The short answer
Multiply length, width and height in inches and divide by 231, since NIST defines a US gallon as 231 cubic inches. In centimeters, divide by 1,000 for liters. Measure inside, to the water line; Aquarium Tracker, an aquarium-care app, warns that “External glass dimensions usually overestimate usable water volume.”
The box formula, in inches or centimeters
NIST, the federal standards agency, gives the US gallon in its Handbook 44 tables as “1 gallon (gal) = 231 cubic inches”. Top Shelf Aquatics, a reef retailer, prints the same rule: “Formula: Gallons = (L × W × H in inches) ÷ 231”.
For metric tanks, NIST’s volume page gives “1 000 cm3 (1 dm3 or 1 000 mL or 1 L)”, so centimeters multiplied together and divided by 1,000 give liters. Aquarium Tracker’s example: “A tank measuring 60 × 30 × 30 cm holds 54 liters before substrate, hardscape, glass thickness, and the fill gap are considered.” NIST gives a US gallon as 3.785 liters.
One tank, worked from outside to inside
TruVu, an acrylic aquarium maker, lists its 10-gallon tank at “20 in. Length x 10 in. Width x 12 in. Height” with “1/4 in. Body – 1/4 in. Top/Btm”. The steps are our arithmetic; we read TruVu’s listed size as the outside size and its 1/4 in. figures as wall thickness, which its page does not say, so inside takes 1/4 inch off each wall, top and bottom included.
| Step | Sum | Result |
|---|---|---|
| Outside box | 20 x 10 x 12 = 2,400 cu in | 10.4 US gal |
| Inside | 19.5 x 9.5 x 11.5 = 2,130 cu in | 9.2 US gal |
| Inside, in liters | 9.22 x 3.785 | 34.9 L |
| Inside, in UK gallons | 2,130 ÷ 277.42 | 7.7 UK gal |
TruVu’s page gives “Actual Gallons 9.25”, close to the inside figure; it does not say how it measured.
Tetra’s setup guide for its kits gives two fill points. Step 4 says to “Fill to within two to three inches of the aquarium rim” (its drawing calls this the initial fill). Step 7, “Finish Filling Aquarium”, says: “Water line should be just above the bottom of the aquarium frame, hidden from view.” Wherever your water line sits, each inch of depth on this tank’s inside footprint is about 0.8 gallon (our arithmetic), so measure to your own water line, as Top Shelf says.
Inside, outside and the gallons on the label
Top Shelf says: “Use inside measurements to the actual water line.” Its figures differ on one page. Its FAQ says “Substrate, live rock, the space below the rim and equipment displacement all reduce the real water volume, often by 10 to 20 percent”, while its size chart subtracts “roughly 10 to 15 percent”. Both are one reef retailer’s estimates, not a rule.
Neither Aqueon’s chart nor Tetra’s kit page gives a glass thickness. Aqueon, an aquarium maker, names its chart sizes 10 Leader, 20 Long and 55 and prints no unit; we read them as gallons. Aqueon’s chart does not say whether its sizes are outside figures. Our box volumes:
| Aqueon tank | Listed size (in) | Box volume (our arithmetic) |
|---|---|---|
| 10 Leader | 20¼ x 10½ x 12 9/16 | 11.6 gal |
| 20 Long | 30¼ x 12½ x 12¾ | 20.9 gal |
| 55 | 48¼ x 12¾ x 21 | 55.9 gal |
A box measured outside includes the glass and any frame or base, so it overstates the water (our reading). Our 10-gallon size guide lists each maker’s 10-gallon dimensions by model.
Round, bow-front and hexagon tanks
- Cylinders: Top Shelf’s cylinder formula is π × r² × H, divided by 231, with r the radius (half the width across). Its example: “A round tank with a 10-inch radius and 24-inch water height holds about 32.6 gallons”.
- Bow fronts: Aquarium Tracker says “bow-front tanks add a curved segment to the straight-sided area”, and its calculator asks for “length, side depth, center depth, and height”.
- Hexagons: Top Shelf’s calculator has hexagon settings, but the page prints no hexagon formula.
US or UK (Imperial) gallons
NIST puts the British Imperial gallon at 277.42 cubic inches, or 4.546 liters, and says it is “about 20 %” larger than the US gallon. Aquarium Tracker says “the two gallon units must not be treated as interchangeable”.
Why the number matters
- Dosing. Aqueon’s page for its own water conditioners (which lists a Betta Bowl Plus version too) says “Add 5mL (1 tsp) per 10 gallons of water”. Other makers’ doses differ, and some name tank water or new water, so use the label of the bottle you hold; our conditioner roundup lists each maker’s dose. Top Shelf, a reef retailer, says “use your measured water level for the most accurate dosing and stocking numbers”. Aquarium Tracker adds: “Check which unit a product label or aquarium guide uses before dosing or comparing tank sizes.”
- Heaters. Our heater roundup works out each pick’s watts per gallon from its maker’s tank-size rating, and notes that some makers tie ratings to how far the water sits above the room.
- Stocking. Aquarium Tracker says: “Volume alone does not decide stocking capacity.” Our guide to fish for a 10-gallon tank lists 13 species that sources recommend.
- Weight. Our guide to what a 10-gallon tank weighs adds up the water, tank and gravel; our set-up guide covers choosing a level spot and a stand made for the tank.
Ready to buy?
Our water conditioner roundup compares eight conditioners, and our Tank Setup & Care section gathers our guides on cycling, water changes, setup steps and routines.
Frequently Asked Questions
Q. How many gallons is a 24 x 12 x 12 tank?
About 15.0 US gallons as a box (3,456 cubic inches ÷ 231, our arithmetic); Top Shelf’s chart lists 24 x 12 x 12 as its 15-gallon size. A 24 x 16 x 12 box is about 20 and a 30 x 12 x 19 box about 29.6; if those are outside sizes, the water is usually less.
Q. How many liters is a 5-gallon tank?
About 18.9 liters, on NIST’s 3.785 liters per US gallon (our arithmetic).
Q. How do I tell the size if I can’t measure inside?
Top Shelf suggests comparing the outside size with a standard size chart, such as the one on its volume calculator page (in Sources), for “a planning estimate”.
Q. How do I work it out in feet?
Multiply the three sides in feet, then multiply by 7.481, NIST’s figure for the gallons in a cubic foot.
Sources (8)
- NIST, Handbook 44 Appendix C (PDF) (federal agency)
- NIST, SI Units – Volume (federal agency)
- Top Shelf Aquatics, Aquarium Volume Calculator (retailer)
- Aquarium Tracker, Aquarium Volume Calculator (aquarium-care app)
- TruVu Aquariums, 10 Gallon Acrylic 20x10x12 (maker)
- Tetra, Aquarium Kit Set-Up Guide (PDF) (maker)
- Aqueon, aquarium sizing chart (PDF) (maker)
- Aqueon, Water Conditioners (maker)
