Aquarium Dimensions by Size
Standard aquarium dimensions in inches and centimetres, with the volume each box actually encloses, the water weight it puts on your floor, the floor area gallons hide, and how many real keepers run each size.
A standard 75-gallon aquarium measures 48 × 18 × 21 inches (122 × 46 × 53 cm); a 55-gallon is 48 × 13 × 21 inches (122 × 33 × 53 cm); a 40-gallon breeder is 36 × 18 × 16 inches (91 × 46 × 41 cm). The full chart is below, with the volume each box encloses, the water weight it puts on your floor, and the floor area the gallon rating hides.
Standard aquarium dimensions chart
| Tank | L × W × H (in) | L × W × H (cm) | Encloses | Water weight | Floor area |
|---|---|---|---|---|---|
| 2.5 gallon | 12 × 6 × 8 | 30 × 15 × 20 | 2.5 gal / 9 L | 21 lb / 9 kg | 72 sq in |
| 5 gallon | 16 × 8 × 10 | 41 × 20 × 25 | 5.5 gal / 21 L | 46 lb / 21 kg | 128 sq in |
| 10 gallon | 20 × 10 × 12 | 51 × 25 × 30 | 10.4 gal / 39 L | 87 lb / 39 kg | 200 sq in |
| 15 gallon | 24 × 12 × 12 | 61 × 30 × 30 | 15.0 gal / 57 L | 125 lb / 57 kg | 288 sq in |
| 20 gallon long | 30 × 12 × 12 | 76 × 30 × 30 | 18.7 gal / 71 L | 156 lb / 71 kg | 360 sq in |
| 20 gallon high | 24 × 12 × 16 | 61 × 30 × 41 | 19.9 gal / 76 L | 166 lb / 75 kg | 288 sq in |
| 29 gallon | 30 × 12 × 18 | 76 × 30 × 46 | 28.1 gal / 106 L | 234 lb / 106 kg | 360 sq in |
| 30 gallon breeder | 36 × 18 × 12 | 91 × 46 × 30 | 33.7 gal / 127 L | 281 lb / 127 kg | 648 sq in |
| 40 gallon long | 48 × 12 × 16 | 122 × 30 × 41 | 39.9 gal / 151 L | 333 lb / 151 kg | 576 sq in |
| 40 gallon breeder | 36 × 18 × 16 | 91 × 46 × 41 | 44.9 gal / 170 L | 374 lb / 170 kg | 648 sq in |
| 55 gallon | 48 × 13 × 21 | 122 × 33 × 53 | 56.7 gal / 215 L | 473 lb / 215 kg | 624 sq in |
| 65 gallon | 36 × 18 × 24 | 91 × 46 × 61 | 67.3 gal / 255 L | 561 lb / 255 kg | 648 sq in |
| 75 gallon | 48 × 18 × 21 | 122 × 46 × 53 | 78.5 gal / 297 L | 655 lb / 297 kg | 864 sq in |
| 90 gallon | 48 × 18 × 24 | 122 × 46 × 61 | 89.8 gal / 340 L | 749 lb / 340 kg | 864 sq in |
| 125 gallon | 72 × 18 × 21 | 183 × 46 × 53 | 117.8 gal / 446 L | 983 lb / 446 kg | 1,296 sq in |
“Encloses” is the geometric volume of the box, calculated from the dimensions rather than copied from a catalogue. Water weight is that volume filled to the brim, at 8.34 pounds per US gallon, and counts water only: no tank, no substrate, no rock. In a real tank the water volume runs below the “Encloses” column and the total weight runs above the “Water weight” column, for the reasons in the next two sections.
The nameplate is a name, not a measurement
A 40-gallon breeder encloses 44.9 gallons. A 40-gallon long encloses 39.9. Same name, five gallons apart, because the name is a product line rather than the result of multiplying three numbers.
