Quick answer: Standard glass fish tanks come in a fairly fixed set of sizes. The most common are the 10 gallon (20 x 10 x 12 inches), 20 gallon long (30 x 12 x 12), 20 gallon high (24 x 12 x 16), 29 gallon (30 x 12 x 18), 40 gallon breeder (36 x 18 x 16), 55 gallon (48 x 13 x 21) and 75 gallon (48 x 18 x 21). A filled tank weighs roughly 10 to 11 pounds per gallon once you add glass, substrate and decor, so a 55 gallon lands around 625 pounds. The thing most people get wrong is judging a tank by gallons alone. For most fish, the length and footprint matter more than the number on the box, which is why a 40 breeder often suits fish better than a taller tank with the same volume.
Fish tank sizes at a glance
| Detail | What to know |
|---|---|
| Most popular beginner sizes | 20 gallon long, 29 gallon, 40 breeder |
| Smallest practical size for fish | 5 gallons (one betta or shrimp) |
| How to calculate gallons | Length x width x height in inches, divided by 231 |
| Real water volume | Usually 10 to 20% less than the label, after substrate and decor |
| Filled weight rule | About 10 to 11 lb per gallon (1.2 to 1.3 kg per liter) |
| Needs a dedicated stand | Anything 20 gallons and up |
| Same footprint, different height | 20 long and 29 gallon (30 x 12); 75 and 90 gallon (48 x 18) |
Fish tank size chart: gallons, dimensions and weight
These are the standard rectangular glass sizes sold by the big US brands like Aqueon, Marineland and Tetra. Rimless, acrylic and European tanks vary more, so always check the maker’s listed outside dimensions before you buy a stand or measure a space.
| Tank size | Inches (L x W x H) | Centimeters | Liters | Filled weight (approx.) |
|---|---|---|---|---|
| 2.5 gallon | 12 x 6 x 8 | 30 x 15 x 20 | 9.5 | 30 lb (14 kg) |
| 5 gallon | 16 x 8 x 10 | 41 x 20 x 25 | 19 | 62 lb (28 kg) |
| 10 gallon | 20 x 10 x 12 | 51 x 25 x 30 | 38 | 111 lb (50 kg) |
| 15 gallon | 24 x 12 x 12 | 61 x 30 x 30 | 57 | 170 lb (77 kg) |
| 20 gallon high | 24 x 12 x 16 | 61 x 30 x 41 | 76 | 225 lb (102 kg) |
| 20 gallon long | 30 x 12 x 12 | 76 x 30 x 30 | 76 | 225 lb (102 kg) |
| 29 gallon | 30 x 12 x 18 | 76 x 30 x 46 | 110 | 330 lb (150 kg) |
| 30 gallon | 36 x 12 x 16 | 91 x 30 x 41 | 114 | 340 lb (154 kg) |
| 40 gallon breeder | 36 x 18 x 16 | 91 x 46 x 41 | 151 | 458 lb (208 kg) |
| 40 gallon long | 48 x 12 x 16 | 122 x 30 x 41 | 151 | 455 lb (206 kg) |
| 55 gallon | 48 x 13 x 21 | 122 x 33 x 53 | 208 | 625 lb (283 kg) |
| 75 gallon | 48 x 18 x 21 | 122 x 46 x 53 | 284 | 850 lb (386 kg) |
| 90 gallon | 48 x 18 x 24 | 122 x 46 x 61 | 341 | 1,050 lb (476 kg) |
| 120 gallon | 48 x 24 x 24 | 122 x 61 x 61 | 454 | 1,400 lb (635 kg) |
| 125 gallon | 72 x 18 x 21 | 183 x 46 x 53 | 473 | 1,400 lb (635 kg) |
| 150 gallon | 72 x 18 x 28 | 183 x 46 x 71 | 568 | 1,700 to 1,800 lb (about 800 kg) |
| 180 gallon | 72 x 24 x 25 | 183 x 61 x 64 | 681 | 2,100 lb (950 kg) |
Filled weights include water, glass, about 1 to 2 inches of substrate and normal decor. Heavy rock work adds more. Liters are based on the label size.
