Not every aquarium is a box. Bowls, cylinders, hexagons and bow fronts are sold in great numbers, and the volume on the label is sometimes a round figure that does not match the geometry. When you need to choose a filter, size a heater or plan a water change, you need the real number. This guide compares the formulas for rectangular and round tanks, gives a cylinder table for common sizes, shows how a round tank compares with a rectangular one of the same footprint, covers hexagons and bow fronts and explains why the water volume is lower than the geometry suggests. The Aquarium Volume Calculator handles rectangles and cylinders.
The formulas
| Shape | Volume in litres (centimetres in) |
|---|---|
| Rectangular | length × width × height ÷ 1,000 |
| Cylinder (round tank) | π × radius² × height ÷ 1,000, where radius = diameter ÷ 2 |
| Regular hexagon | 2.598 × side² × height ÷ 1,000 |
| Bow-front | Rectangle plus the extra from the curve; best measured by filling |
The height is the water height, not the glass height, because the water level sits below the rim. Use inside measurements, since the glass takes up space; see how to calculate aquarium volume for how much.
Worked examples
A rectangular tank, 60 × 30 cm and 36 cm of water: 60 × 30 × 36 = 64,800 cm³ = 64.8 litres, about 17.1 US gallons.
A cylinder, 40 cm diameter and 50 cm water height: radius 20 cm, area π × 20² = 1,256.6 cm², volume 62,832 cm³ = 62.8 litres, about 16.6 US gallons.
A larger rectangle, 90 × 45 × 45 cm: 182.25 litres, about 48.1 US gallons.
| Tank | Inside size (cm) | Volume (L) | US gallons |
|---|---|---|---|
| Small rectangle | 40 × 25 × 30 | 30.0 | 7.9 |
| Rectangle | 60 × 30 × 36 | 64.8 | 17.1 |
| Cylinder | 40 diameter × 50 | 62.8 | 16.6 |
| Rectangle | 90 × 45 × 45 | 182.3 | 48.1 |
| Rectangle | 120 × 50 × 50 | 300.0 | 79.3 |
The 60 × 30 × 36 rectangle and the 40 × 50 cylinder hold almost the same volume, though they look nothing alike.
A cylinder table
| Diameter | 40 cm high | 50 cm high | 60 cm high |
|---|---|---|---|
| 30 cm | 28.3 L (7.5 gal) | 35.3 L (9.3 gal) | 42.4 L (11.2 gal) |
| 40 cm | 50.3 L (13.3 gal) | 62.8 L (16.6 gal) | 75.4 L (19.9 gal) |
| 50 cm | 78.5 L (20.8 gal) | 98.2 L (25.9 gal) | 117.8 L (31.1 gal) |
| 60 cm | 113.1 L (29.9 gal) | 141.4 L (37.4 gal) | 169.6 L (44.8 gal) |
Figures are for the full height. Subtract the gap below the rim to get the water: for a water level 5 cm below the top, use 5 cm less.
Why a cylinder holds less than its box
A cylinder inscribed in a square box occupies π ÷ 4 = 78.5% of the box’s footprint. If you compare a 50 × 50 cm square tank with a 50 cm diameter cylinder of the same height, the cylinder holds 21.5% less water.
| Same height (50 cm) | Footprint | Volume |
|---|---|---|
| Square 50 × 50 cm | 2,500 cm² | 125.0 L |
| Cylinder 50 cm diameter | 1,963 cm² | 98.2 L |
That is useful when you compare a round tank with a square one of the same width: the round one holds about 79% as much. It also means that a round tank needs a larger diameter than the width of a square one to hold the same volume.
Same volume, different footprint
For 100 litres, how big must each shape be?
| Shape | Dimensions for about 100 L |
|---|---|
| Cylinder, 50 cm diameter | about 51 cm high |
| Rectangle 60 × 30 cm | about 56 cm high |
| Rectangle 80 × 40 cm | about 31 cm high |
A tall, narrow tank has a small footprint and a small water surface; a long, low tank has a large surface. The surface area matters for gas exchange and for how the tank looks, and a specialist can advise on what suits your stock. A round tank also gives less room along the glass for swimming. The calculator gives volume; it does not tell you which shape is suitable for the fish.
Hexagons and bow fronts
Hexagon. A regular hexagon with side s has an area of about 2.598 × s². A tank with a 25 cm side and 50 cm water height: 2.598 × 625 = 1,623.8 cm² × 50 = 81,188 cm³ = 81.2 litres. A smaller hexagon of 20 cm side and 40 cm height is 41.6 litres.
Bow front. The front is curved, so the volume is more than the rectangle through the corners and less than a rectangle at the widest point. The most reliable way to find the volume is to fill the tank from a measured container, or to use the maker’s stated capacity, then allow for displacement.
Corner and quarter-circle tanks. A quarter circle of radius r has an area of π × r² ÷ 4. Combine with any straight part.
Real water volume
The geometry gives gross volume. Real water is less, because:
- Glass thickness takes space from the inside.
- The water line sits below the rim, often by a few centimetres.
