PLAN YOUR AQUARIUM

Aquarium volume calculator

Estimate your tank’s capacity in litres, US gallons or Imperial gallons. Measure inside the glass and use the water height for an estimate of the water volume.

ESTIMATED CAPACITY

Enter your dimensions

Your volume estimate will appear here.

Substrate, rocks, driftwood and equipment displace water. Allow for these when using the estimate.

TRY IT WITH YOUR TANK

How to use the aquarium calculator

Choose the shape that matches your aquarium, select your measurement unit, then enter the internal dimensions. The result shows litres, US gallons and Imperial gallons together. These two examples show how to go from dimensions to capacity, or work backwards from a target volume.

EXAMPLE 01 · FIND THE VOLUME

A bow-front aquarium

A curved front needs one extra measurement: the full depth at the centre, as well as the depth at the straight sides.

  1. Choose Bow-front and Inches (in) in the calculator above.
  2. Enter a length of 35 in, a side width of 15 in, and a water height of 20 in.
  3. Enter 18 in for the full width at the centre, then calculate.
195.14 litres51.55 US gallons · 42.92 Imperial gallons

This assumes a circular bow. The estimate describes the water space before gravel, hardscape and equipment displace water. An unusual curved front may need a different calculation.

EXAMPLE 02 · FIND A DIMENSION

How big is a 75-gallon tank?

A volume alone cannot tell you a tank’s dimensions: a tall narrow aquarium and a long shallow aquarium can hold the same amount.

  1. For a rectangular tank, choose a target of 75 US gallons in the tool below.
  2. Choose Length as the dimension to find, and Inches (in) as the measurement unit.
  3. Enter a width of 22 in and a water height of 22 in, then calculate.
35.80 in longAbout 35.80 × 22 × 22 in internally

That is about 90.92 × 55.88 × 55.88 cm and 283.91 litres. Use the unrounded length for an exact calculation. A manufacturer's external dimensions or advertised capacity may differ.

WORK BACKWARDS

Find a missing tank dimension

For a rectangular aquarium, enter the capacity you want and two known internal dimensions. We’ll calculate the third in your chosen unit.

The calculation uses length × width × water height. When measuring in centimetres, divide cubic centimetres by 1,000 to get litres. To find a missing length, divide the required cubic centimetres by width × water height.

This is a geometry estimate, not a tank construction specification. Leave space above the waterline and allow for glass thickness, substrate and decor when planning a real aquarium.

ESTIMATED INTERNAL DIMENSION

Your missing dimension will appear here.

COMMON QUESTIONS

Aquarium volume FAQs

How do I calculate the volume of an aquarium?

For a rectangular aquarium, multiply the internal length by width by water height. With centimetres, divide the answer by 1,000 for litres: a 60 × 30 × 30 cm water space holds 54 litres before anything is added. Measure to the actual waterline if the tank is partly filled.

A cylinder uses π × radius² × water height. Our cylinder option asks for the diameter, which is twice the radius. Substrate, rocks, driftwood and equipment reduce the water the tank actually holds.

Does the shape of an aquarium affect its volume?

Yes. A rectangular tank, cube, cylinder and curved-front aquarium use different geometry. Two tanks with the same maximum outer dimensions may have different capacities. Select the correct shape and measure inside the glass.

This calculator supports four shapes. For a corner bow-front, half-cylinder or quarter-cylinder, use the extended Omni Calculator ↗. For an irregular tank, ask its manufacturer for the internal capacity.

How many fish can I keep in a 50-litre aquarium?

There is no reliable single number based on litres alone. Adult size, activity level, schooling needs, temperament, tank footprint, filtration, oxygen and water quality all affect suitable stocking. A simple “centimetres of fish per litre” rule can overcrowd a tank.

Start with a fully cycled aquarium, choose compatible species for the available swimming space and introduce livestock gradually while checking water quality. Tell our team your tank dimensions, filter and intended species so we can help you plan a suitable community. See the care guides or ask our team.

How many gallons can a 3 m³ aquarium hold?

Three cubic metres is 3,000 litres, or approximately 792.52 US gallons and 659.91 Imperial gallons. One cubic metre equals 1,000 litres; one US gallon equals 3.785411784 litres, while one Imperial gallon equals 4.54609 litres.

Check which gallon unit a manufacturer uses before comparing aquarium capacities. These conversions describe geometric volume; the actual water added will be lower once you allow for the waterline, substrate and equipment.

Using your result

Measure the water space

Use internal dimensions to exclude glass thickness. For a partly filled tank, enter the actual water height. For a cube, the result assumes all three dimensions are equal.

Check the gallon type

One US gallon is 3.785411784 litres. One Imperial gallon is 4.54609 litres. The calculator labels both clearly.

Planning livestock?

Ask our team about suitable species, filtration and stocking for your aquarium. A volume estimate alone is not a stocking recommendation.

Need another shape?

The original extended calculator is available from Omni Calculator ↗, including more tank shapes and dimension calculations.