About the Aquarium Volume Calculator
Everything in fish keeping depends on how much water your tank holds: how many fish you can keep, how much conditioner or medicine to add, how big a heater or filter you need, and whether your stand can carry the load. This calculator takes the inside length, width and height of a rectangular aquarium and the level you plan to fill it to, and gives the volume in litres, US gallons and UK gallons. It also shows the weight of the water, which is easy to underestimate, since each litre weighs a kilogram.
Updated October 2026
How to use the Aquarium Volume Calculator
- 1Choose centimetres or inches.
- 2Measure the inside length, width and height of the glass.
- 3Enter how full you will fill the tank. 90 percent is typical.
- 4Read the volume in litres and gallons.
- 5Check the weight of the water against what your stand can support.
Why volume matters
Water volume sets the dose for water conditioners, salt, fertilisers and medicines, which are all written per litre or per gallon. A guess that is 30 percent too low or too high leaves treatment weak or harmful. It also affects water stability: larger volumes change temperature and chemistry more slowly, which is why beginners often do better with a bigger tank.
Heaters are usually sized at 1 watt per litre in a heated room, and filters should turn the tank volume over three to ten times an hour depending on the stock.
Placing a heavy tank
- Use a stand rated for the weight, not a normal cabinet.
- Put the stand on a level floor, ideally over a wall or joist.
- Add foam or a levelling mat under the tank.
- Check the floor loading rules if you live in an apartment.
- Fill slowly to check for leaks before adding fish.
Related calculators
For round tanks and barrels try the Cylinder Calculator. The Volume Calculator handles boxes and prisms, the Pool Volume Calculator is for larger water features and the Unit Converter switches litres, gallons and cubic centimetres. The Density Calculator helps when you want to check how much a decoration weighs.
Features
- Litres, US gallons and UK gallons
- Weight of the water in kg and lb
- Adjustable fill level
- Centimetres or inches
- Shows the volume of a completely full tank
- Quick to use on a phone at the shop
Frequently asked questions
How do I calculate how many litres my tank holds?
Multiply length, width and height in centimetres, then divide by 1,000. A tank of 100 by 40 by 50 cm is 200,000 cubic centimetres, which is 200 litres when full to the brim.
How many gallons is a 100 cm tank?
It depends on the width and height, but a 100 by 40 by 50 cm tank holds about 200 litres, which is 52.8 US gallons or 44 UK gallons. US and UK gallons differ, so check which one a guide is using.
Why is the real volume lower than the calculated volume?
Substrate, rocks, wood, equipment and the space left at the top all displace water. Real water volume is usually 10 to 20 percent lower than the number for a full tank. For medication, dose by the real volume if you can measure it.
How heavy is a filled aquarium?
Water alone is 1 kg per litre, or about 8.3 lb per US gallon. A 200 litre tank holds 200 kg of water, and glass, gravel and decoration can add another 20 to 40 percent. Always put a big tank on a stand built for it.
Does this work for a bow-front or hexagonal tank?
No, it is for rectangular tanks. For a cylinder use the Cylinder Calculator, and for oddly shaped tanks measure by adding known volumes of water from a bucket.
What is a good fill level?
Most keepers fill to about 2 to 3 cm below the rim to stop fish jumping out and to leave room for the filter outlet. In a 50 cm high tank, that is about 94 to 96 percent, and 90 percent is a safe round number.