A retailer’s capacity, geometric water space and the water physically in your setup can all be different numbers.
A model name may use a rounded nominal capacity. Geometric gross volume describes a rectangular or shaped water space. Actual water volume is what remains after the water line, substrate, rocks, wood and internal equipment are considered.
Water-change percentages, filter-turnover comparisons and some product instructions become more useful when they start from a realistic water quantity rather than a marketing label.
Original diagram supporting the gross volume vs actual water volume explanation.
A blanket 10% deduction can be a useful scenario, but it is not a measurement. Bare tanks and heavily scaped aquariums can differ dramatically. The main calculator therefore lets you choose the allowance rather than forcing one value.
If a marked 25% water change consistently requires far less than a quarter of the advertised capacity, that is evidence the working water quantity differs from the label. Consistent real-world measurements can improve the estimate.
Gross volume is useful for comparing physical tanks. Actual water volume is more useful for changes and water-based calculations. Neither number alone tells you what animals the aquarium can responsibly house.
Nominal or gross capacity is useful for identifying the aquarium and comparing manufacturer information. Actual-water estimates are useful for percentage water changes and flow calculations. Writing down which number you used prevents later confusion when two perfectly reasonable “tank sizes” differ.