To get an aquarium’s volume in litres, multiply length, width and water height in centimetres and divide by 1,000. A 2 x 1 x 1 ft tank works out to 56.6 litres and a 3 x 1 x 1.5 ft tank to 127.4 litres, both measured to the full outside height. The calculator below does the sum in inches or feet and adds a sump’s water if you run one.

Treat the size printed on a tank’s box as a starting point when you dose medication, CO2 or fertiliser. Real water volume depends on the waterline and what sits in the tank, and a sump adds a second volume that is easy to forget. Heaters, pumps, filters and CO2 systems are all sized from litres, so a wrong volume skews each of them.

How aquarium water volume is worked out

Water volume is length × width × water height, converted to litres. One litre is 1,000 cubic centimetres, which is why the centimetre formula divides by 1,000.

Volume (litres) = (Length cm × Width cm × Water height cm) ÷ 1000

If your tank is measured in feet or inches, convert each side to centimetres first: 1 ft = 30.48 cm and 1 in = 2.54 cm. For a 2 x 1 x 1 ft tank the sides are 60.96 cm, 30.48 cm and 30.48 cm. Multiplied together that is 56,633.7 cubic cm, and dividing by 1,000 gives 56.6 litres.

When all three sides are in feet there is a shortcut. Multiply them together, then multiply by 28.317, the number of litres in one cubic foot (30.48 × 30.48 × 30.48 = 28,316.8 cubic cm). A 2 x 1 x 1 ft tank is 2 cubic feet, and 2 × 28.317 = 56.6 litres.

The calculator below accepts inches or feet, and the Units setting applies to every field, sump included. Set “Water height” to the level you actually fill to. For centimetres, use the formula above.

Aquarium Water Volume Calculator

Enter your tank’s dimensions in inches or feet. If you run a sump, tick the box below to add its water volume to the total.

Units

Main tank

Length Width Water height

I have a sump

Tank volume in litres for common tank sizes

The table shows the volume of the full box at each size, with every side converted using 1 ft = 30.48 cm. These figures are the ceiling. Real water volume is lower, for the reasons in the next section.

Tank size (ft)	Inches	Centimetres	Volume (litres)
2 x 1 x 1	24 x 12 x 12	60.96 x 30.48 x 30.48	56.6
2.5 x 1 x 1	30 x 12 x 12	76.2 x 30.48 x 30.48	70.8
3 x 1 x 1.5	36 x 12 x 18	91.44 x 30.48 x 45.72	127.4
4 x 1.5 x 1.5	48 x 18 x 18	121.92 x 45.72 x 45.72	254.9
5 x 2 x 2	60 x 24 x 24	152.4 x 60.96 x 60.96	566.3
6 x 2 x 2	72 x 24 x 24	182.88 x 60.96 x 60.96	679.6

The first row answers the 2 x 1 x 1 ft question directly: a 2 x 1 x 1 ft tank holds 56.6 litres when full to the top edge.

Why real water volume is lower than the glass volume

The table and the calculator both multiply the dimensions you give them, so they return the volume of a box. Four things make the water in the tank smaller than that box.

Waterline: if you leave a gap below the rim, use the actual waterline as the water height. For the 2 x 1 x 1 ft tank, a waterline 1 in (2.54 cm) below the top edge gives 24 x 12 x 11 in, which is 51.9 litres instead of 56.6.
Glass thickness: if you measured the outside of the tank, the inside is smaller by the glass thickness. Taking 5 mm glass on every panel as an example, the inside of that 2 x 1 x 1 ft tank is 59.96 cm long and 29.48 cm wide, and a waterline 2.54 cm below the rim stands 27.44 cm above the inner floor. 59.96 × 29.48 × 27.44 ÷ 1000 = 48.5 litres, about 8 litres less than the full box.
Substrate: sand or gravel takes up room at the bottom. A 5 cm layer across the floor of that tank is 60.96 × 30.48 × 5 = 9,290 cubic cm, or 9.3 litres of bulk material. That is an upper limit, because water fills the gaps between the grains, so the grains displace less than their bulk. Subtracting the substrate depth from the water height gives a figure that is low.
Rock, wood and equipment: live rock, dry rock, driftwood, heaters and internal filters each displace water equal to their own volume. No standard percentage covers this, and a heavily rocked tank holds noticeably less water than its dimensions suggest.

