A tank sold as 55 gallons that measures 48 by 13 by 21 inches on the outside holds 53.3 US gallons to the rim and 50.8 gallons at a water line an inch below it. Put in 2 inches of gravel and the water you actually own is 48.2 gallons, 182 litres, or 11,133 cubic inches. Filled and decorated it weighs about 538 lb, which is 124 lb on every square foot of floor it covers. The calculator does that sum for eight tank shapes and shows each step under the result.
How aquarium volume is calculated
Every tank here is a prism: a footprint area multiplied by a water height. A rectangle is length times width, a cylinder is pi over four times the diameter squared, and the curved shapes use the same areas that any geometry table gives. One US gallon is 231 cubic inches, or 3.785 litres, so inches turn into gallons by dividing by 231.
Gallons = length (in) × width (in) × water height (in) ÷ 231
Measure the outside if that is easier and switch the toggle to "Outside of the glass". The calculator then takes one pane of glass off each side and off the bottom, using the glass thickness you enter. For the 55 gallon example, 48 by 13 by 21 inches outside with 1/4 inch glass becomes 47.5 by 12.5 by 20.75 inches inside. With the water 1 inch below the rim the water height is 19.75 inches: 47.5 × 12.5 × 19.75 ÷ 231 = 50.8 gallons.
Net water after substrate and rock
Substrate does not remove its full volume from the tank, because water fills the gaps between grains. Only the grains displace water. The calculator takes the bulk density from the maker (CaribSea publishes 96 lb per cubic foot for its natural gravel and 85 for aragonite reef sand) and divides by the mineral density: aragonite is 2.94 times as dense as water and quartz 2.65. That gives grain shares of roughly 46% for reef sand and 60% for quartz gravel. For products that do not publish a grain material the share starts at 50% and you can change it.
For the example, 2 inches of substrate over the floor is 5.1 gallons of bulk gravel, half of it grain, so 2.6 gallons displaced and 48.2 gallons of net water. Rock, wood and equipment go in the separate volume field. Dunk them in a bucket and measure the water they push out, since the volume of a rock is hard to guess.
Volume by tank shape
| Shape | Size | Volume |
|---|---|---|
| Rectangle | 47.5 × 12.5 in inside, 19.75 in water | 50.8 gal |
| Cube | 24 in on every edge | 59.8 gal |
| Cylinder | 24 in across, 30 in high | 58.8 gal (222 L) |
| Bowfront | 36 in long, 14 in at the ends, 18 in at the bow, 16 in high | 41.6 gal |
| Cut corner | 39 × 27 in, 12 in cut, 23 in high | 97.7 gal |
| Quarter round | 24 in walls, 24 in high | 47.0 gal |
| Hexagon | 24 in across the flats, 24 in high | 51.8 gal |
| Half round | 36 in wide, 18 in deep, 20 in high | 44.1 gal |
A bowfront is a rectangle plus a circular segment: the calculator finds the circle through the two front corners and the widest point of the bow. Enter the bow depth as the total front to back; it must exceed the end depth.
Filled weight and floor load
Water weighs 8.34 lb per US gallon, so the 48.2 gallons of net water in the example are 402 lb. Glass is 2,500 kg per cubic metre (156 lb per cubic foot), which puts the panes of this tank at 70 lb, and 2 inches of gravel adds 66 lb. Add the stand, hood and equipment in the last field if you want the whole load.
Reef Builders quotes typical residential live loads of 40 lb per square foot on a ground floor, 30 upstairs and 20 in an attic. Those figures are averages for a whole room. A tank concentrates its weight on its own footprint: the example is 124 lb per square foot, above all three. That is normal for an aquarium and why the advice is to place it over a wall or across several joists, and to ask a structural engineer before putting a large tank upstairs.
Glass thickness
The page shows a guide table by water column height rather than a thickness calculation. No table we could read comes from a design standard we can cite. Use it to sanity check a build or a purchase, and ask the maker or a glass fabricator for anything custom. Unbraced tanks can need up to double the thickness.
Next steps
Once you know the volume, the fish tank calculator sizes the heater and filter for it, the aquarium substrate calculator turns your depth into bags, and the salinity calculator covers salt mix for a marine tank. To see how many fish the water can carry, use the aquarium stocking calculator. All the tank tools are on the aquarium hub.