One of the most frequently asked questions in the aquarium world is “what size is your tank?”. Many people either 
forget, never really knew or have a custom size which needs to be calculated. So in order to help you in answering that question with accuracy…the formula to find the number of gallons for a rectangle aquarium is pretty simple. Length x Width x Height (in inches) divided by 231 . A cylinder would be ∏ x radius squared x height divided by 231.
The formula to find the number of gallons in a rectangle aquarium is pretty simple: Length x Width x Height (in inches) divided by 231.
We use the internal dimensions to come up with the volume in gallons, which makes the calculation even more accurate. We subtract the thickness of the glass, the difference from the top of the tank and the actual operating height of the water to come up with this more precise volume. In addition, we take into account the displacement from the rock, substrate & decor when figuring the total operating volume for any system.