Way 1: measure it (two minutes)
You need a tape measure and nothing else.
- Measure how long the tank is, left to right.
- Measure how wide it is, front to back.
- Measure how tall it is, bottom to top.
Measure the glass, not the plastic trim. Then do this sum:
Example. Your tank is 30 inches long, 12 inches wide and 18 inches tall. 30 × 12 × 18 = 6,480. Divide by 231 and you get 28. That is a 29 gallon tank.
Do not want to do the sum? Type the three numbers into the aquarium volume calculator and it does it for you. It also handles round, hexagon, corner and bow-front tanks, which need different sums.
Way 2: match the length to a standard size
Most glass tanks are made in a small number of standard sizes. If you know how long the tank is, you are already most of the way there. Measure the length, find it below, then check the height to pick between the options.
| Tank length | It is probably a... |
|---|---|
| 12 inches (30 cm) | 2.5 gallon |
| 16 inches (41 cm) | 5 gallon |
| 20 inches (51 cm) | 10 gallon |
| 24 inches (61 cm) | 15 gallon (12 in tall) or 20 gallon high (16 in tall) |
| 30 inches (76 cm) | 20 gallon long (12 in tall), 29 gallon (18 in tall) or 37 gallon (22 in tall) |
| 36 inches (91 cm) | 30 breeder (12 in tall), 40 breeder (16 in tall) or 65 gallon (24 in tall) |
| 48 inches (122 cm) | 40 gallon long (12 in wide), 55 gallon (13 in wide), 75 gallon (18 in wide, 21 in tall), 90 gallon (18 in wide, 24 in tall) or 120 gallon (24 in wide) |
| 72 inches (183 cm) | 125 gallon (18 in wide, 21 in tall), 150 gallon (18 in wide, 28 in tall), 180 gallon (24 in wide, 25 in tall) or 210 gallon (24 in wide, 29 in tall) |
Here is the full chart of standard sizes. Tap any tank to see its weight, heater, filter and how many fish it holds.
| Tank | L × W × H (in) | L × W × H (cm) | Liters | Empty | Filled, about |
|---|---|---|---|---|---|
| 2.5 gallon | 12 × 6 × 8 | 30 × 15 × 20 | 9 | 5 lb2 kg | 30 lb15 kg |
| 5.5 gallon | 16 × 8 × 10 | 41 × 20 × 25 | 21 | 8 lb4 kg | 60 lb30 kg |
| 10 gallon | 20 × 10 × 12 | 51 × 25 × 30 | 38 | 13 lb6 kg | 105 lb50 kg |
| 15 gallon | 24 × 12 × 12 | 61 × 30 × 30 | 57 | 16 lb7 kg | 155 lb70 kg |
| 20 gallon high | 24 × 12 × 16 | 61 × 30 × 41 | 76 | 25 lb11 kg | 205 lb95 kg |
| 20 gallon long | 30 × 12 × 12 | 76 × 30 × 30 | 76 | 19 lb9 kg | 205 lb95 kg |
| 29 gallon | 30 × 12 × 18 | 76 × 30 × 46 | 110 | 39 lb18 kg | 295 lb135 kg |
| 30 gallon breeder | 36 × 18 × 12 | 91 × 46 × 30 | 114 | 27 lb12 kg | 315 lb145 kg |
| 37 gallon | 30 × 12 × 22 | 76 × 30 × 56 | 140 | 61 lb28 kg | 380 lb170 kg |
| 40 gallon breeder | 36 × 18 × 16 | 91 × 46 × 41 | 151 | 42 lb19 kg | 405 lb185 kg |
| 40 gallon long | 48 × 12 × 16 | 122 × 30 × 41 | 151 | 52 lb24 kg | 410 lb185 kg |
| 55 gallon | 48 × 13 × 21 | 122 × 33 × 53 | 208 | 89 lb40 kg | 570 lb260 kg |
| 65 gallon | 36 × 18 × 24 | 91 × 46 × 61 | 246 | 90 lb41 kg | 650 lb295 kg |
| 75 gallon | 48 × 18 × 21 | 122 × 46 × 53 | 284 | 102 lb46 kg | 755 lb345 kg |
| 90 gallon | 48 × 18 × 24 | 122 × 46 × 61 | 341 | 140 lb64 kg | 910 lb415 kg |
| 120 gallon | 48 × 24 × 24 | 122 × 61 × 61 | 454 | 161 lb73 kg | 1,190 lb540 kg |
| 125 gallon | 72 × 18 × 21 | 183 × 46 × 53 | 473 | 142 lb64 kg | 1,220 lb555 kg |
| 150 gallon | 72 × 18 × 28 | 183 × 46 × 71 | 568 | 265 lb120 kg | 1,535 lb695 kg |
| 180 gallon | 72 × 24 × 25 | 183 × 61 × 64 | 681 | 228 lb103 kg | 1,770 lb805 kg |
| 210 gallon | 72 × 24 × 29 | 183 × 61 × 74 | 795 | 305 lb138 kg | 2,080 lb945 kg |
Way 3: count as you fill
If the tank is an odd shape and empty, fill it with a bucket or jug you know the size of, and count. A standard household bucket holds 2 to 3 gallons (about 10 liters). Check the mark on the side. This is slow, but it gives you the true amount of water the tank holds, which is the number that matters most.
The size on the label is not the water inside
Here is something most people never hear. A "55 gallon" tank does not hold 55 gallons of water. The label size is the tank filled to the very brim, measured from the outside of the glass.
In real life:
- the glass itself takes up room,
- you stop filling an inch below the top,
- the gravel takes up room,
- and so do rocks, wood and the heater.
Add those up and a set-up tank holds about 15% less than its label. A 10 gallon tank holds about 8.5 gallons. A 55 holds about 47.
This matters when you add medicine or water conditioner. Those are measured per gallon of water. If you dose for the label size, you put in too much.
The volume calculator gives you both numbers: the label size, and the real water inside.
US gallons or UK gallons?
They are not the same. A UK gallon is about a fifth bigger than a US gallon. American tank sizes are always in US gallons. If you are in the UK, Canada or Australia, it is simpler to think in liters: one US gallon is 3.785 liters. The gallons to liters converter switches between all three.
Questions and answers
How do I know what size my fish tank is without a label?
Measure its length, width and height in inches, multiply the three numbers, and divide by 231. The answer is the size in US gallons. Or match the length to a standard size: 20 inches long is a 10 gallon, 30 inches is a 20 long or 29, and 48 inches is a 55 or 75.
How many gallons is a 24 × 12 × 16 tank?
24 × 12 × 16 = 4,608 cubic inches. Divided by 231, that is 19.9 gallons. It is a standard 20 gallon high tank.
How many gallons is a 36 × 18 × 16 tank?
36 × 18 × 16 = 10,368 cubic inches. Divided by 231, that is 44.9 gallons brim-full. It is sold as a 40 gallon breeder.
How many gallons is a 4 foot fish tank?
It depends on the width and height. The common four-foot (48 inch) tanks are the 55 gallon (13 inches wide), the 75 gallon (18 inches wide) and the 90 gallon (18 inches wide and 24 inches tall).
Does a 10 gallon tank really hold 10 gallons?
No. Brim-full it holds about 10.4 gallons, but set up with gravel, decor and a gap at the top it holds about 8.5 gallons of water.