For the most accurate number, count what you pour in. Fill the tank from a bucket of known size after the substrate and rock are in, and note how many buckets it takes. If you are still choosing a tank, our complete aquarium setup guide covers how volume, filtration and stocking fit together.

How to measure sump water volume and add it to the tank

A sump is a second tank plumbed to the first, so its water counts toward the system total. Our guide to sump filters covers what a sump does.

A sump holds less water than its outer box suggests. Baffles, a filter sock holder and the return pump take up room, and the working chamber normally runs well below the sump’s full height, which leaves space for water that drains back from the display tank when the pump stops. Measure what the water actually fills:

Measure the inside length and width of the chamber, then the height from the floor to the waterline with the return pump running.
If baffles split the sump into chambers with different water levels, measure each chamber separately and add them together.
Ignore small items such as a heater.
Enter the numbers in the calculator’s sump fields, in the same unit as the tank, or use the centimetre formula and add the result to the tank’s volume.

Take both readings with the pump running. When the pump stops, water drains from the display into the sump, so the sump reads higher and the display lower. The total is the same either way, but mixing one reading from each state would count some water twice.

Worked example: a 36 x 12 x 18 in display tank (3 x 1 x 1.5 ft) filled to 1 in below the rim has a water height of 17 in. In centimetres that is 91.44 × 30.48 × 43.18 ÷ 1000 = 120.3 litres. Its sump is 24 in long and 12 in wide, with the water 8 in deep while the pump runs: 60.96 × 30.48 × 20.32 ÷ 1000 = 37.8 litres. The total is 120.3 + 37.8 = 158.1 litres. The calculator rounds each figure to whole litres, so it shows 120, 38 and 158.

If that sump box is 12 in tall and you enter the full height by mistake, it adds 56.6 litres instead of 37.8. The total becomes about 177 litres, which is 19 litres too high. In feet, the same sump is 2 ft by 1 ft with 8 in (0.6667 ft) of water, which is 1.3333 cubic feet, and 1.3333 × 28.317 = 37.8 litres.

What to use the total volume for

The combined figure, display tank plus sump, is the input for most dosing and equipment decisions:

Supplements and medication: doses are usually given per litre or per set volume of water, so dose against the total. The display and sump are one body of water joined by the return pump. Always follow the dosage printed on the product’s label. Our freshwater fish disease and medication guide and marine medicine and quarantine guide cover the treatments themselves.
Salt mix: pack instructions give the salt amount for a set volume of water, so the total in litres tells you how much a full system needs.
Heaters: size wattage from total volume. Our heater wattage guide gives roughly 1 watt per litre as a rule of thumb, between 0.75 and 1.25 watts per litre depending on room conditions. For the 158.1 litre example that is about 119 to 198 watts.
Pumps and flow: size flow as a multiple of total volume. Our submersible pump guide puts a freshwater community tank at roughly 4 to 6 times its volume per hour. For 158.1 litres that is 632 to 949 litres per hour.
Filters: turnover is also a multiple of total volume. See our canister filter buying guide and filter sizing and turnover guide.
CO2 injection: bubble rate starting points scale with tank volume. See our CO2 system comparison.
Protein skimmers: match the rating to total system volume, meaning display plus sump plus any refugium. See our protein skimmer sizing guide.

Common questions about tank and sump volume

How many litres is a 2x1x1 ft tank?

A 2 x 1 x 1 ft tank (60.96 x 30.48 x 30.48 cm) holds 56.6 litres when filled to the top edge. With the water 1 in below the rim it holds 51.9 litres, and glass thickness, substrate and rockwork lower it further.

How do I measure a sump?

Measure the inside length, width and running waterline height of each water-filled chamber. Multiply the three in centimetres, divide by 1,000 for litres, and add the chambers together. Do not use the outer height of the sump box.

Does substrate reduce the water volume of a tank?

Yes. A 5 cm layer across the floor of a 2 x 1 x 1 ft tank is 9.3 litres of bulk material at most, and the real displacement is lower because water fills the gaps between the grains.

Should I include the sump when dosing?

Yes, when the sump is plumbed to the display tank. A dose spreads through all the connected water, so use display plus sump. Follow the product label where it says anything different.

Does the calculator give litres, and can I enter centimetres?

The results are in litres, rounded to whole litres. The inputs are inches or feet. For centimetres, multiply length, width and water height and divide by 1,000, or divide each side by 2.54 to get inches first.