AquariumCalcs: Fin & Figures — for the fish aficionado
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Tank Volume Calculator

Enter your tank's inside dimensions to get exact gallons and liters — plus a realistic "as filled" volume once substrate, decor and headspace are accounted for.

Your numbers
Diameter, if cylinder
Accounts for substrate, decor and headspace below the rim
Full (to the rim)
Realistic, as filled

Measure to your real fill line, not the rim

Measure at the water line you'll actually fill to, not the rim — most tanks are filled an inch or two below the top edge to prevent splashing during water changes.

How this calculator works

For a rectangular tank, volume is length × width × height, divided by 231 (the number of cubic inches in one US gallon). For a cylinder or bowl, it's π × radius² × height ÷ 231. Enter the height of the water line you actually plan to fill to, not necessarily the full height of the glass — most tanks are filled an inch or two below the rim.

Why the "realistic" number is lower

The full-to-the-rim figure is the number printed on the box, but it's rarely the number of gallons actually holding water and fish. Substrate, rockwork, driftwood and the gap most keepers leave below the rim to stop splashing all displace water. A 90% fill factor is a reasonable planning default for a lightly-decorated tank; heavily aquascaped or reef tanks with a deep sand bed and lots of rock can realistically end up closer to 75–80% of the on-paper volume.

What this number is used for

Aquarium volume is the input every other calculator on this site builds from — heater wattage, dosing amounts, stocking levels and water-change volume are all expressed per gallon or per liter. Getting this number right first makes everything downstream more accurate.

Source: The US gallon is defined as exactly 231 cubic inches (the standard US customary liquid volume unit) — this calculator applies that definition directly to your entered dimensions.

Frequently asked questions

Why is my tank's real capacity less than the size printed on the box?

The advertised size (e.g. "55 gallon") is the full-to-the-rim volume of the empty glass. Once substrate, decor and a safe headspace below the rim are in place, the actual water volume is typically 80–92% of that figure.

Do I measure the outside or inside of the glass?

Always measure the inside dimensions. Glass thickness on a large tank can be half an inch or more per pane, which adds up to a meaningfully smaller inside volume than the outside dimensions suggest.

How do I calculate the volume of a bowfront or corner tank?

Bowfront and irregular tanks don't reduce to a single clean formula. The closest estimate is to measure it as a rectangle using the average of the front bow's minimum and maximum depth, which typically comes within a few percent of the true volume.

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