How to measure your tank
Measure the inside length, width (front to back), and height of the tank in centimetres or inches, then read off the volume in both litres and US gallons. Multiplying the three dimensions gives the total glass volume; in practice a tank holds a little less once you account for the rim, substrate, and equipment, so treat this figure as the full-to-the-brim maximum. For stocking, use the real filled water volume in the stocking calculator.
This calculator is for rectangular tanks, which are by far the most common. For a bow-front or cylindrical tank the true volume is smaller than the bounding box, so the rectangular figure will read slightly high.