Aquarium Water Volume Calculator
Pets & AnimalsCalculate your rectangular or cylindrical aquarium's water volume, usable water range, and water weight, in gallons or liters.
Water Volume
The gross water volume of a tank at this size, in US gallons, before subtracting substrate, decorations, and equipment.
Estimated Usable Water
40.4 - 45.4 gal
A more realistic range for dosing and stocking, since substrate, rock, decorations, and equipment typically displace 10 to 20 percent of the gross volume.
Water Weight
421.1 lbs
The weight of the water alone at this volume, using freshwater's density of 8.34 lb per US gallon. Add the empty tank's own weight for total floor load.
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Pets & Animals calculators
Wondering how many gallons or liters of water your fish tank actually holds? This calculator finds your aquarium's water volume from its dimensions, supporting both standard rectangular tanks and cylindrical (round) tanks, the two most common aquarium shapes.
Enter your tank's length, width, and height for a rectangular tank, or diameter and height for a cylinder, all in inches. The math is simple: a rectangular tank's volume is length times width times height divided by 231 (the number of cubic inches in a US gallon), while a cylinder uses pi times the radius squared times the height, divided by that same 231.
Along with the main volume result, you'll get an estimated usable water range and the approximate weight of that water, both useful for real fishkeeping tasks like sizing a heater or filter, planning a stand's load, or dosing medication and salt mix accurately.
Keep in mind that your tank's rated size and its actual water volume are rarely the same number. Substrate, rocks, decorations, and equipment all displace water, and most aquarists do not fill all the way to the rim, so real usable water typically runs 10 to 20 percent below the tank's full gross volume. This calculator does not currently support bow-front, hexagon, or other non-standard shapes; for those, the gross rectangular or cylinder estimate above is the closest approximation available here.
How this aquarium volume calculator works
For a rectangular tank, volume equals length times width times height, divided by 231. For example, a common 40-gallon breeder tank measuring 36 by 18 by 16 inches works out to (36 x 18 x 16) / 231, or about 44.9 gallons.
For a cylindrical tank, volume equals pi times the radius squared times the height, divided by 231. A round tank with a 20-inch diameter (10-inch radius) and 24-inch water height works out to (3.14159 x 10 x 10 x 24) / 231, or about 32.6 gallons. Measure the diameter across the widest point of the tank, at the water line, for the most accurate result.
Every result also converts to liters with one click using the gal/L toggle next to the main result, so you can work in whichever unit your equipment, medication, or local fish store uses. If you're setting up a new tank and want to plan feeding alongside water volume, our Pet Food Portion Calculator can help with the feeding side of a new aquarium too.
Standard aquarium sizes and dimensions
If you know a tank's common name, like a "55 gallon" or a "20 gallon long," but can't measure it directly yet, this chart gives typical exterior dimensions and the estimated usable water range this calculator's own formula produces for each. Actual dimensions can vary slightly by manufacturer, so measure your own tank when you can.
| Common Tank Size | Typical Dimensions (L x W x H) | Estimated Usable Water |
|---|---|---|
| 10 gallon | 20 x 10 x 12 in | 8.3-9.4 gal |
| 20 gallon Long | 30 x 12 x 12 in | 15.0-16.8 gal |
| 20 gallon High | 24 x 12 x 16 in | 16.0-18.0 gal |
| 29 gallon | 30 x 12 x 18 in | 22.4-25.2 gal |
| 40 gallon Breeder | 36 x 18 x 16 in | 35.9-40.4 gal |
| 55 gallon | 48 x 13 x 21 in | 45.4-51.1 gal |
| 75 gallon | 48 x 18 x 21 in | 62.8-70.7 gal |
| 90 gallon | 48 x 18 x 24 in | 71.8-80.8 gal |
| 125 gallon | 72 x 18 x 21 in | 94.3-106.0 gal |
Why usable water volume matters
A tank's gross volume, the number printed on the box, is not the same as how much water is actually in it day to day. Substrate, rockwork, driftwood, and equipment like heaters and filters all displace real water, and most aquarists leave a gap below the rim rather than filling completely. Using the gross number instead of a realistic usable-water estimate can lead to over- or under-dosing medication, fertilizer, or salt mix, and to under- or over-stocking fish relative to the water your tank actually holds.
As a rough starting point, many fishkeepers use about one inch of adult fish length per gallon of usable water for small, calm community fish, though this rule of thumb ignores bioload, filtration, and adult fish size, and is treated as an oversimplification by many experienced aquarists. Water changes, filtration capacity, and species-specific needs all matter as much as raw volume. If accurate medication or treatment dosing is part of why you're calculating volume, our Vet Medication Dosage Calculator can help with the dosing side once you know your real water volume.
Aquarium and stand weight
Water alone weighs about 8.34 lb per US gallon, so a full 55-gallon tank holds roughly 460 lb of water before adding the glass or acrylic tank itself, substrate, rock, and equipment. For any tank over about 40-55 gallons, it's worth confirming your stand and, for upper floors, the floor structure itself can support the combined weight, since a large aquarium's total loaded weight is a genuine, real structural consideration, not just a footnote.
Frequently Asked Questions
Why does the formula divide by 231?
One US gallon equals exactly 231 cubic inches, so dividing a tank's total volume in cubic inches by 231 converts the measurement into gallons, the standard unit for aquarium sizing in the US.
How do I calculate a cylindrical tank's volume?
Use pi times the radius squared times the height, then divide by 231 for gallons. The radius is half the diameter, so measure straight across the widest point of the tank and divide by two if you only have the diameter.
Why is my actual water volume less than my tank's rated size?
Substrate, rock, decorations, and equipment like heaters, filters, and powerheads all displace water, and most aquarists don't fill all the way to the rim. Real usable water volume typically runs 10 to 20 percent below a tank's full gross, labeled size, which is why this calculator shows both numbers.
How much does a full aquarium weigh?
Water alone weighs about 8.34 lb per US gallon. Add the empty tank's own weight, plus substrate and rock, for the true total load on a stand or floor. A full 75-gallon tank, for example, holds close to 650 lb of water alone.
How many fish can I keep in my aquarium?
A commonly cited starting rule is about one inch of adult fish length per gallon of usable water for small, calm community fish, though this ignores bioload, filtration capacity, and the adult size of larger or more active species. Treat it as a rough starting point rather than a precise limit.
How do I convert aquarium gallons to liters?
One US gallon equals about 3.785 liters. Use the gal/L toggle next to this calculator's result to switch units instantly, or multiply your gallon result by 3.785 by hand.
Does this work for bow-front, hexagon, or other unusual tank shapes?
Not currently. This calculator supports standard rectangular and cylindrical tanks, which cover most aquariums. Bow-front, hexagon, and other irregular shapes need a different, shape-specific formula that this calculator does not include yet.
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