Conversions · Oct 17, 2026 · By Fish Tank Volume Calculator Team
How Many Gallons Is My Fish Tank? Calculate Aquarium Gallons
Work out how many gallons your fish tank or aquarium holds from its length, width and height, in US and UK gallons, and identify an unknown tank by its size.
Second-hand tanks rarely come with a label, and even new ones are sold by a rounded “nominal” size. If you need to dose medication, buy a heater or plan your stocking, you need to know how many gallons your aquarium really holds. The good news is that a tape measure and one division are all it takes.
This guide shows you how to calculate the gallons in any fish tank, why the answer is often lower than the size printed on the box, and how to identify a standard tank just from its footprint.
The quick answer
Measure the length, width and height in inches, multiply them together, then divide by 231. The result is the volume in US gallons.
US gallons = (length x width x height) / 231
A tank measuring 24 x 12 x 16 inches works out as 24 x 12 x 16 = 4,608 cubic inches, and 4,608 / 231 = 19.9 US gallons. That is a standard 20 gallon high tank.
If you prefer metric, measure in centimetres, multiply, and divide by 1,000 to get litres. Then divide litres by 3.785 for US gallons.
Or skip the arithmetic: the fish tank volume calculator gives you US gallons, UK gallons and litres at the same time, for rectangular and curved tanks alike.
How to measure your tank for gallons
- Measure the length along the front of the tank, from one side panel to the other.
- Measure the width (front to back) across the top.
- Measure the height from the base to the top rim.
- Multiply the three numbers and divide by 231.
Measure the inside of the glass if you can. Outside measurements include the glass thickness and any plastic trim, so they overstate the volume slightly. For the most realistic number, measure the height to the water line rather than the rim, since nobody fills a tank to the very top.
Worked examples for common tanks
| Outside dimensions (in) | Cubic inches | Calculated gallons | Sold as |
|---|---|---|---|
| 20 x 10 x 12 | 2,400 | 10.4 | 10 gallon |
| 24 x 12 x 16 | 4,608 | 19.9 | 20 gallon high |
| 30 x 12 x 12 | 4,320 | 18.7 | 20 gallon long |
| 30 x 12 x 18 | 6,480 | 28.1 | 29 gallon |
| 36 x 18 x 16 | 10,368 | 44.9 | 40 gallon breeder |
| 48 x 13 x 21 | 13,104 | 56.7 | 55 gallon |
| 48 x 18 x 21 | 18,144 | 78.5 | 75 gallon |
| 48 x 18 x 24 | 20,736 | 89.8 | 90 gallon |
Notice that the calculated figure does not always match the name. The 40 gallon breeder works out at almost 45 gallons from its outside dimensions, while the 20 long is closer to 19. Manufacturers round to familiar sizes, so always calculate from your own measurements.
Why your tank holds fewer gallons than its size
Three things reduce the real amount of water in a tank:
- Glass thickness. A 20 gallon high measured outside at 24 x 12 x 16 inches, with 1/4 inch glass, is about 23.5 x 11.5 x 15.75 inches inside. That is 18.4 gallons, not 19.9.
- The gap at the top. Most people fill to about an inch below the rim. In the same tank, that brings the water down to around 17.3 gallons.
- Substrate and decor. Gravel, sand, rock and wood displace water. A typical setup loses another 5 to 10 percent.
As a rule of thumb, the real water volume of a furnished tank is about 85 to 90 percent of the calculated tank volume. Use the water volume calculator when you need that net figure for dosing.
US gallons vs UK gallons
A gallon is not the same everywhere:
- 1 US gallon = 3.785 litres
- 1 UK (imperial) gallon = 4.546 litres
To get UK gallons from inches, divide the cubic inches by 277.4 instead of 231. The same 24 x 12 x 16 inch tank is 19.9 US gallons but only 16.6 UK gallons. If a product’s dosing chart is written for one kind of gallon and you measure in the other, you will dose roughly 17 to 20 percent too little or too much. The UK fish tank volume calculator shows imperial gallons directly.
Converting litres to gallons
If you already know your tank’s volume in litres:
- Litres to US gallons: divide by 3.785 (or multiply by 0.264)
- Litres to UK gallons: divide by 4.546 (or multiply by 0.220)
A 100 litre tank is 26.4 US gallons or 22.0 UK gallons. A 200 litre tank is 52.8 US gallons or 44.0 UK gallons.
Identify an unknown tank by its footprint
If you inherited a tank and want to know what size it was sold as, measure the length and width first. Many standard sizes share a footprint and differ only in height:
- 20 x 10 inches: 10 gallon (12 in tall)
- 24 x 12 inches: 20 gallon high (16 in tall)
- 30 x 12 inches: 20 gallon long (12 in tall) or 29 gallon (18 in tall)
- 36 x 18 inches: 40 gallon breeder (16 in tall)
- 48 x 13 inches: 55 gallon (21 in tall)
- 48 x 18 inches: 75 gallon (21 in tall) or 90 gallon (24 in tall)
See the standard fish tank dimensions guide and the fish tank size chart for the full list.
Frequently asked questions
How many gallons is a 24 x 12 x 16 tank? 24 x 12 x 16 = 4,608 cubic inches, divided by 231 = 19.9 US gallons.
How do I calculate gallons for a round or bow front tank? Curved tanks need a different formula for the base area. Pick the shape in the fish tank volume calculator and enter the measurements it asks for; it handles cylinders, bow fronts, corner tanks and hexagons.
Should I use the rated gallons or my calculated gallons for medication? Use your calculated real water volume. Dosing by the rated size usually means slightly overdosing, because the real water volume is lower.
Once you know how many gallons your tank really holds, every other decision, from heater wattage to how many fish you keep, becomes easier.