Aquarium Volume Calculator

How the calculation works

Everything runs in cubic inches. One US gallon is 231 cubic inches exactly, and one gallon is 3.785411784 liters 1. If you enter centimeters, the calculator converts at 2.54 cm per inch 1 first. Every tank here is a straight-sided prism, so the volume is the floor area times the height.

• Rectangular: length × width × height ÷ 231. A 20 × 10 × 12 in tank: 2,400 ÷ 231 = 10.4 gal.
• Cylinder: π × (diameter ÷ 2)² × height ÷ 231. A 10 in wide, 12 in tall cylinder: 4.1 gal.
• Regular hexagon: (3√3 ÷ 2) × side² × height ÷ 231. The first part is about 2.598. A 6 in side and 10 in height: 4.0 gal. All six sides have to be equal.
• Bow front (estimate): floor area = length × end width + ⅔ × length × (middle width − end width), then × height ÷ 231. The curve is treated as a parabola, which is close to a real bow but not identical. We found no manufacturer formula for it, so this is our own approximation. A 36 in long tank, 15 in at the middle, 12 in at the ends and 16 in tall: 34.9 gal.

Measure inside the glass

Outside measurements include the glass, and sometimes trim, so they make the tank look bigger than the space for water. Measure from glass to glass. For the real water volume, measure the height up to the water line, or use the gap field to say how far below the rim you fill.

Here's how much it matters. Aqueon lists its 20 Long at 30¼ × 12½ × 12¾ inches, outside 3. Put those numbers through the formula and you get 4,821 cubic inches, about 20.9 gallons. That figure is our own calculation from outside measurements, not an Aqueon number. It's already above the 20 in the tank's name, and the water space inside the glass is smaller still. We don't know how Aqueon arrives at the 20, so we don't claim to match it.

Real water volume

Water depth is the tank height minus the substrate depth minus the gap at the top. The calculator then uses that depth in the same formula. It treats the substrate as solid, so it undercounts slightly: water also sits between the grains, and we have no sourced figure for how much, so we don't guess. Rocks, wood and decorations aren't counted either. Measure those separately if you need to account for them.

Water weight

The USGS gives the weight of water as 8.33 pounds per US gallon, or 1 kilogram per liter 2. We multiply the gallons by 8.33 and the liters by 1. That's the water alone. A filled tank also weighs the glass, the substrate, the rocks and the decorations, and those depend on your setup, so the calculator doesn't try to total them. Whatever the tank sits on needs to carry all of it.

Worked example

Take a tank that is 36 inches long, 18 wide and 16 tall inside, with 2 inches of sand and water filled to 1.5 inches below the rim.

1. Tank volume. 36 × 18 × 16 = 10,368 cubic inches. Divided by 231, that's 44.9 gallons, or 169.9 liters.
2. Water depth. 16 − 2 − 1.5 = 12.5 inches.
3. Real water volume. 36 × 18 × 12.5 = 8,100 cubic inches. Divided by 231, that's 35.1 gallons, or 132.7 liters.
4. Water weight. 35.1 × 8.33 = 292.4 pounds, or 132.7 kilograms.

The glass box is 44.9 gallons, but the water in it is 35.1. That's 9.8 gallons less, so anything you dose by volume should use 35.1.

Standard tank sizes

If you're checking a tank you own, or shopping for one, here are six common sizes from Aqueon's standard glass line, with the dimensions Aqueon publishes 3.

These are outside measurements: the diagram in the document marks length, width and height on the outside of the tank. The document is dated April 2023 (last modified April 24, 2023), and Aqueon links it as current on its standard glass aquarium page as of October 8, 2026. For the water space, measure the inside of your own tank.

Why the real volume matters

Dosing. Water conditioner and medication labels dose per gallon of water. Dose for the 44.9 gallons in the example and you'd be treating water that isn't there. Follow the label, with the real volume.

Water changes. A 25% change is a quarter of the water, not a quarter of the tank. In the example that's 8.8 gallons, not 11.2. Our water change guide covers how often to do it.

Cycling. The fishless cycle calculator dose depends on volume. It asks for tank gallons and a displacement percentage for gravel and rocks. If you type the real water volume from this page, set displacement to 0 so the substrate isn't subtracted twice. If you type the full tank volume, leave displacement to do the job.

Heater and filter. Heaters and filters are sold by tank size, so you need your own number to read the box. The heater size chart is arranged by gallons, though room temperature matters more than the gallons.

Fish. Stocking isn't a formula, and we won't give you one here. How many fish fit depends on the species and the floor space. Our stocking guide shows how to check.

How we got these numbers

Every constant on this page comes from a published source we opened on October 8, 2026. Nothing else is assumed.

• 231 cubic inches per gallon, 3.785411784 liters per gallon and 2.54 cm per inch: NIST Handbook 44, Appendix C (2026). Consulted October 8, 2026 1.
• 8.33 pounds per gallon and 1 kilogram per liter: USGS Water Science School, Facts About Water. Consulted October 8, 2026 2. The page is dated August 24, 2026. It's a single round figure for water, so treat the weights as approximate.
• Aqueon tank sizes and outside dimensions: Aqueon, Information, page 66 (PDF). Consulted October 8, 2026 3. Dated April 2023, so older than three years; we use it because Aqueon still links it as current. Dimensions are copied as printed, and the 20.9 gallon figure is our calculation.
• Cylinder, hexagon and bow front areas: standard geometry, no outside source. The bow front formula is our approximation, as explained above.

The math lives in one small file and every shape is checked by tests against hand-worked cases, such as 20 × 10 × 12 inches = 2,400 cubic inches = 10.4 gallons.

FAQ

How many gallons is my fish tank?

Measure the inside length, width and height in inches, multiply the three, then divide by 231. A tank measuring 20 × 10 × 12 inches is 2,400 cubic inches, which is 10.4 US gallons.

How do I convert gallons to liters?

Multiply US gallons by 3.785411784. Ten gallons is 37.9 liters. Going the other way, divide liters by 3.785411784.

How do I calculate the volume of a round, hexagonal or bow front tank?

Work out the floor area, multiply by the height, then divide by 231 for gallons. A cylinder’s area is π × (diameter ÷ 2)². A regular hexagon’s is about 2.598 × side². A bow front is an estimate: length × end width, plus two thirds of length × (middle width − end width).

How much does a gallon of water weigh?

The USGS gives 8.33 pounds per US gallon, or 1 kilogram per liter. That is the water only. A filled tank also carries the glass, the substrate and everything you put in it.

Do I subtract gravel and decorations from the tank volume?

For the water you actually have, yes. This calculator takes off the substrate depth and the gap at the top. Rocks, wood and decorations aren’t counted, so the real figure is a bit lower still.

Should I measure inside or outside the glass?

Inside. Outside measurements include the glass, so they overstate how much water the tank holds. Measure from glass to glass, and to the water line if you want the water volume.