How it works
A rectangular tank's volume is length × width × height. In inches, divide by 231 to get US gallons. In centimetres, divide by 1,000 to get litres.
Measure the outside and the answer runs slightly high, because glass has thickness and nobody fills a tank to the brim. Substrate, rock and wood take up room too. Expect to hold about 10% less water than the number here.
This works for rectangular tanks only. Bow-front and hexagonal tanks need the maker's stated volume.