Setup & Sizing
Fish Tank Volume Calculator
Shape-aware aquarium volume calculator. Pick rectangular, cylindrical or spherical, enter the inside dimensions, and read off litres, UK gallons and US gallons.
Fish Tank Volume Calculator
From substrate line to water line, not total glass height.
Water volume
240L
- UK gallons
- 52.79
- US gallons
- 63.41
- Cubic cm
- 240,000 cm3
How the volume is worked out
Each shape uses a different geometry formula, but every one gives you a cubic-centimetre volume that we divide by 1,000 to get litres.
- Rectangular: length x width x height.
- Cylindrical: pi x (diameter / 2)^2 x height.
- Spherical: (4 / 3) x pi x (diameter / 2)^3.
1 UK gallon = 4.546 litres. 1 US gallon = 3.785 litres. We convert using those exact values so the number matches what you'll see on a UK product spec sheet.
Why accurate volume matters
Stocking advice (inches of fish per gallon), filter turnover (4-6x per hour), heater wattage (2-5 W per litre) and dosing all depend on a reliable volume number. Guessing by "it looks about 100 litres" is the most common reason a fishkeeper ends up with the wrong heater or under-medicated fish.
Frequently asked questions
Does the shape of the tank really change the volume?
Yes, significantly. A cylinder of 40 cm diameter and 60 cm height looks similar to a 40 x 40 x 60 cube, but it only holds about 75 litres versus the cube's 96 litres, because a circle fits inside a square with corners left over. Always pick the shape that matches your tank, even if the difference feels small.
UK gallons vs US gallons - which should I use?
UK fishkeepers almost always work in litres. Older UK filter ratings and heater guides sometimes quote Imperial (UK) gallons, while many online US guides and products quote US gallons, which are smaller (about 3.78 L vs 4.55 L). If in doubt, litres are safest.
Why is my "real" volume lower than this number?
Substrate, rocks, driftwood, pumps and the gap under the rim all reduce the water volume. A typical planted tank holds 10-15% less water than its nominal volume. Subtract around 10% for dosing medications or fertilisers to avoid overshooting.
How do I measure a cylinder tank?
Measure the inside diameter across the top (not the circumference), and the water height from the substrate to the water line. Plug those two numbers in and the calculator handles the rest.
What about bow-front tanks?
Bow-fronts sit between a rectangle and a cylinder. A rough estimate is to use the rectangular mode with the widest width, then subtract about 3%. For exact numbers, check the manufacturer's spec sheet - most list the true volume.
Related calculators
Water Volume
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Heater Size
Get a recommended aquarium heater wattage from your tank volume and the temperature offset you need to hold.
Flow Rate
Find the filter turnover (litres per hour) you should be aiming for based on tank volume and stocking style.