volume

When you own an outdoor swimming pool, in the spring you need to "shock" it, which means add a large amount of chlorine, in order to kill all the backteria and algea that accumulated over the fall/winter/spring. But chlorine is expensive, so you don't want to add too much (and waste it). In order to figure out how much to add, you need to figure out the volume of your pool, since the label on the chlorine tells you how much to add per litre of water.