To find out how much water you have, measure the inner part of your tank, so you'll discard the glass thickness. Do this with lenght and width. As for height, measure from the bottom to the water surface (not the border of the wall). You'll them multiply the 3 values and you'll get the almost exact amount of water you have. (Remember that gravel, filters, ornaments and fishes have their volume. You'll have to take this into consideration if you have too many things into your aquarium and you are planning on medicating).
If you do it in meters, multiply the value by 1000 and you'll get liters. (Divede by 3.785 to get gallons)
If you measure inches, divide the total by 231 and you'll get the gallons right away.