Common aquarium sizes run from 2.5 gallons (12 x 6 x 8 inches) up to 225 gallons (72 x 27.5 x 27.5 inches). The gallon number is a nominal volume, so the footprint, height and weight vary from one tank style to another. The chart below lists length, width and height in inches along with empty and filled weight in pounds.
Fish tank dimensions chart
Dimensions are length x width x height, in inches. Weights are rough estimates for a tank filled with water, because glass thickness, rock and substrate all change the total. Treat every figure as nominal and measure your own tank before buying a stand, lid or light.
Small aquariums
| Tank size | L x W x H (in) | Empty (lb) | Filled (lb) |
|---|---|---|---|
| 2.5 gallon | 12 x 6 x 8 | 3 | 27 |
| 5 gallon | 16 x 8 x 10 | 7 | 62 |
| 10 gallon | 20 x 10 x 12 | 11 | 111 |
| 15 gallon | 24 x 12 x 12 | 21 | 170 |
| 15 gallon high | 20 x 10 x 18 | 22 | 170 |
Mid-sized aquariums
| Tank size | L x W x H (in) | Empty (lb) | Filled (lb) |
|---|---|---|---|
| 20 gallon high | 24 x 12 x 16 | 25 | 225 |
| 20 gallon long | 30 x 12 x 12 | 25 | 225 |
| 25 gallon | 24 x 12 x 20 | 32 | 282 |
| 29 gallon | 30 x 12 x 18 | 40 | 330 |
| 30 gallon breeder | 36 x 18 x 12 | 48 | 348 |
| 40 gallon breeder | 36 x 18 x 16 | 58 | 458 |
| 40 gallon long | 48 x 12 x 16 | 55 | 455 |
Large aquariums
| Tank size | L x W x H (in) | Empty (lb) | Filled (lb) |
|---|---|---|---|
| 50 gallon | 36 x 18 x 19 | 100 | 600 |
| 55 gallon | 48 x 13 x 21 | 78 | 625 |
| 65 gallon | 36 x 18 x 24 | 126 | 772 |
| 75 gallon | 48 x 18 x 21 | 140 | 850 |
| 90 gallon | 48 x 18 x 24 | 160 | 1,050 |
| 125 gallon | 72 x 18 x 21 | 206 | 1,206 |
| 150 gallon | 72 x 18 x 28 | 338 | 1,838 |
| 180 gallon | 72 x 24 x 25 | 430 | 1,870 |
| 225 gallon | 72 x 27.5 x 27.5 | 358 | 2,158 |
Other charts round a few of these filled weights differently. A 120 gallon tank (48 x 24 x 24 inches) is listed at roughly 1,400 lb filled, and a 10 gallon with water and gravel is often quoted nearer 121 lb than 111 lb. Use the figures as a planning range, not an exact number.
How many gallons is my fish tank?
Multiply length, width and height in inches, then divide by 231. That is the number of cubic inches in a US gallon.
Gallons from inches
- Formula
- Length x width x height (in) / 231
- Example
- 48 x 18 x 21 = 18,144 cubic inches / 231 = about 78 gallons (sold as a 75)
- 24 x 12 x 12 tank
- 3,456 / 231 = about 15 gallons
- Water weight
- About 8.34 lb per gallon of fresh water
The result is the full-to-the-brim volume of the glass box. In real use the water sits an inch or so below the rim, and substrate and decor take up space, so the actual water volume is lower. Plan feeding and water-conditioner amounts around the water you actually have.
How much does a fish tank weigh?
Water is the heavy part. At about 8.34 lb per gallon, a 20 gallon tank holds roughly 167 lb of water on its own, and the filled total of about 225 lb in the chart includes the glass, substrate and decor. A 55 gallon tank is about 625 lb filled and a 125 gallon is about 1,206 lb.
Check your floor, not just your stand. A large tank concentrates hundreds of pounds on a small footprint, so place it against a load-bearing wall or over joists when you can.
Popular sizes at a glance
- 10 gallon: 20 x 10 x 12 inches and about 111 lb filled. Small enough for a desk or sturdy dresser.
- 20 gallon long: 30 x 12 x 12 inches. A shallow tank with plenty of swimming length.
- 29 gallon: 30 x 12 x 18 inches. About 330 lb filled.
- 40 gallon breeder: 36 x 18 x 16 inches. A wide front-to-back footprint and a lower height than a 55 give more usable floor space and easier access.
- 55 gallon: 48 x 13 x 21 inches. More length in a narrower footprint.
Is a 10 or 20 gallon aquarium better?
For most beginners, a 20 gallon is the easier tank. It holds roughly twice the water of a 10 gallon, which dilutes waste and keeps temperature and water chemistry steadier. A 20 long measures 30 x 12 x 12 inches against 20 x 10 x 12 inches for a 10 gallon, so you also get more swimming room. The 10 gallon wins on cost, weight (111 lb filled against 225 lb) and space. If you are planning a betta home, see our betta tank setup guide and the minimum size in our betta fish care guide.
Bigger is easier to keep stable. If you have room for a 20 gallon, it is a safer first tank than a 10.
High, long and breeder tanks: why the same gallons look different
Two tanks with the same gallon rating can have very different shapes. A 20 gallon high (24 x 12 x 16 inches) and a 20 gallon long (30 x 12 x 12 inches) hold the same water but stand on different footprints. A high tank takes less shelf space but gives fish less length to swim. A long tank is shallower, which makes the water surface larger and the inside easier to reach.
Breeder tanks, such as the 30 gallon (36 x 18 x 12 inches) and the 40 gallon (36 x 18 x 16 inches), are wide front to back. That depth leaves room to arrange plants and hiding places so fish have more territory. They also need a deeper stand and shelf than a narrow tank of the same length, so check the depth before you buy. When shopping for a stand or lid, always match the exact length and width, not only the gallon number.
How to choose a tank size
- Species first. Adult size and swimming habits decide the minimum, not the number on the box.
- Footprint over height. Length and width give surface area for gas exchange and room for fish.
- Stand and floor. Match the stand to the footprint and plan for the filled weight.
- Maintenance. Smaller tanks need steadier attention. A new setup should be cycled before fish arrive, and routine care is covered in how to clean a fish tank.
FAQ
What are the dimensions of a 10 gallon tank?
20 x 10 x 12 inches.
What are the dimensions of a 20 gallon long?
30 x 12 x 12 inches. A 20 gallon high is 24 x 12 x 16 inches.
What are the dimensions of a 29 gallon tank?
30 x 12 x 18 inches.
What are the dimensions of a 40 gallon breeder?
36 x 18 x 16 inches.
What are the dimensions of a 55 gallon tank?
48 x 13 x 21 inches.
How many gallons is a 24 x 12 x 12 tank?
About 15 gallons (3,456 cubic inches / 231).
The Pet Almanac editorial team researches, writes and edits every care guide on the site. Each guide is fact-checked before publishing and reviewed again when care guidance or equipment changes.