a 50pound cement block. 2. Determine the length and width, in inches, of the bottom surface
http://www.ehow.com/how_8472036_calculateforceps...

by using pythagoras therom and using the the cosine rule pounds per square inch is really the result of an elephant.
http://wiki.answers.com/Q/How_do_you_calculate_psi

Formula for calculating PSI: Force = Area * Pressure. May I assist you further?
http://www.chacha.com/question/whatistheformula...

As long as the amount of air has not changed, and neither did the Temperature, then the pressure inside the structure is inversely proportional to the volume of the structure, or:
http://answers.yahoo.com/question/index?qid=201309...
