Don't forget to factor in glass thickness for actual volume.
Ex: 24" x 12" x 24" ....say glass is 1/2 inch thick per wall. Your calculation would be more like 23" x 11" x 23".
There is a site that actually has a calculator that factors in actual gallons. If I can find it I'll post it here. It allows you to put in the wall thickness of your tank.
Its funny that people pitch a fit about the gallons not being true. Of course not! Manufacturers use the outside dimensions, lol. There was a court case on TV where a bone headed dude wanted to sue a guy for selling him a tank that had less actual gallons than advertised.