Aquarium Volume Calculator

Which formula does this aquarium calculator use for each tank shape?

For a rectangular tank it multiplies length × width × height in centimetres and divides by 1,000. For a cylinder it uses π × (diameter ÷ 2)² × height ÷ 1,000. For a bow-front tank it takes the rectangular volume and multiplies it by 0.80, a fixed approximation built into this tool. Whichever shape you pick, the result is then multiplied by the fill percentage.

Does it show the tank’s full capacity or the water it actually holds?

It shows water volume, meaning full capacity reduced by the fill level you enter. A 60 × 30 × 36 cm tank has 64.8 L of glass volume, and at the default 90% fill the tool reports 58.3 L. It does not subtract anything for substrate, rocks or wood, so measure your real water volume if you need precision for dosing.

How are US gallons worked out?

This tool multiplies litres by 0.264172 and rounds to one decimal. That is the US gallon, not the imperial gallon, which is larger. The default tank of 58.3 L therefore shows as 15.4 gal.

How does the fish count estimate work?

It is a simple division of the water volume. Small fish of about 2.5 cm count as one per 4.5 L, and medium fish of about 5 cm as one per 9 L, each rounded down. For 58.3 L that gives up to 12 small fish or up to 6 medium ones. It knows nothing about species, adult size, temperament, bioload or the filter, so treat it as a ceiling to compare against, not a recommendation.

What happens with a blank or zero fill level, or invalid dimensions?

Clearing the fill box counts as 0, and a fill level of 0 gives 0 L; the 90% figure is only the starting value. Values above 100 are capped at 100, and negative or non-numeric dimensions are treated as 0, which produces a volume of 0 L. The tool does not display an error message, so a result of 0.0 L means one of your inputs was cleared or zero.

Which inputs are ignored for each shape?

Rectangular and bow-front calculations ignore the diameter field. Cylindrical calculations ignore length and width and use only diameter and height. Make sure the unused fields do not confuse you when you switch shapes, because their old values stay in the form.

What is the stocking note based on?

Only on the water volume: under 40 L it describes the tank as very small and suited to nano fish, from 40 L up to just under 100 L it describes a small community tank, and 100 L or more it describes a community aquarium and mentions filter turnover. These are fixed text bands in the code, not an assessment of your setup.

What does it not do?

It does not account for glass thickness, so measure inside dimensions if you want a close figure. It does not model sumps, corner or hexagonal tanks, or the water displaced by decor. It does not check filter ratings, heating, water chemistry or whether particular species can live together.