Aquarium Volume Calculator Litres: How to Use It

This calculator works out the water volume of a rectangular aquarium in litres. You enter the tank’s length, width, and height in centimetres, and it returns the volume in litres, along with the volume in US gallons. The result is the total capacity of the tank, not the actual water fill, which is usually slightly less due to substrate, decorations, and the glass thickness.

Formula

The volume of a rectangular prism is length × width × height. For an aquarium, all three dimensions are measured in centimetres, so the result is in cubic centimetres (cm³). Since 1 litre equals 1000 cubic centimetres, divide by 1000 to get litres:

Volume (L) = (Length × Width × Height) / 1000

• Length – the longest horizontal side of the tank, in cm.
• Width – the shorter horizontal side, in cm.
• Height – the vertical distance from bottom to top, in cm.

For US gallons, divide the cubic centimetres by 3785.41 (the number of cm³ in a US gallon).

Worked Example

Using the default values from the calculator: length = 60 cm, width = 30 cm, height = 40 cm.

1. Multiply length × width × height: 60 × 30 × 40 = 72,000 cm³.
2. Divide by 1000 to get litres: 72,000 / 1000 = 72.0 litres.
3. For US gallons: 72,000 / 3785.41 = 19.02 gallons.

The calculator shows 72.0 litres and 19.02 US gallons, matching the result above.

Why Measure Volume in Litres?

Most aquarium equipment – heaters, filters, pumps – is rated for a specific water volume. If you under-size a heater, it won’t keep the water warm enough; if you over-size a filter, it may create too much current. Knowing your tank’s volume in litres lets you choose the right equipment and dose medications or water conditioners correctly. Many liquid treatments list dosages per litre, so an accurate volume is essential for the health of your fish.

Volume also matters for stocking levels. A common rule of thumb is “1 cm of fish per litre of water,� though this is a rough guide and varies by species, filtration, and aquascaping. Always research the specific needs of your fish rather than relying solely on volume.

How to Measure Your Tank Correctly

To get accurate results, measure the inside dimensions of the tank, not the outside. Glass thickness can add 0.5–1 cm on each side, which changes the volume by several litres on a large tank.

• Use a metal tape measure or a ruler that reaches across the tank.
• Measure length from the inside left to inside right, at the top or bottom.
• Measure width from front to back, inside the glass.
• Measure height from the bottom glass to the top edge – but note that you won’t fill the tank to the very top.

If your tank is not a perfect rectangle (e.g., bow-front or hexagonal), this calculator won’t be accurate. For those shapes, you may need a more specific formula or a different tool.

Actual Water Volume vs. Tank Capacity

The calculator gives the total volume of the empty tank. The actual amount of water you can pour in is less because of:

• Substrate – gravel or sand at the bottom displaces water.
• Decorations – rocks, driftwood, and ornaments take up space.
• Equipment – heaters, filters, and pumps sit inside the tank.
• Air gap – you typically leave 2–5 cm empty at the top to prevent fish jumping out and to allow for water evaporation.

As a rough estimate, subtract 10–20% from the calculated volume to get the actual water volume. For a heavily decorated tank, the difference can be more. When dosing medications, always use the actual water volume, not the tank capacity.

Common Mistakes and Unit Mix-Ups

The most frequent error is mixing units. If you measure in inches, you must convert to centimetres before using this formula (1 inch = 2.54 cm). Alternatively, you can calculate in cubic inches and then convert to litres (1 litre = 61.0237 cubic inches).

Another mistake is using the outside dimensions. A tank that is 60 cm long externally might be only 58 cm internally, which reduces the volume by about 5%.

Rounding too early can also cause errors. For example, if you round 72,000 cm³ to 70,000 cm³, you get 70 litres instead of 72 – a 2.8% error that matters when dosing medication.

Limitations of This Formula

This calculator only works for rectangular tanks. Common non-rectangular shapes include:

• Bow-front – the front curves outward, so volume is higher than a straight rectangle with the same width.
• Hexagonal – volume depends on the specific geometry; a simple L×W×H formula overestimates.
• Cylindrical – volume = Ï€ × radius² × height, not L×W×H.

For these shapes, you need a different formula or a dedicated calculator. Also, this calculator assumes the tank is filled to the brim, which is not realistic for a working aquarium.

Frequently Asked Questions

How many litres is a 4-foot by 2-foot by 2-foot tank?

First convert feet to centimetres: 4 ft = 121.92 cm, 2 ft = 60.96 cm. Then multiply: 121.92 × 60.96 × 60.96 = 453,000 cm³, which is 453 litres. This is a common large tank size, often sold as a “450-litre� aquarium.

Does the calculator account for glass thickness?

No, it assumes you enter the inside dimensions. If you only know the outside measurements, subtract twice the glass thickness from each dimension (or just measure the inside) to get an accurate volume.

How much water should I actually put in my tank?

Leave 2–5 cm of air at the top. Also subtract the volume displaced by substrate and decorations. A safe estimate is 80–90% of the total capacity for a typical planted tank with gravel.

Why is my tank’s volume different from what the store says?

Manufacturers often list the “nominal� volume based on outside dimensions or rounded numbers. For example, a “60-litre� tank might actually hold 58 litres when measured inside. This calculator gives the true geometric volume based on your measurements.

Can I use this for a saltwater tank?

Yes, the volume calculation is the same. However, saltwater has a higher density, so weight per litre is about 1.025 kg instead of 1 kg. Volume in litres is still the same.

How do I convert litres to gallons for aquarium equipment?

1 US gallon = 3.78541 litres. Divide the number of litres by 3.78541 to get US gallons. This calculator already does that for you in the second output.

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in l | Length | cm | 60 | 0
in w | Width | cm | 30 | 0
in h | Height | cm | 40 | 0
out vol_l | Volume | litres | (l * w * h) / 1000 | 1
out vol_gal | Volume (US gal) | gal | (l * w * h) / 3785.41 | 2