Here ,
here , and
here for explosions.
You can find help on calculations if you go to Guidelines -} Policies. There's a lot of good stuff there.
Also, pixel measurements work by knowing something's real size. For example, in a reccent calc I did with pixel measurements, I used Mr. Incredible's canonical height of 200 cm and in-video height of 372 pixels to scale up to everything in the area. When you have measurements like that, divide the real height by the pixel height (200/372) and you get the width of a pixel in centimeters. Using that, you can measure everything in frame, and scale up to almost anything in the area.