How to calculate aquarium volume
Every rectangular tank follows the same three-step process. Measuring inside the glass (glass-to-glass) gives the most accurate result, because nominal sizes like “20 gallon” refer to the filled volume before glass thickness, trim and the gap below the waterline.
- Measure length, width and height of the tank interior. A tape measure or ruler is fine — accuracy to a few millimetres is plenty.
- Multiply the three numbers. For centimetres the result is cubic centimetres (cm³); for inches it is cubic inches.
- Convert to the unit you need: divide cm³ by 1,000 for litres, or multiply litres by 0.2642 for US gallons, 0.2200 for UK gallons. If you measured in inches, first multiply cubic inches by 0.0164 to get litres.
The calculator above performs all of this instantly and also lets you subtract the space taken by substrate and hardscape — see gross vs actual water volume below.
Aquarium volume formula
For a rectangular (box-shaped) aquarium the gross volume is simply the volume of a cuboid:
V (cm³) = length (cm) × width (cm) × height (cm) litres = cm³ ÷ 1,000 · US gal = litres × 0.2642 · UK gal = litres × 0.2200 Worked example — a 90 × 45 × 45 cm tank
90 × 45 × 45 = 182,250 cm³ → 182.25 litres. In US gallons: 182.25 × 0.2642 ≈ 48.2 US gal. In UK gallons: 182.25 × 0.2200 ≈ 40.1 UK gal. If that tank holds about 12% substrate and rock, actual water volume is roughly 160 litres.
Odd-shaped tanks
Bowfront, hexagonal and cylinder tanks need different formulas (a cylinder is π × r² × height). A practical trick: measure the tank in sections and add them up, or approximate with the average width. The result will be within a few percent of reality — good enough for heater and filter sizing.
Liters to US and UK gallons
The two gallons are not interchangeable: a UK (imperial) gallon is about 20% bigger than a US gallon. Use these exact factors:
| Conversion | Multiply by |
|---|---|
| litres → US gallons | 0.264172 |
| US gallons → litres | 3.785412 |
| litres → UK gallons | 0.219969 |
| UK gallons → litres | 4.546090 |
| US gallons → UK gallons | 0.832674 |
Common aquarium sizes
Typical manufacturer dimensions and their calculated volumes — the same presets available at the top of this page:
| Tank | Dimensions (cm) | Litres | US gal | UK gal |
|---|---|---|---|---|
| 10 gallon | 51 × 25 × 31 cm | 39 L | 10 | 9 |
| 20 gallon | 61 × 31 × 41 cm | 76 L | 20 | 17 |
| 29 gallon | 76 × 31 × 46 cm | 106 L | 28 | 23 |
| 40 gallon | 91 × 41 × 41 cm | 151 L | 40 | 33 |
| 55 gallon | 122 × 33 × 56 cm | 225 L | 59 | 49 |
| 75 gallon | 122 × 46 × 53 cm | 297 L | 78 | 65 |
| 90 gallon | 122 × 46 × 64 cm | 354 L | 93 | 78 |
| 120 gallon | 122 × 61 × 64 cm | 472 L | 125 | 104 |
| 60 cm (metric) | 60 × 30 × 30 cm | 54 L | 14 | 12 |
| 80 cm (metric) | 80 × 30 × 35 cm | 84 L | 22 | 18 |
| 90 cm (metric) | 90 × 45 × 40 cm | 162 L | 43 | 36 |
| 100 cm (metric) | 100 × 40 × 50 cm | 200 L | 53 | 44 |
| 120 cm (metric) | 120 × 45 × 50 cm | 270 L | 71 | 59 |
| 150 cm (metric) | 150 × 60 × 60 cm | 540 L | 143 | 119 |
Volumes assume exact inside dimensions; real tanks vary by a few percent between manufacturers.
Gross volume vs actual water volume
Gross volume is the full cuboid volume — what the tank would hold filled to the very brim with nothing inside. Actual (net) water volume is what remains once the aquascape displaces water and the level sits below the rim. It matters because equipment sizing, medication doses, fertiliser dosing and stocking rules all depend on the water that is actually there.
- Substrate bed (3–5 cm deep): 3–8% of volume
- Rockwork and driftwood: 3–10%
- Gap below the rim (2–5 cm): 2–6% on small tanks
- Filter volume inside the tank: 1–3%
A lightly planted community tank typically ends up 8–12% below gross; a heavily aquascaped iwagumi-style layout can lose 20% or more. Set the substrate & decorations slider in the calculator to model your own setup.
Frequently asked questions
How do I calculate aquarium volume?
Measure the inside length, width and height of the tank in centimetres, multiply the three numbers together to get cubic centimetres, then divide by 1,000 to get litres. Multiply litres by 0.2642 for US gallons or 0.2200 for UK (imperial) gallons. Our calculator does all conversions automatically.
How many litres is a 55 gallon aquarium?
A US 55 gallon tank holds about 208 litres of gross volume; in UK gallons it is about 46 gallons. The exact figure depends on the real dimensions — nominal sizes like “55 gallon” are rounded and usually slightly lower once glass thickness and trim are considered.
What is the difference between gross and actual water volume?
Gross volume is the full geometric volume of an empty tank filled to the brim. Actual (net) volume is what you get after subtracting substrate, rocks, driftwood, filters and the gap below the waterline — typically 5–15% less than gross in a planted tank, more in heavily aquascaped ones.
Are US and UK gallons the same?
No. One US gallon is 3.785 litres, while one UK (imperial) gallon is 4.546 litres — about 20% larger. Tank ratings in the US use US gallons; aquariums sold in the UK are often described with imperial gallons, so always check which unit is meant.
Does substrate and decoration displacement matter?
Yes, especially for medication dosing and stocking. A deep soil substrate plus large rocks can easily displace 10–20% of the water. Use the displacement slider to estimate true water volume; 5–15% is a typical planted-tank range.
How heavy is my aquarium once filled?
Water weighs almost exactly 1 kg per litre (8.34 lb per US gallon). Add roughly 10–15% for glass, stand, substrate and equipment, then make sure the floor and stand are rated for the total. A 240 litre tank means about 240 kg of water alone.
Should I measure inside or outside dimensions?
Measure the inside glass-to-glass dimensions. Glass thickness and trim can remove several litres from the rated volume, especially on smaller tanks. If you can only measure outside, subtract roughly twice the glass thickness per dimension.
Next steps
Once you know your volume, the natural follow-ups are heater size, filter turnover and stocking levels. Those guides — plus practical articles on measuring and maintenance — live in the guides section. Learn more about the project on the about page, or get in touch if something looks wrong.