Long vs high vs breeder: what the names mean
Many sizes come in more than one shape, and the name tells you which way the extra space goes.
| Type | Example | Shape | Best for |
|---|---|---|---|
| Long | 20 long (30 x 12 x 12), 40 long (48 x 12 x 16) | Extra length, low height | Fast schooling fish, bottom dwellers, shrimp, easy maintenance |
| High or tall | 20 high (24 x 12 x 16), 29 (30 x 12 x 18) | Extra height, smaller footprint | Small spaces, tall plants, mid-water fish |
| Breeder | 40 breeder (36 x 18 x 16) | Extra width and depth front to back | Community tanks, aquascapes, breeding, angelfish and gouramis |
| Standard | 10, 55, 75 gallon | Balanced proportions | All-round community tanks |
Here is the honest part most charts skip. Fish use floor space and length, not gallons. A 20 long and a 29 gallon have exactly the same 30 x 12 inch footprint, so for corydoras and loaches they are nearly identical. A 40 breeder has 50% more floor space than a 40 long, even though both hold about the same water. And a tall tank with a small footprint can hold more gallons while giving fish less room to swim. When you compare two tanks, compare the length and the footprint first, then the volume.
How many gallons is my fish tank?
Measure the inside length, width and height in inches, multiply them, and divide by 231. That gives you US gallons.
- Example: a 30 x 12 x 12 inch tank is 30 x 12 x 12 = 4,320 cubic inches. Divide by 231 and you get about 18.7 gallons, which is why it is sold as a 20 gallon long.
- In centimeters: multiply length x width x height and divide by 1,000 to get liters. Divide liters by 3.785 for US gallons.
- Your real water volume is lower than the label. Substrate, rocks, wood and the air gap at the top usually take 10 to 20% away, and that real number is the one to use for medications and fertilizer.
Our free aquarium volume calculator does this for rectangular, cube, bow front and cylinder tanks, and it subtracts substrate for you.
How much does a fish tank weigh?
Water weighs about 8.34 pounds per US gallon. Add the glass, the substrate and decor, and a set-up tank usually weighs 10 to 11 pounds per gallon. That is why even a 20 gallon reaches about 225 pounds and a 75 gallon passes 800.
What it means for your stand. Any tank from 20 gallons up needs a stand made for aquariums. Dressers, desks and bookshelves are not built to carry that much weight on their top edge, and they sag over time.
What it means for your floor. Tanks up to about 55 gallons are fine on most ground-floor and well-built upper floors. For 75 gallons and up, place the tank against a load-bearing wall and across the floor joists rather than along them, and ask a contractor if you are unsure, especially in an apartment.
Never move a filled tank. Drain it completely and take out the substrate first. The weight can twist the frame and crack a seam.
Which fish tank size should you choose?
Bigger is easier, within reason. More water dilutes waste and changes temperature slowly, so mistakes take longer to hurt your fish. Here is a quick guide to what each size suits.
| Size range | Good for | Not suitable for |
|---|---|---|
| 2.5 to 5 gallon | One betta, cherry shrimp, a few nerite snails | Schooling fish, goldfish, beginners who want several fish |
| 10 gallon | A small school of nano fish, a betta with tankmates, shrimp colonies | Goldfish, plecos other than very small species |
| 20 to 30 gallon | Beginner community tanks, neon tetras, corydoras, honey gouramis, cherry barbs | Large cichlids, silver dollars, common plecos |
| 40 to 55 gallon | Bigger communities, tiger barbs, bristlenose plecos, angelfish (40 breeder or taller) | Oscars, adult silver dollar schools |
| 75 to 125 gallon | Large schools, medium cichlids, silver dollars, big planted scapes | Very large species like adult common plecos and arowanas |
| 150 gallon and up | Large South American and African cichlids, big community showpieces | Small homes without floor support |
The best first tank for most beginners is a 20 long or a 29 gallon. Both are small enough to fit most homes and budgets and big enough to keep water stable while you learn. A 10 gallon works too, but it gives you fewer fish options and less room for mistakes. For a list of fish that suit these sizes, see our best aquarium fish for beginners guide, and check any mix with our aquarium stocking calculator.
Small fish tank sizes: 1 to 5 gallons
1 gallon tanks and bowls. There is no true standard size, but they are usually around 8 x 8 x 8 inches. They are too small for any fish to live in long term, because the water swings in temperature and fills with waste very fast. Use them for a single moss ball, a few plants or as a temporary hospital container.
2.5 gallon. At 12 x 6 x 8 inches, it is fine for a few cherry shrimp, snails and plants, but tight for a betta.
5 gallon. The smallest size we recommend for a betta. At 16 x 8 x 10 inches, it fits a heater, a gentle filter and a few plants. Our 5 gallon fish tank guide covers the best setups.
