Tank volume calculator
Work out the total volume of your aquarium and a realistic estimate of how much water it actually holds.
Updated 1 August 2026
Results
The estimated water volume accounts for typical substrate depth (around 5 cm) and the gap between the water line and the rim (around 3 cm). Your actual water volume will vary with your specific setup.
Standard Australian tank sizes
Australian shops and keepers size tanks by length in feet. A "4ft tank" is the standard unit of conversation here, where American sources talk about a 55 gallon. The figures below are typical dimensions for each size, with the water volume worked out the same way the calculator does it.
Treat them as a starting point rather than a specification. Height in particular varies between brands, and the difference between a standard and a high model at the same length can be 30% of the volume. If you have the tank in front of you, measure it and use the calculator above.
| Size | Typical dimensions | Total volume | Actual water |
|---|---|---|---|
| 1ft cube | 30 × 30 × 30 cm | 27 L (7 US gal) | about 20 L |
| 45 cm nano | 45 × 27 × 30 cm | 36 L (10 US gal) | about 27 L |
| 2ft | 60 × 30 × 38 cm | 68 L (18 US gal) | about 54 L |
| 3ft | 90 × 38 × 45 cm | 154 L (41 US gal) | about 127 L |
| 4ft | 120 × 45 × 45 cm | 243 L (64 US gal) | about 200 L |
| 5ft | 150 × 45 × 60 cm | 405 L (107 US gal) | about 351 L |
| 6ft | 180 × 45 × 60 cm | 486 L (128 US gal) | about 421 L |
The 4ft tank is worth knowing as a reference point because so much stocking advice is written around it. At roughly 200 litres of actual water it is the smallest tank that comfortably suits most medium community fish, and it is the size at which goldfish stop being an unreasonable idea.
How the calculations work
The total volume is a straightforward geometric calculation based on the shape you've selected. Rectangular and cube tanks use length × width × height. Bow-front tanks add an approximation of the curved front (treated as a half-cylinder along the length of the tank). Cylindrical tanks use π × radius² × height, where the radius is half the diameter you enter.
The estimated water volume is the total volume minus typical losses: roughly 5 cm of substrate at the bottom, and around 3 cm of air gap at the top between the water surface and the tank rim. These are sensible defaults for community freshwater tanks. Tanks with deep planted substrate, large decorations, or unusually high water lines will deviate from the estimate.
Why a tank holds less water than its stated size
A tank's advertised volume is the volume of the glass box. It is calculated before any substrate goes in, and it assumes the water fills the tank to the very top, which no working aquarium does. The real figure is always lower, and the gap is larger than most people expect.
Two deductions do the work. Substrate takes up roughly the bottom 5 cm, and the water line sits about 3 cm below the rim to keep it off the trim and away from the light. That is 8 cm of the tank's height gone, applied across the whole footprint.
The proportional loss depends entirely on how tall the tank is, and this is the part worth paying attention to:
- A 6ft tank at 60 cm tall loses about 13% of its stated volume.
- A 4ft tank at 45 cm tall loses about 18%.
- A 30 cm nano cube loses more than 25%.
The smaller and shallower the tank, the bigger the fraction that disappears, because a fixed 8 cm is a much larger share of a 30 cm height than of a 60 cm one. A nano cube sold as 27 litres is really holding about 20.
This matters most for dosing. Medications, dechlorinator and fertilisers are all dosed per litre of water, not per litre of glass. Dosing a nano tank on its stated volume means overdosing it by a quarter, and that margin is the difference between a safe treatment and a dead shrimp colony. Work from the actual water volume, particularly in small tanks and particularly with anything containing copper.
Tanks this calculator doesn't cover
The shapes above handle the large majority of freshwater tanks. Hexagonal, corner pentagon and L-shaped tanks are not covered, because their geometry varies enough between manufacturers that a generic formula would give a worse answer than the two methods below.
For an angular tank, break the footprint into rectangles and triangles, work out the area of each, add them together, and multiply by the height. A corner pentagon is usually close enough to a quarter circle with two straight sides to approximate that way.
For anything genuinely awkward, measure it by filling instead of calculating. Fill the empty tank using a container of known volume, a 10 litre bucket is ideal, counting as you go and stopping at your intended water line. It takes ten minutes, it needs no geometry, and it is more accurate than any formula because it measures the tank you actually own rather than the one the specification describes.
Why this matters
Tank volume is the single most useful number in fishkeeping. It determines appropriate stocking, heater wattage, filter flow rate, and dosing for medications, fertilisers, and dechlorinator. Manufacturer-stated tank sizes are often rounded or marketing-friendly; actual water volume is what matters for fish welfare and equipment sizing.
Once you know your water volume, you can choose appropriate species for your tank size, calculate water change volumes accurately using our water changes guide, and match equipment to actual capacity rather than stated capacity.