It is a minor point but one that can have large effects when calculating doses when adding medication to water.
The first point to take into consideration is you never fill the tank to the brim, so you need to take an inch (2.5cm) or so of the height of the tank. Then you have the thickness of the glass to subtact, so if your tank has 5mm glass you need to subtact 10mm from the length and 10mm from the width, and only measure the height from the top of the base glass. An easier way is to just measure the size on the inside of the tank from the inner walls, still remembering to subtract in inch from the height.
So if the external measurements of just a small tank as follows:
50cm (L) x 25cm (W) x 28cm (H) the tank volume would be 35litres
A lot of difference in what seems a very small change in overall size. This calculatiing process becomes more vital when dosing medication as it can make a big difference to the amount of medication given. This becomes more apparent when the tanks become bigger and the glass gets thicker as the tank dimensions grow.
For water volume always measure the internal dimensions, the external dimensions are only any good to know if it will fit in that alcove in the lounge...
As another example take a 6 foot tank 72" x 26" x 26":
180cm x 66.5cm x 66.5cm = 796 litres
182.4cm (L) x 68.9cm (W) x 75cm (H) = 942.5 litres
If you calculated by external measurements you would calculate medicine by 942.5 litres and overdose by 146 litres which could lead to the death of your fish.