How to use the fish tank volume calculator
- Select your tank shape: rectangular or cylindrical.
- Enter the outer dimensions in inches. For rectangular tanks, provide length, width, and water height. For cylindrical tanks, provide diameter and water height.
- Read the calculated water volume in cubic inches, US gallons, UK gallons, and liters.
- Use the water height field to account for substrate, decorations, or an unfilled rim rather than entering the total glass height.
How the volume is calculated
The fish tank volume calculator uses standard geometry formulas based on the shape you select. For a rectangular tank, the formula is Volume = length x width x height. If your tank measures 30 by 12 by 12 inches, the calculation is 30 x 12 x 12 = 4,320 cubic inches.
For a cylindrical tank, the formula is Volume = pi x radius^2 x height, where the radius is half the diameter. A cylinder with a 12-inch diameter and 10-inch water height has a radius of 6 inches, giving pi x 6 x 6 x 10 = 1,130.973 cubic inches.
Once the volume in cubic inches is known, it is divided by 231 to convert to US gallons, since one US gallon is defined as exactly 231 cubic inches. To get liters, the cubic inch value is multiplied by 2.54 cubed and then divided by 1,000.
Common aquarium sizes reference
Fish keepers often refer to tanks by their nominal US gallon size, but actual water capacity depends on the exact dimensions. Here are typical rectangular tank measurements and their approximate volumes when filled to the brim.
| Nominal size | Length (in) | Width (in) | Height (in) | Actual US gallons |
|---|---|---|---|---|
| 5 gallon | 16 | 8 | 10 | 5.54 |
| 10 gallon | 20 | 10 | 12 | 10.39 |
| 20 gallon long | 30 | 12 | 12 | 18.70 |
| 29 gallon | 30 | 12 | 18 | 28.05 |
| 55 gallon | 48 | 13 | 21 | 56.57 |
| 75 gallon | 48 | 18 | 21 | 78.31 |
These figures assume the tank is filled completely. In practice, substrate, rocks, driftwood, and leaving space at the top reduce the actual water volume by 10 to 15 percent.
Why accurate water volume matters
Knowing the exact aquarium volume is essential for dosing medications and water conditioners. Most treatments specify a dosage per gallon or per liter. Overestimating the water volume leads to under-dosing, which may fail to treat parasites or bacterial infections. Underestimating it can result in toxic overdoses that harm fish and invertebrates.
Heater sizing also depends on water capacity. A general rule is 5 watts per gallon for a temperature increase of about 10 degrees Fahrenheit above room temperature. A 20-gallon tank typically needs a 100-watt heater, while a 55-gallon tank requires 250 to 300 watts. Using this fish tank volume calculator ensures you buy the correct equipment.
Filtration flow rates are another consideration. Canister filters and hang-on-back filters are rated by gallons per hour. Matching the filter output to your actual water volume keeps the tank cycled and clear without creating excessive current that stresses small fish.
Accounting for substrate and decorations
The dimensions you enter should reflect the actual water level, not the total height of the glass. Gravel, sand, and aquasoil displace a significant amount of water. One inch of gravel across a 30 by 12 inch footprint displaces 360 cubic inches, which is about 1.56 US gallons.
To get the most accurate reading from the fish tank volume calculator, measure from the top of the substrate to the water surface and use that number as the water height. Large rock formations or driftwood pieces further reduce capacity. If precision is critical for medication dosing, fill the empty tank with a known quantity of water using a measuring jug before adding hardscape, or subtract the estimated displacement of solid objects.
For related calculations involving other container shapes, such as finding the capacity of plumbing lines, see the volume of a pipe calculator. For broader industrial or agricultural containers, the volume calculator for tank covers additional geometries.
US gallons versus UK gallons
Aquarium literature sometimes mixes up US and UK (imperial) gallons, leading to confusion. A US gallon is exactly 231 cubic inches, or 3.785411784 liters. A UK gallon is larger, defined as exactly 4.54609 liters. This means one UK gallon equals about 1.20095 US gallons.
If you follow a stocking guide written by a UK author that recommends one inch of fish per UK gallon, applying that rule directly to US gallons will overstock your tank by roughly 20 percent. Always check which gallon unit the source uses. This tool displays both values so you can match the unit to your local standards or the origin of your reference material.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| Standard 20-gallon long | Tank shape: rectangular; Length (inches): 30; Width (inches): 12; Water height (inches): 12 | Volume (cubic inches): 4,320; US gallons: 18.7013; UK gallons: 15.5721; Liters: 70.7921 |
| 10-gallon tank | Tank shape: rectangular; Length (inches): 20; Width (inches): 10; Water height (inches): 12 | Volume (cubic inches): 2,400; US gallons: 10.3896; UK gallons: 8.6512; Liters: 39.329 |
| Small cylindrical bowl | Tank shape: cylindrical; Diameter (inches): 12; Water height (inches): 10 | Volume (cubic inches): 1,130.9734; US gallons: 4.896; UK gallons: 4.0768; Liters: 18.5333 |
| Large display cylinder | Tank shape: cylindrical; Diameter (inches): 24; Water height (inches): 36 | Volume (cubic inches): 16,286.0163; US gallons: 70.5022; UK gallons: 58.7054; Liters: 266.88 |
FAQ
How do I measure my fish tank for this calculator?
Use a tape measure to find the inside length, width, and water height in inches. Measure the water height from the top of the substrate to the water surface, not the total glass height.
Why does my 20-gallon tank show less than 20 gallons?
Nominal tank sizes are rounded marketing numbers. The actual water capacity depends on the precise interior dimensions, and substrate or decorations further reduce the volume.
Can I use this fish tank volume calculator for saltwater tanks?
Yes. The geometric volume is identical for freshwater and saltwater. However, saltwater is slightly denser, weighing about 8.55 pounds per US gallon compared to 8.34 pounds for freshwater.
Does the calculator include the volume taken up by the filter or heater?
No. Internal equipment like sponge filters, heaters, and powerheads displace a small amount of water. For most home aquariums this difference is negligible, but you can estimate their volume and subtract it if extreme precision is needed.
What is the difference between US and UK gallons?
A US gallon is 3.785 liters, while a UK (imperial) gallon is 4.546 liters. The UK gallon is about 20 percent larger. Always verify which unit a care guide uses.
Is my data sent to a server when I use this tool?
No. The fish tank volume calculator performs all math locally in your browser. Your dimensions are never uploaded or stored.