Medium fish tank sizes: 10 to 40 gallons
This is where most people keep their first community tank. The 10 gallon is the classic starter kit, the 20 gallon comes as a long or high tank, and the 29 gallon adds 6 inches of height to a 20 long’s footprint.
The 40 breeder deserves a special mention. Its 36 x 18 inch footprint gives you room for a proper aquascape, a big school and bottom dwellers, while still fitting on a 3 foot stand. Many experienced keepers call it the best value tank you can buy.
Large fish tank sizes: 55 gallons and up
The 55 gallon is the most common big tank, at 48 inches long but only 13 inches front to back. The 75 gallon has the same length with 18 inches of depth, which makes a huge difference for aquascaping and for wider-bodied fish. The 90 gallon is simply a taller 75.
Once you go past 4 feet, the 125 and 150 gallon tanks stretch to 6 feet long. These suit large cichlids and big schools, but they need a strong stand, solid floors and a few people to move them.
Before you buy: setup and equipment by size
Filter. Choose one rated for about 4 to 6 times your tank volume per hour. A 20 gallon needs roughly 80 to 120 GPH, a 55 gallon about 220 to 330 GPH.
Heater. Plan on about 3 to 5 watts per gallon for tropical fish. Bigger tanks often run two smaller heaters instead of one large one, so a single failure is less dangerous.
Light and lid. Lights and lids are sold by tank length, not gallons, so a 20 long and a 29 gallon use the same 30 inch light, and a 55, 75 and 90 all use 48 inch lights.
Cycle first. Whatever size you choose, cycle the tank for a few weeks before adding fish. Our fish tank setup guide walks through every step, from rinsing substrate to adding your first fish.
Common fish tank size mistakes
Buying the smallest tank to be safe. Small tanks are harder, not easier. Water quality swings fast, so a 20 gallon is usually more forgiving than a 5.
Choosing by gallons instead of footprint. A tall tank can hold plenty of water and still be too short for active swimmers. Check the length your fish need.
Forgetting adult size. Common plecos, silver dollars, oscars and goldfish are sold small but outgrow most home tanks. Plan for the adult fish, not the one in the bag.
Putting a tank on furniture. A 29 gallon weighs over 300 pounds when set up. Use a stand rated for your tank.
Dosing by the label size. Medications and fertilizers should be dosed for your real water volume, which is usually 10 to 20% less than the number on the box.
FAQ
What are the standard fish tank sizes?
The most common standard sizes are 5, 10, 20 long, 20 high, 29, 40 breeder, 55, 75, 90, 125 and 150 gallons. Each one has a fairly fixed set of dimensions across the major US brands, listed in the chart above.
How big is a 10 gallon fish tank?
A standard 10 gallon tank measures 20 x 10 x 12 inches (51 x 25 x 30 cm) and weighs about 111 pounds filled. It is one of the most common starter tanks.
How many gallons is a 30 x 12 x 12 tank?
About 18.7 US gallons. Multiply 30 x 12 x 12 to get 4,320 cubic inches and divide by 231. It is sold as a 20 gallon long.
How do I know how many gallons my aquarium is?
Measure the inside length, width and height in inches, multiply them together and divide by 231. Or enter the measurements in our aquarium volume calculator, which also subtracts substrate.
How long is a 120 gallon tank?
A standard 120 gallon tank is 48 inches long, 24 inches wide and 24 inches tall. The 125 gallon is longer, at 72 inches, but only 18 inches front to back.
What size fish tank is best for beginners?
A 20 gallon long or a 29 gallon is the best starting size for most beginners. They hold enough water to stay stable, fit most homes, and suit a wide range of easy community fish.
How much does a 55 gallon fish tank weigh?
About 625 pounds once it is filled with water, substrate and decor. The empty glass tank weighs around 75 to 80 pounds.
What is the difference between a 40 breeder and a 40 long?
Both hold about 40 gallons, but a 40 breeder is 36 x 18 x 16 inches and a 40 long is 48 x 12 x 16. The breeder has more floor space and depth for aquascaping, while the long gives fast swimmers more length.
Final word
Use this chart to find a tank that fits your space, then choose by footprint and the adult size of the fish you want, not by gallons alone. If you are just starting out, a 20 long, 29 gallon or 40 breeder gives you the best mix of stability, space and choice. Then put it on a proper stand, cycle it, and stock it slowly.
