Is your tower made of steel or aluminum? You should find some info on your case and then do some guess math on the hardware accessories, or just weigh it on a scale of some sort.
weigh yourself on the scale, then weigh yourself on the scale with the computer. As long as the scale is consistant (probably is) then you'll get an accurate reading. Don't just measure the box on the scale, because your scale seems to be off, and they tend not to be terribly accurate under 50 pounds.