The gap does not even run in one direction. A 5-gallon encloses 5.5, eleven percent over its name. A 20-gallon long encloses 18.7, six percent under. A 125 encloses 117.8, almost six percent under. Whoever named these tanks was picking round numbers a customer would remember, and the arithmetic landed where it landed.
Then reality takes some back. You fill an inch or two below the rim, substrate displaces water, and rock and hardscape displace more. A planted 75 with three inches of substrate can hold closer to 65 gallons of actual water.
Which direction that leaves you in depends on the tank:
- For dosing and water changes, use your actual water volume, never the nameplate. Dose a “75-gallon” that really holds 65 gallons and you overdose it by 15 percent. On a 20-gallon long the two errors compound, because it encloses less than its name and you still fill it below the rim, so treating it as 20 gallons can overdose by a quarter.
- For stocking, the nameplate is close enough on most tanks, because the enclosed volume and the fill line pull in opposite directions and roughly cancel.
Measure yours with the volume calculator and subtract roughly 10 percent for substrate and hardscape.
Breeder, long and high: what the suffixes mean
Same nominal volume, different box. The suffix tells you which dimension the maker traded away.
| 40 long | 40 breeder | |
|---|---|---|
| Dimensions | 48 × 12 × 16 in | 36 × 18 × 16 in |
| Encloses | 39.9 gal | 44.9 gal |
| Floor area | 576 sq in | 648 sq in |
| Suits | Mid-water swimmers, rainbowfish, long schools | Corydoras, loaches, shrimp, aquascaping |
- Long trades width and height for length. Good for schooling fish that swim horizontally.
- Breeder trades length for width. More floor area at the same nominal volume, which is what bottom-dwellers actually use.
- High trades footprint for height. It looks impressive and gives the least usable fish space per gallon, because a tall column adds volume without adding floor or surface area.
Footprint is the number to compare when you are stocking bottom-dwellers, and it is the number gallons hide. A 20-gallon long gives 360 square inches of floor; a 20-gallon high gives 288. Identical rating, a quarter less floor. See how many corydoras in a tank for why floor area drives cory group size.
Why published filled weights disagree
The two dimension charts that hold the editorial slots for this query give the same 10-gallon tank as 111 pounds and about 94 pounds filled. That is a 17 percent spread on a tank both describe as 20 × 10 × 12 inches.
Neither is wrong. They are answering different questions, and neither row says which:
- The 94-pound figure is water and glass. It is the nominal 10 gallons at 8.34 lb, plus the 11-pound empty tank. Clean arithmetic, stated in that chart’s own caption.
- The 111-pound figure includes substrate and rock. That chart’s preamble says its weights vary with “the amount of rock, thickness of glass and type of water”. It is a furnished-tank estimate, and every row in it runs 7 to 24 percent above what the box can physically hold in water.
Here is the third number, the one neither chart gives: the ceiling. A 10-gallon is 20 × 10 × 12 inches, which is 2,400 cubic inches. One US gallon is 231 cubic inches, so the box encloses 10.4 gallons. At 8.34 pounds per gallon that is 87 pounds of water, brim-full, and no 10-gallon tank on earth holds more. Add the 11-pound tank and 98 pounds is the hard maximum before a single grain of sand goes in.
One more thing worth knowing before you trust any of these tables. One of the two charts states its method in its own caption, gallons × 8.34 plus the empty weight, and seventeen of its eighteen rows follow it to the pound. The exception is the 55-gallon, where the formula gives 537 pounds and the row says 625: the same figure, to the pound, that the other chart prints. Charts in this niche inherit from each other, and the inherited numbers are the ones that do not reconcile.
If you are checking whether a floor can take it, start from the water weight in our chart, add the tank and stand, add substrate, and treat the result as a minimum.
What size do real keepers actually run?
Dimension charts describe what is sold. Our dataset describes what people own.
