20 Gallon Long vs 20 High: Differences Explained
Both hold about 20 gallons, but one is long and low and the other is short and tall. Here's when each one makes sense.
Aquarium volume calculator
Use this fish tank volume calculator to find how many gallons your aquarium holds, in US gallons, litres and UK gallons, plus how much the water weighs.
What is aquarium volume?
Aquarium volume is the amount of water a fish tank can hold, usually given in US gallons in North America and litres elsewhere. It decides almost everything about setting up a tank: how many fish it can support, what size heater and filter it needs, how much water conditioner to dose and how heavy it will be once full.
Tanks are sold by a rounded nominal size, such as "55 gallon", so two tanks sold at the same size can differ by a gallon or more. This aquarium calculator works as an aquarium gallon calculator for your own tank, and doubles as an aquarium size calculator: it shows the closest standard tank size for your measurements.
How to calculate aquarium volume
- Measure length, width and height in inches or centimetres. Inside measurements are most accurate.
- Multiply them together. This gives cubic inches or cubic centimetres.
- Convert the result. Divide cubic inches by 231 for US gallons, or cubic centimetres by 1,000 for litres.
Not sure how to measure? See how to measure a fish tank, including bowfront and corner tanks.
Aquarium volume converter
Already know your tank's volume in one unit? Convert between US gallons, UK gallons, litres, cubic inches and cubic feet. Handy when a product label, a fertiliser dose or a forum post uses a different unit to your tank.
For example, a 40 gallon breeder holds about 151 litres, and a 200 litre European tank is about 53 US gallons.
How much water at a given fill level
Most tanks are filled an inch or two below the rim. Move the slider to see how much water your tank holds at that level. It uses the measurements from the calculator above.
Real water volume
The brim-full figure is always more than the water you actually have. Glass, substrate, rock and the gap below the rim all take space. Adjust these for your setup to find the volume to use for medication and fertiliser doses. Works for rectangular tanks.
Volume formulas for every tank shape
Rectangle or cube
V = L x W x H
Bowfront
V = (L x side depth + bow area) x H
Cylinder
V = 3.1416 x r x r x H
Hexagon
V = 2.598 x side x side x H
Corner tank
V = 0.785 x side x side x H
Popular tank sizes to scale
The most common aquariums drawn at the same scale, front view. Select a tank for its full dimensions, weight and gravel needs.
Fish tank calculators and size guides
Aquarium volume calculatorGallons, litres and water weight for any tank shape.Substrate calculatorPounds, kilos and bags of gravel, sand or aquasoil.Tank size chartEvery standard size in inches, centimetres and litres.Fish tank weight guideHow heavy each tank gets once it's full.Heater size guideHeater watts and filter flow for every tank size.Gravel for a 29 gallon tankHow many pounds and bags a 30 x 12 in floor needs.Gravel for a 40 gallon breeder tankHow many pounds and bags a 36 x 18 in floor needs.Gravel for a 55 gallon tankHow many pounds and bags a 48 x 13 in floor needs.Gravel for a 75 gallon tankHow many pounds and bags a 48 x 18 in floor needs.
Dimensions by tank size
Latest guides
20 Long vs 29 Gallon: Which Fish Tank to Choose
Same footprint, 6 inches of height apart. How the 20 long and 29 gallon compare on volume, weight, equipment and fish.
40 Breeder vs 55 Gallon: Which Tank Is Better?
The 40 breeder is wide and shallow, the 55 is long and narrow. Here's how they compare and which one suits your plans.
Frequently asked questions
How do I calculate the volume of a fish tank?
Multiply length x width x height in inches, then divide by 231 to get US gallons. A 48 x 13 x 21 inch tank holds 13,104 cubic inches, or about 56.7 gallons.
How many litres is my aquarium?
Multiply US gallons by 3.785 to get litres. Working in centimetres, multiply length x width x height and divide by 1,000 to get litres directly.
Should I use inside or outside measurements?
Inside measurements give the most accurate volume, because aquarium glass is commonly 3 to 12 mm thick. Outside measurements are fine for a quick estimate and are what size charts list.
How much does the water in an aquarium weigh?
Fresh water weighs about 8.34 lb per US gallon, or 1 kg per litre. Saltwater is about 3% heavier. Add the tank, stand, substrate and rock for the total.
Why is the real water volume lower than the calculator says?
Substrate, rocks, decor, the gap below the rim and the glass itself all take up space. Real water volume is often 10 to 20% lower than the brim-full figure.
How do I measure a bowfront aquarium?
Measure the length along the back, the depth at the sides, and the depth at the middle of the bow. Choose Bowfront in the calculator and enter all three with the height.