Fish Tank Water Volume Calculator

Tank volume vs water volume

The calculated volume is the full capacity. The actual water volume is lower because substrate, rocks and decor displace water and you leave a gap at the top. Subtract about 10 to 15 percent for a realistic figure.

How to calculate the water volume in a fish tank

The full size of a tank and the amount of water in it are two different numbers. To get the real water volume with this calculator:

1. Measure the inside length and width of the tank, from glass to glass.
2. Measure the water height, not the tank height. Measure from the top of the gravel or sand to the water surface. This removes the substrate and the empty gap at the top in one step.
3. Enter those three numbers in the calculator above. The result is the water in the tank, not the size of the box.
4. Take off a little more for large rocks or wood. If the tank has big hardscape, subtract a further 5 percent or so.

Worked example

A 60 x 30 x 30 cm tank (outside) holds 54 litres when it is empty. Here is what happens once it is set up:

• The inside, with 6 mm glass, is about 58.8 x 28.8 cm.
• There is a 5 cm layer of gravel, and the water stops 2 cm below the rim.
• The water depth is about 29.4 - 5 - 2 = 22.4 cm.
• 58.8 x 28.8 x 22.4 = 37,933 cubic centimetres, or about 37.9 litres.

So the “54 litre” tank holds about 38 litres of water. That is 30 percent less than the label, and the 5 cm gravel bed accounts for most of the difference. With a thin 2 cm layer, the same tank holds about 43.0 litres.

Water does fill the small gaps between grains of gravel, so the true amount of water is a little higher than this method gives. If you need the exact figure, use the bucket method below.

Why the real water volume matters

• Medication and treatments. Doses are given per litre or per gallon of water. Dosing for 54 litres in a tank that holds 38 means about 40 percent too much.
• Salt. Aquarium salt is dosed by water volume. See the aquarium salt calculator.
• Stocking. How many fish a tank can support depends on the water, not the glass. See the aquarium stocking calculator.
• Water changes. A 25 percent change in the example tank is about 9.5 litres, not 13.5.

A rule of thumb when you cannot measure

If the tank is full and you cannot get a tape measure inside, take the full tank volume and use 85 to 90 percent of it for a tank with a normal 3 to 5 cm substrate bed. Use the lower end if you have deep substrate or a lot of rock. The table above shows the 85 percent figure for common sizes.

Measure it exactly with a bucket

The most accurate method is to count the water itself. Next time you do a full refill, use a bucket with a marked 5 or 10 litre line and count the buckets. Write the number down; you will use it for every dose.

For the full tank capacity in litres or gallons, use the litres calculator or the gallons calculator.