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 millimeters is plenty.
- Multiply the three numbers. For inches the result is cubic inches (in³); for centimeters it is cubic centimeters (cm³).
- Convert to the unit you need: divide cubic inches by 231 for US gallons (a US gallon is defined as exactly 231 in³). If you measured in centimeters, divide cm³ by 1,000 for liters, then multiply liters by 0.2642 for US gallons or 0.2200 for UK gallons.
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:
US gallons = length (in) × width (in) × height (in) ÷ 231 liters = length (cm) × width (cm) × height (cm) ÷ 1,000 · US gal = liters × 0.2642 · UK gal = liters × 0.2200 Worked example: a 36 × 18 × 16 in tank (40 breeder)
36 × 18 × 16 = 10,368 in³ ÷ 231 ≈ 44.9 US gallons (about 170 liters). That is the gross volume over the outside of the glass; with the glass and a gap below the rim taken off, a 40 breeder holds closer to 40 gallons of water.
Worked example: a 90 × 45 × 45 cm tank
90 × 45 × 45 = 182,250 cm³ → 182.25 liters. 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 liters.
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 |
|---|---|
| liters → US gallons | 0.264172 |
| US gallons → liters | 3.785412 |
| liters → UK gallons | 0.219969 |
| UK gallons → liters | 4.546090 |
| US gallons → UK gallons | 0.832674 |
Common aquarium sizes
Standard outside dimensions and the gross volume they work out to. These are the same presets available in the calculator above. Each US size links to its own page with real water volume, filled weight and heater size, and the aquarium sizes chart compares them all:
| Tank | Dimensions (L × W × H) | US gal | Liters | UK gal |
|---|---|---|---|---|
| 10 gallon | 20 × 10 × 12 in (51 × 25 × 30 cm) | 10 | 39 L | 9 |
| 20 gallon high | 24 × 12 × 16 in (61 × 30 × 41 cm) | 20 | 76 L | 17 |
| 29 gallon | 30 × 12 × 18 in (76 × 30 × 46 cm) | 28 | 106 L | 23 |
| 40 breeder | 36 × 18 × 16 in (91 × 46 × 41 cm) | 45 | 170 L | 37 |
| 55 gallon | 48 × 13 × 21 in (122 × 33 × 53 cm) | 57 | 215 L | 47 |
| 75 gallon | 48 × 18 × 21 in (122 × 46 × 53 cm) | 79 | 297 L | 65 |
| 90 gallon | 48 × 18 × 24 in (122 × 46 × 61 cm) | 90 | 340 L | 75 |
| 120 gallon | 48 × 24 × 24 in (122 × 61 × 61 cm) | 120 | 453 L | 100 |
| 60 cm (metric) | 60 × 30 × 30 cm | 14 | 54 L | 12 |
| 80 cm (metric) | 80 × 30 × 35 cm | 22 | 84 L | 18 |
| 90 cm (metric) | 90 × 45 × 40 cm | 43 | 162 L | 36 |
| 100 cm (metric) | 100 × 40 × 50 cm | 53 | 200 L | 44 |
| 120 cm (metric) | 120 × 45 × 50 cm | 71 | 270 L | 59 |
| 150 cm (metric) | 150 × 60 × 60 cm | 143 | 540 L | 119 |
Why does a "55 gallon" show 57 here? Nominal sizes are measured over the outside of the glass and up to the top of the frame. The water inside sits behind 5–10 mm of glass and an inch or so below the rim, so the real water volume of most US tanks ends up a few gallons under the rating, and lower again once substrate and rock go in. Dimensions also vary slightly between brands.
Glass thickness & fill level (estimating the real volume)
Manufacturer ratings assume brimful, invisible-glass tanks. The calculator's “Fine-tune: glass & water level” section closes the gap:
- Glass thickness (mm): most tanks are built with each pane inside the perimeter, so measuring outside-to-outside you lose two panes per horizontal dimension and one base pane from the height. Typical float glass is 4–10 mm for hobby tanks (roughly 1 mm per 10 cm of height, more for tall ones).
- Distance below rim (cm): the water line usually sits 2–6 cm under the top edge. Enter it and the calculator scales the volume by the real fill height (bowls use the spherical cap formula).
Combine both with the displacement slider and the results panel shows the estimated actual water volume, the number that matters for medication dosing, stocking and heater sizing. These are geometric estimates: the rated volume on the box, or a bucket and a scale, always beat any calculator.
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, fertilizer 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 inches, multiply the three numbers together to get cubic inches, then divide by 231 to get US gallons. In centimeters, multiply the three numbers and divide by 1,000 to get liters (liters × 0.2642 = US gallons). Our calculator does all conversions automatically.
What is the volume of a 60x30x30 cm tank in liters?
A 60 × 30 × 30 cm aquarium has a theoretical volume of 54 liters, calculated as 60 × 30 × 30 ÷ 1,000 = 54 L. For the most accurate result, use the inside dimensions of the tank rather than the outside dimensions. The actual water volume will be slightly lower due to substrate, decorations, equipment, and the space left at the top.
How many liters is a 55 gallon aquarium?
A standard 55 gallon tank measures 48 × 13 × 21 inches on the outside. Multiplying those gives about 215 liters (57 US gallons), but that includes the glass and the frame. Inside the glass, with the water about an inch below the top, it holds roughly 190 liters (about 50 US gallons, 42 UK gallons). Use the calculator with your glass thickness and water level for your exact tank.
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 liters, while one UK (imperial) gallon is 4.546 liters, 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 liter (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 liter tank means about 240 kg of water alone.
Should I measure inside or outside dimensions?
Measure the inside glass-to-glass dimensions for the most accurate result. If you only have outside measurements, open the “Fine-tune: glass & water level” section in the calculator and enter the glass thickness; it estimates the interior by deducting two panes per horizontal side and one base pane from the height. The result is an estimate; a tape measure inside the empty tank always wins.
How do I calculate the water volume if the tank is not filled to the top?
Enter the distance from the water surface to the rim in the “Fine-tune” section. The calculator scales the gross volume by the fill height (and uses the spherical cap formula for bowl-shaped tanks), then applies your displacement setting, giving the actual liters of water rather than the brimful rating. Every result shown this way is an estimated water volume, clearly labelled in the panel.
Next steps
Once you know your volume, size the equipment that depends on it: heater wattage, how much gravel or sand to buy with the substrate calculator, then what that heater and the rest of your gear cost to run with the aquarium electricity cost calculator. Every tool is listed in the guides section. Learn more about the project on the about page, or get in touch if something looks wrong.