Remember that any aquarium decorations will occupy space that would otherwise be used by water. So if for whatever reason you want to know exactly how much water is in your tank at a given time, you need to subtract the water displaced by the decorations. That would be a hassle, but I can explain it if that's what you want to do...

But if you just want to be able to say "I have a 29 gallon tank" then no need to stress over displacement. The above posting gave you the metric. If you want gallons not liters then do the same length x width x height with measurements in inches. That will give you cubic inches. You can type that number in

here to get gallons.