Setup & Sizing
Water Volume Calculator
The simplest way to work out how many litres a rectangular fish tank holds. Enter the internal length, width and height in centimetres and get an instant volume.
Water Volume Calculator
Inside glass, front edge to back edge.
Inside glass, left edge to right edge.
From substrate line to water line, not the glass height.
Total water volume
64.8L
- Cubic centimetres
- 64,800 cm3
- UK gallons
- 14.25
- US gallons
- 17.12
How this calculator works
For a rectangular aquarium, the water volume is simply length times width times height. When your dimensions are in centimetres, dividing the total by 1,000 gives you litres, because one litre is exactly 1,000 cubic centimetres.
Formula: litres = (length x width x height) / 1000
Worked example
A tank that is 90 cm long, 45 cm wide and filled to 40 cm of water holds 90 x 45 x 40 = 162,000 cm3, which is 162 litres. Subtracting around 8% for substrate and hardscape, the actual treatable water volume would be nearer 149 litres - that is the number you should use when dosing medications or fertilisers.
Why accurate volume matters
Medication, dechlorinator and plant-fertiliser doses are all measured per litre. Overdosing is stressful or fatal for fish; underdosing leaves parasites and disease to bounce back. Spending 30 seconds on an accurate volume number can be the difference between a quick recovery and a wiped-out tank.
Frequently asked questions
How do I measure a fish tank for this calculator?
Use a soft tape or ruler and measure the inside of the glass at the longest point (length), the front-to-back distance (width) and from the substrate line up to the normal water line (height). Measuring the inside rather than the outside avoids counting the glass thickness, which on a bigger tank can easily add 10-15 litres of phantom water.
Why is my real volume lower than the calculator says?
A real aquarium never fills to the brim. Subtract 2-3 cm from your height measurement to account for the gap below the rim, and expect 5-10% less again once substrate, rocks, wood and equipment are in the tank. For dosing accuracy, always work from the actual water line, not the glass height.
Does this work for rimless or bow-front tanks?
For rimless rectangular tanks this is fine - just use the water line for the height input. For bow-front, cylinder or sphere tanks the maths is different; use our shape-aware Fish Tank Volume Calculator instead.
How do I convert cm to litres manually?
Multiply length x width x height in centimetres, then divide by 1000. For example, 60 x 30 x 36 = 64,800 cm3, divided by 1000 = 64.8 litres.
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