Fish Tank Size Calculator

What is fish tank size?

Tank size is its internal volume, the amount of water it can hold. Measure the inside of the glass, multiply length by width by height, and convert to litres or gallons.

How to find the size of your tank

“Size” can mean two things: the outside measurements (will it fit?) and the capacity (how much water does it hold?). This calculator gives you the second from the first.

1. Measure the length along the front, the width from front to back and the height from the base to the rim.
2. Enter them in the unit you measured in. Inches, centimetres and feet all work.
3. Read the size in litres, US gallons and UK gallons. Compare it with the table above to see which standard size your tank is closest to.

If you measured the outside, the result is a little bigger than the true capacity, because the glass takes up space. On small tanks the difference is usually 5 to 10 percent.

Working backwards: what size tank fits my space?

If you know how much floor or shelf space you have and the volume you want, divide the volume by the footprint to get the height you need:

height (cm) = litres x 1,000 / (length x width in cm)

• 200 litres on a 100 x 40 cm stand: 200,000 / 4,000 = 50 cm tall.
• About 125 litres on an 80 x 35 cm shelf: 125,000 / 2,800 = about 45 cm tall.
• 54 litres on a 60 x 30 cm desk: 54,000 / 1,800 = 30 cm tall.

Remember to leave room above the tank for the lid, the light and your arm. A tall tank on a short shelf is hard to maintain.

Same volume, different shape

Two tanks can hold the same water and still suit very different fish. A 20 gallon long (30 x 12 x 12 inches) and a 20 gallon high (24 x 12 x 16 inches) are both sold as 20 gallons, but the long version has 360 square inches of floor against 288. Bottom dwellers and active swimmers use length and floor area, so for the same volume a longer, lower tank usually gives them more usable space. Taller tanks suit tall plants and fish that use the upper water, such as angelfish.

Choosing a size

• Start with the fish. Look up the adult size of the species you want, then use the aquarium stocking calculator to check the tank can support them.
• Check the weight. Water alone weighs about 1 kg per litre, so a 200 litre tank puts more than 200 kg on its stand before glass and gravel. The weight calculator gives the full figure.
• Bigger is easier to keep stable. Temperature and water quality change more slowly in a larger volume of water.
• Budget for the equipment. Heaters, filters and lights are all sized by volume. See the heater and filter calculators.

Identify a tank you already own

If the tank has no label, measure its length and width and compare them with the standard fish tank dimensions. Many standard sizes share a footprint and only differ in height, for example the 20 long and the 29 gallon (both 30 x 12 inches). The fish tank size chart lists both US and metric sizes.