- Substrate and decor displace water.
- Equipment takes up space.
For a cylinder of 62.8 litres, a 10% reduction for water level and displacement gives about 56.5 litres; for the 90 × 45 × 45 rectangle, 164 litres instead of 182. If you need to be accurate, measure by filling. Our guides on substrate volume and water changes use the real volume.
Weight
Water weighs about 1 kg per litre, so the 62.8 litre cylinder holds 62.8 kg of water, and the 182 litre rectangle 182 kg. Round tanks often stand on a small base, so the load is concentrated; check the stand and the base. See how much a fish tank weighs for the full sum and fish tank size chart in gallons for common rectangular sizes.
Choosing between shapes
| Consideration | Rectangular | Round | Hexagon or other |
|---|---|---|---|
| Volume for the footprint | Highest | About 79% of the box | Between |
| Surface area | Larger for a given volume | Smaller | Varies |
| Filtration and equipment | Wide choice | Fewer options | Fewer |
| View | Standard | Distorted at the edges | Varies |
| Cost | Usually lower | Often higher | Often higher |
Ask a specialist what suits the animals you want to keep.
Working in inches and US gallons
If your tank is sold in inches, convert at the end, not at the start. A US gallon is 231 cubic inches.
| Tank | Inside size (in) | Volume (in³) | US gallons |
|---|---|---|---|
| Cylinder | 16 diameter × 20 high | π × 8² × 20 = 4,021 | 17.4 |
| Rectangle | 24 × 12 × 14 | 4,032 | 17.5 |
Here a cylinder of 16 inches diameter and 20 inches height holds almost exactly the same as a 24 × 12 × 14 inch rectangle, even though their footprints differ: 201 square inches against 288. That is the same pattern as in the metric table, where the cylinder needs more height to match a box.
Filling to a level
A tank is rarely filled to the brim. At 90% fill, the 62.8 litre cylinder holds 62.8 × 0.9 = 56.5 litres of water. Use that figure for treatments and water changes, and keep the full figure for choosing the stand. The Aquarium Water Change Calculator takes the water volume you give it.
Common mistakes
- Using diameter as radius.
- Using glass height as water height.
- Forgetting to subtract glass thickness.
- Comparing a cylinder with a box of the same width.
- Trusting the labelled volume of a curved tank.
- Mixing centimetres and inches.
- Treating gross volume as water volume.
Quick summary
- Rectangle: L × W × H ÷ 1,000 litres; cylinder: π × r² × H ÷ 1,000 (centimetres).
- A 60 × 30 × 36 cm rectangle is 64.8 litres; a 40 cm wide, 50 cm high cylinder is 62.8 litres.
- A cylinder holds about 78.5% of the box around it.
- Use inside measurements and the water height; fill to measure curved tanks.
- Real water volume is lower than gross volume, and water weighs about 1 kg per litre.
- Use the Aquarium Volume Calculator.
Frequently asked questions
How do I calculate the volume of a round tank?
Use π × radius² × water height. For a diameter of 40 cm and a height of 50 cm that is about 62.8 litres.
How do I calculate a rectangular tank?
Length × width × height in centimetres divided by 1,000 gives litres.
How do I calculate a hexagonal tank?
Area = 2.598 × side², then multiply by the water height. A 25 cm side and 50 cm height is about 81 litres.
Why does a round tank hold less than its box?
A cylinder fills about 78.5% of the square box around it, so it holds less than a rectangle of the same width, depth and height.
What height do I use?
The water height, which is below the glass height.
Is the stated volume exact?
No. Glass thickness, the water line, substrate and decor all reduce real water volume.
Sources
- Pure geometry: rectangle L × W × H; cylinder π × r² × H; regular hexagon area = 3√3/2 × side² ≈ 2.598 × side²; 1 litre = 1,000 cm³
- NIST Guide to the SI, Appendix B.8 (exact US liquid gallon = 3.785411784 litres)
Related calculators
- Aquarium Volume CalculatorWork out your fish tank's volume in litres and gallons from its dimensions.
- Aquarium Water Weight CalculatorWork out how much the water in a fish tank weighs so you can check your stand and floor.
- Aquarium Water Change CalculatorWork out how much water to replace for the percentage change you choose.
Keep reading
- How to Calculate Aquarium Volume (Litres/Gallons)How to calculate aquarium volume from length, width and height in cm or inches: the formulas, US and UK gallon conversions and real water volume.
- Fish Tank Size Chart: Gallons, Litres and WeightA fish tank size chart for common US gallon sizes with typical dimensions, volume in gallons and litres and water weight, plus how to read nominal sizes.
- How Much Does a Fish Tank Weigh? Full Tank WeightHow to estimate the weight of a filled fish tank: water at 1 kg per litre or 8.35 lb per US gallon, plus glass, substrate, rock and stand, with examples.
- US Gallons vs UK Gallons for AquariumsA US gallon is 3.785 litres and a UK gallon 4.546, a 20% gap. Convert tank sizes both ways, see why 55 gallons can mean 208 or 250 litres, and fix water sums.