Most water tanks will be labled by size. That means how many gallons the tank hold is what it is called. A 100 gallon tank will be sold as such and a ten gallon tank will be labled that way. So just check the labels. Most water tanks hold about thirty gallons of water. Of course the bigger the tank the more water it will hold. And you can purchase apartment sized water tanks that will hold a lot less then the average water tank.