How to measure your tank
Measure the inside length, width (front to back), and height in centimetres or inches, then read off the volume in both litres and US gallons. Measuring the outside is the most common mistake: aquarium glass runs roughly 6 mm on a small tank and 12 mm or more on a large one, and across three dimensions that error compounds into several percent.
The arithmetic, if you want to check it
Volume is simply length × width × height. The conversions are where people slip. In metric, centimetres multiply out to cubic centimetres, and there are exactly 1,000 cm³ in a litre, so divide by 1,000. In imperial, inches multiply out to cubic inches and there are 231 in³ in a US gallon, so divide by 231. A 36 × 18 × 16 inch tank is 10,368 in³, which is 44.9 US gallons - the tank sold as a "45 gallon".
Glass volume is not water volume
This calculator gives the full-to-the-brim figure, and your tank never holds that. You lose the freeboard between the waterline and the rim, typically an inch or two. You lose the substrate, which is pure displacement: two inches of sand across a 36 × 18 inch footprint is around 1,296 in³, or 5.6 gallons gone before a single fish arrives. You lose rock, wood and equipment. In practice a planted tank with a substrate bed holds roughly 10 to 20% less water than its rated volume.
That gap matters in two places, and one of them is dangerous. For stocking, using the rated volume makes you overstock by exactly the amount you have displaced - so use the real filled figure in the stocking calculator. For dosing, it is a safety issue: medications, dechlorinator and fertilisers are dosed per unit of water, and dosing a 45-gallon tank that really holds 38 gallons is an 18% overdose. With most fertilisers that is harmless. With copper-based medication it is not.
Tanks that are not rectangular
A bow-front tank sits between a rectangle and a rectangle-plus-half-cylinder, so the rectangular figure using the maximum depth reads high - the usual working estimate is a few percent under the bounding box. A cylindrical tank is π × radius² × height, which is about 79% of the square box it would sit in; using the rectangular calculator on a cylinder will overstate the volume by roughly a fifth. Hexagonal and corner tanks vary too much for a rule of thumb.
When the shape is awkward and the number matters, measure it directly rather than calculating: fill the tank with a container of known volume and count. It is tedious once and correct forever.