Guide
How many gallons is a 48x13x21 aquarium?
By the Aquarium Calculator team
A 48 × 13 × 21 inch aquarium holds 56.7 US gallons (214.7 liters, or 47.2 UK gallons) when measured from the outside. It's the standard 55-gallon tank. Once you allow for the glass, the gap below the rim and a substrate bed, it really holds about 44–49 gallons of water.
At a glance
- Outside dimensions
- 48 × 13 × 21 in · 122 × 33 × 53 cm
- US gallons
- 56.7
- UK gallons
- 47.2
- Liters
- 214.7
- Water weight (full)
- 215 kg / 473 lb
Gross volume of the glass box, calculated from the outside dimensions.
The calculation
Volume for a rectangular tank is length × width × height. For this tank that is 48 × 13 × 21 = 13,104 cubic inches. One US gallon is exactly 231 cubic inches, so 13,104 ÷ 231 = 56.7 US gallons. In metric terms, the same box is about 122 × 33 × 53 cm, which works out to 214.7 liters, or 47.2 imperial (UK) gallons.
That 56.7 figure is slightly more than the 55 gallons on the label, and that's normal. Manufacturers name tanks in round numbers from the outside dimensions, and the 48 × 13 × 21 footprint has been sold as the "55" for decades. You'll see small variations between brands (a 55 might be 48¼ × 12¾ × 21 inches), but they all land within a gallon or two.
How much water a 55-gallon really holds
The outside measurement includes the glass itself, which on a framed 55 is about a quarter of an inch (6 mm) thick. Inside, the tank is closer to 47.5 × 12.5 × 20 inches, about 51 gallons. Nobody fills to the very top edge either: with the water line an inch below the rim, you're at about 49 gallons. Add a 2-inch sand or gravel bed and the water drops to roughly 44 gallons, and rock and driftwood take a little more.
That real (net) volume is the number to use for medication doses, water conditioner, fertilizers and equipment sizing. Dosing for 55 gallons when the tank holds 44 means overdosing by about 25%. The volume calculator works this out for you: load the 55-gallon preset, enter your substrate depth and a décor percentage, and it shows gross and net volume side by side.
How much does a 55-gallon tank weigh?
Fresh water weighs about 8.34 pounds per US gallon, so the water alone in a full 55 is around 400–450 pounds. The empty glass tank weighs roughly 75–80 pounds, and a 2-inch sand bed adds about 70 pounds more. Running with rock and equipment, a 55-gallon tank typically weighs about 600 pounds (270 kg), all resting on a footprint of four feet by a little over one foot.
That weight is why a 55 needs a purpose-built aquarium stand rather than ordinary furniture, and why the stand must be perfectly level. On an upper floor, place it against a load-bearing wall and across the floor joists rather than parallel to them. The tank weight calculator gives a line-by-line breakdown for your exact setup.
Heater, filter and substrate for a 48x13x21
Using a net volume of about 185 liters (49 gallons), a heater needs around 0.2 watts per liter for each °C above room temperature. Holding 25 °C (77 °F) in a room that drops to 20 °C (68 °F) calls for about 185 W, so a 200 W heater, or two 100 W heaters at opposite ends for more even heat and a safety margin if one fails. In a colder room, step up to 250–300 W total.
For filtration, aim for the full volume to pass through the filter four to six times an hour: roughly 200–300 GPH (750–1,100 liters per hour) of real flow. Box ratings are measured with no media, so choose a filter rated for 75 gallons or more, or run two filters. For a 2-inch sand bed across the 48 × 13 inch base you'll need about 33 kg (72 lb) of sand; gravel is slightly lighter per bag of the same volume.
What fish suit a 55-gallon tank?
Four feet of swimming length is the 55's biggest strength. It suits active schooling fish that need room to move: a school of 15–20 tetras or rasboras, a group of 6–8 corydoras on a sand bed, and a centerpiece such as a pair of dwarf gouramis or a bristlenose pleco makes a classic peaceful community. It's also a popular choice for African mbuna cichlids, which need the length to spread out aggression, and for a small group of rainbowfish.
The 13-inch depth (front to back) is the 55's weakness: it limits aquascaping room and is too narrow for fish that grow large or turn slowly, such as oscars, adult common plecos or large goldfish. Check every species' adult size and minimum tank size, and run your plan through the stocking calculator to see bioload, filtration and compatibility before you buy.
Calculate any tank in seconds
The same method works for any rectangular tank: multiply length × width × height in inches and divide by 231 for US gallons, or multiply in centimeters and divide by 1,000 for liters. For bow fronts, corner, hexagon and cylinder tanks, or to find your real net volume, use the free volume calculator. Enter the dimensions and it shows US gallons, UK gallons, liters and water weight instantly.
Check your own tank
Enter your real dimensions to get exact figures for your setup in a few seconds.
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