How many gallons of water does a 30 foot round pool that is 48 inches deep hold?

Hi~ I just read on the back of a chlorine bottle by which you can get an approximate for a round pool. Try: diameter x diameter x depth x 5.9 gallons. I happen to know how much my pool holds, and did the math to see how close it came. It was about 9 gallons off. I would think if you have a rectangular pool you could probably use good ole math and replace the two diameters with length and width and finish the equation. Good luck! dahl