How much water is in your aquarium?

What the number means

A tank sold as 55 gallons is a name, not a measurement. The glass has thickness, the water stops below the rim, and the substrate and rocks take the water's place, so the water you really own is smaller than the gallons printed on the box. That water volume is what you need for dosing medication, sizing a heater or filter, and planning how much to change. The volume mode finds it step by step and the other two modes use it: how much to change each week, and whether the number of fish fits the tank.

How the volume and weight are found

Volume in US gallons is length × width × height in inches ÷ 231, since a US gallon is 231 cubic inches or 3.785 liters (NIST Handbook 44). The height used is the depth of the water, the inside height minus the gap below the top. The substrate layer is length × width × depth, the same formula, and is subtracted whole; rocks and wood are subtracted as a percentage of what remains. Weight is water at 8.32 pounds per gallon at 77 °F (USGS lists 62.300 pounds per cubic foot at 70 °F and 62.217 at 80 °F, and a cubic foot is 1,728 ÷ 231 gallons) plus the substrate, decor and empty tank weights that you enter.

Worked example

A tank 48 inches long, 13 wide and 20 high inside holds 54 gallons to the rim. With the water 1 inch below the top the space to fill is 51.3 gallons. A 2-inch layer of substrate takes 5.4 gallons, and rocks and wood at 5 percent take another 2.3, leaving 43.6 gallons of water, or 165 liters. That water weighs 363 pounds; with 40 pounds of substrate and decor and a 50-pound tank, the filled aquarium weighs 453 pounds. A tank that is rated 55 gallons on the box thus holds about 44 gallons of usable water here.

How the weekly water change is found

Fish and feed add nitrate to the water; a water change removes a fraction of what is there and replaces it with tap water. Say nitrate rises by P mg/L every week and you change a fraction f once a week with tap water at T. After many weeks the level settles into a cycle, and the level just before each change is P ÷ f + T. Solving for the fraction that keeps that peak at or under a ceiling C gives f = P ÷ (C − T). In the example, P = 10, C = 40 and T = 0, so f = 0.25, about 25 percent, or 10.9 gallons of the 43.6. If your tap water already carries 10 mg/L of nitrate, the same ceiling needs 33 percent. Where P is large and C − T is small the fraction passes 100 percent, and the calculator says so rather than printing an impossible number: with P = 30, C = 40 and T = 20 it would be 150 percent. The model assumes one change a week and a well-mixed tank, and the peak you get is only as good as the weekly rise you measured.

Where the water change advice comes from

Extension and agency advice is a range, not a formula. Penn State's classroom aquarium lesson drains about a third of the water at first and then cuts the weekly change to about one quarter of the tank. Connecticut DEEP says to change 20 to 50 percent every two to four weeks. UF/IFAS gives 25 to 50 percent as a response to an ammonia spike and warns that nitrate builds up in closed systems. The mass-balance mode lets you see what a given habit does to your own tank: a 25 percent weekly change holds a tank that gains 10 mg/L a week to 40 mg/L before each change, and a 10 percent change lets it climb to 100.

Limits of the inch per gallon rule

Connecticut DEEP, a state agency, calls an inch of fish per gallon a rule of thumb that is pretty close for most of the state's freshwater fish. The arithmetic is simple: six fish that will grow to 2 inches add up to 12 inches, which is exactly the rule for a 20-gallon tank, and ten more inches already living there would put the total at 22, 2 inches over. Three limits keep it from being a standard. First, it counts length, but the waste a fish makes depends on what it eats: UF/IFAS names overfeeding as a major cause of high ammonia, and a heavy-bodied fish of the same length eats and excretes more than a slim one. Second, it must use the adult length, which is why the field asks for it. Third, nothing in it describes oxygen, filtration or the space a swimming species needs; Connecticut DEEP itself recommends an external filter and an air pump with an air stone. Treat a result near or over one inch per gallon as a reason to look at the fish you plan to keep, and not as permission or a ban.

How many gallons is my fish tank?

Multiply the inside length, width and height in inches and divide by 231, the cubic inches in a US gallon (NIST Handbook 44). A tank measuring 24 × 12 × 16 inches inside holds 19.9 gallons to the rim. The size on the box is usually rounded and often measured outside the glass, so measure the inside yourself; this calculator also takes off the space above the water line, the substrate and the decor to give the water you can really use.

How much does a filled aquarium weigh?

Water at 77 °F weighs about 8.32 pounds per gallon, found from the USGS density and weight of water table. The 43.6 gallons of water in the example above weigh 363 pounds, and adding 40 pounds of substrate and rock and a 50-pound empty tank gives 453 pounds. The calculator uses the empty weight and the substrate weight you enter, because they depend on the tank and the material; check the maker's rating of the stand and your floor before you fill a large tank.

How much water should I change each week?

It depends on how fast nitrate builds up, which you can measure with a test kit. Penn State's classroom aquarium lesson cuts weekly changes to about one quarter of the water once the tank is established. Connecticut DEEP, a state agency, says changing 20 to 50 percent of the water every two to four weeks keeps it in good condition. The mass-balance mode finds the fraction from your own numbers: with nitrate rising 10 mg/L a week, a 40 mg/L ceiling and nitrate-free tap water, 25 percent a week is enough.

What nitrate level is safe for fish?

There is no single number. UF/IFAS says nitrate has traditionally been considered non-toxic to fish even at levels up to 200 mg/L, but that more recent research shows it may be more harmful than believed, and that in closed systems with little water exchange it can pile up above 250 mg/L, which is why water changes are needed. Because the documents we opened set no aquarium ceiling, the calculator asks you to choose one. The 40 mg/L in the field is an example, not a rule.

Does the one inch of fish per gallon rule work?

Connecticut DEEP calls it a rule of thumb that is pretty close for most of the state's freshwater fish. With six fish that will each grow to 2 inches, the arithmetic gives 12 inches for a 20-gallon tank, 0.6 inch per gallon, and no room for more. It is a starting point and the next section explains where it breaks.

Do I subtract the gravel from the tank volume?

Yes, if you want the water volume, for example to dose a medication or plan a water change. Gravel, rocks and wood take the place of water. The calculator subtracts the whole substrate layer, which slightly overstates the loss, because water also fills the spaces between grains, and a percentage of the water for rocks and wood that you estimate. The truth lies between your result and the volume with no substrate at all.

Sources: NIST Handbook 44, Appendix C (US gallon = 231 cubic inches = 3.785411784 liters); USGS Water Science School, “Density and weight of water” (table, July 18, 2018) and “Water density”; UF/IFAS Extension EDIS FA031, “Ammonia in Aquatic Systems” (Francis-Floyd, Watson, Petty, Pouder; revised June 2022); Penn State Extension, “Your Own Freshwater Aquarium in the Classroom” (Saylor); Connecticut DEEP, “Home Aquariums” (freshwater fishes of Connecticut; a state agency page, not a university publication), all read October 28, 2026. The nitrate mass balance is our own arithmetic on stated assumptions. For a pond rather than a tank see the pond calculator; for round and box tanks of any kind the tank volume calculator; for the room it stands in, the volume calculator and water heater calculator. How we check our calculators.