I've never done the math on it, but I'm sure it's something like concrete. The pad is supposed to be 3 1/2 inches thick, but if you bought exactly how much concrete you needed by WxLxD, you'll end up short every time.
When I figure how much gravel to buy, I keep it simple and estimate 100 feet per load. If I end up with areas that are 5 to 6 inches deep, that's fine. It's not one of those things that will hurt anything, and thicker is better.
The biggest mistake anybody can make when buying rock is to put it on too thin. Four inches really is an important number for thickness. Depending on soil compaction and drainage, it also might not even be close to enough.
Eddie