How do you calculate the compressive strength of a structural steel beam w150 x 30?

I think you mean beam W30x150; where 30 is the nominal depth in inches, and 150 is the weight in pounds per linear foot. To answer the question about the compression strength, it is all relative to the type of steel, the length of the structural member, and how it is fixed at either end. Is it A-36 or A-50 steel; is it 10 ft. long, or 20 ft. long; is it pinned (bolted) or fixed (welded)? These unknown variables must be addressed before this theoretical value can be calculated.