OH, it is so much easier to use the calculator in the header above. Using your brain is exercise, and I for one, am on strike!!!
Click on articles up at the top of the page, and at the bottom of the list is the calculator. Just plug in the dimensions of your tank and PRESTO, a 20 gall is easy, what hurts my head is trying to figure the volume for a
DIY project, when you know about what you want to build, but then have to know how many gallons, to be able to figure filtration and weight etc..........