Across 5,134 real tanks that stated a volume (July 2026 snapshot), the median is 20 gallons and 58 percent are 20 gallons or smaller. The mean is 39 gallons, pulled up by a long tail; the largest tank in the set is 1,200 gallons.
| Size | Share of real tanks |
|---|---|
| Under 5 gallons | 4% |
| 5 gallons | 14% |
| 6 to 9 gallons | 3% |
| 10 gallons | 18% |
| Over 10 gallons | 61% |
Those five bands cover every tank in the set. Within the last one, the 20-gallon is the single most common size at 14 percent, which is why 5, 10 and 20 gallons together account for nearly half the hobby. The 75-gallon and 125-gallon tanks that dominate dimension charts are a small minority of real aquariums.
Tanks are self-reported by keepers who chose to post, so read this as what the hobby runs and talks about rather than a census of what it owns. Full method on the aquarium statistics page.
Which dimensions suit which fish
Choosing between two tanks of the same volume comes down to what your fish use.
- Bottom-dwellers (corydoras, loaches, shrimp) use floor area. Pick the wider box: breeder over long, long over high.
- Schooling mid-water fish (tetras, rasboras, rainbowfish) use length. A 20 long beats a 20 high for the same money.
- Tall-bodied fish (angelfish, discus) need height. This is the one case where a high tank earns its shape.
- Surface breathers (bettas, gouramis) need surface area and a short swim to the top, so height works against them.
If you are still deciding which size to buy rather than what a given size measures, start at what size aquarium do you need, which weighs cost, stability and stocking.
How we calculated this
Volumes are geometric: length × width × height in inches, divided by 231 cubic inches per US gallon. Metric conversions use 2.54 cm per inch and 3.785411784 litres per US gallon. Water weight uses 8.34 pounds per US gallon of fresh water, the same figure both charts we cite use, converted at 0.45359237 kg per pound. Floor area is length × width.
Dimensions are the standard trade sizes, and every row is one both cited charts agree on. Where the two disagreed on a size we left that size out rather than pick a winner. Where their arithmetic disagrees with ours, we show ours.
Real-tank figures come from the AquaGauge dataset (July 2026 snapshot, 5,134 tanks that declared a volume). See our methodology.
FAQ
- What are the dimensions of a 75-gallon aquarium?
- A standard 75-gallon measures 48 x 18 x 21 inches (122 x 46 x 53 cm). Those dimensions enclose about 78.5 gallons (297 litres), so the box holds more than its name suggests, and you fill below the rim anyway.
- What are the dimensions of a 55-gallon aquarium?
- 48 x 13 x 21 inches (122 x 33 x 53 cm), enclosing about 56.7 gallons (215 litres). It has the same 48-inch length as a 75-gallon but is five inches narrower, which is why a 55 feels cramped for bottom-dwellers despite the length.
- What is the difference between a 40-gallon breeder and a 40-gallon long?
- Same nominal volume, different box, and they do not even hold the same water. The breeder is 36 x 18 x 16 inches and encloses 44.9 gallons; the long is 48 x 12 x 16 inches and encloses 39.9. The breeder also gives 648 square inches of floor against the long's 576, which is 12 percent more of the surface corydoras, loaches and shrimp actually live on.
- How much does a filled aquarium weigh?
- Fresh water weighs about 8.34 pounds per US gallon, so a 75-gallon holds roughly 655 pounds (297 kg) of water alone when filled to the brim. Add the tank, substrate and rock. Published filled weights disagree by 15 percent or more because charts differ on whether the figure covers water only or water plus substrate and rock, and almost none of them say which.
- Does tank shape matter more than volume?
- For stocking, footprint often matters more than gallons. A 20-gallon long gives 360 square inches of floor and a 20-gallon high only 288, a quarter less, at the same nominal volume. Bottom-dwellers use floor, not water column.
Sources
- Standard Aquariums: Weights & Sizes, SaltwaterAquarium.com. View source ↗
- Aquarium Dimensions & Size Chart: All Standard Tanks, FishStores.org. View source ↗
Freshwater aquarist of 8+ years.