How to calculate the volume of an aquarium?

To calculate the volume of a rectangular aquarium, multiply its length, width, and height in centimeters, then divide by 1,000. You get the volume in liters. A 60 x 30 x 30 cm tank therefore holds 54 gross liters.

However, this figure is not the amount of water in your aquarium. The glass, the space below the rim, the substrate, and the décor take up a good portion of it. Here is how to obtain both values, and which one to use depending on what you want to do.

The basic formula

One liter corresponds to 1,000 cm³. Simply calculate the volume in cubic centimeters, then divide by 1,000:

Volume in liters = length x width x height / 1000, with all measurements in centimeters.

Here are a few examples calculated with this formula. These are gross volumes, based on the dimensions shown.

If your measurements are in meters, the product gives cubic meters: multiply it by 1,000 to get liters. Thus, 0.6 x 0.3 x 0.3 m gives 0.054 m³, or 54 liters. If they are in millimeters, divide the product by 1,000,000.

Gross volume and actual volume

The gross volume comes from the tank's dimensions. This is the figure often used to describe an aquarium. The actual volume, or net volume, is the water inside once the aquarium is set up.

An example from our catalog shows this well. The Medaka Aqua One measures 40 x 30 x 24 cm. The gross calculation gives 28.8 liters, while its stated capacity is 25 liters. The difference appears even before adding the substrate and décor.

Four elements lower the volume: the thickness of the glass, the space left below the rim, the substrate, and the décor. On a well-furnished tank, the difference becomes significant.

Calculate the actual volume, step by step

1. Measure inside. Subtract two thicknesses of glass from the outer length and width, or measure directly between the glass panels.
2. Take the water height. Measure from the bottom of the tank to the surface, not up to the rim. An aquarium is rarely filled to the brim.
3. Subtract the substrate. Calculate inner length x inner width x substrate thickness / 1000.
4. Subtract the décor. Measure the volume of the rocks and roots by immersion, as explained below.
5. Round off. A result rounded to the nearest liter is sufficient for all common uses.

Let's take a 60 x 30 x 35 cm tank with 6 mm glass, filled to 3 cm from the rim, with 5 cm of substrate. The inner dimensions are 58.8 x 28.8 cm, and the water height is 31.4 cm, or 35 cm minus 0.6 cm for the bottom and 3 cm of space. The 2-liter décor is an example value.

This 63-gross-liter tank therefore contains about 43 liters of water, or roughly one-third less.

The case of gravel

Water also fills the spaces between the grains. Therefore, removing the entire layer of substrate slightly underestimates the actual volume of water. This calculation remains deliberately cautious, and it is very suitable for dosing a product. To choose the thickness, see our article what substrate height in an aquarium.

The same calculation gives you the amount to buy. Our example requires 8.5 liters of substrate. As a rough guide, one liter of dry gravel weighs about 1.5 kg: so count on nearly 13 kg, or three to four 4 kg bags. Always check the packaging instructions.

Measure a rock or a root

Fill a bucket halfway, submerge the object completely, and mark the water level. Remove the object, then add water with a measuring cup up to the mark. The amount poured corresponds to the volume of the object. A root tends to float: hold it just below the surface, without dipping your hand in.

The most reliable check: count the water

The calculation gives a very good estimate, but nothing beats direct measurement. When you fill the tank with the substrate and décor already in place, count the buckets. With a graduated 10-liter bucket, the result is easy to read. Pour slowly onto a plate placed on the floor so as not to dig into it.

If you fill with a hose, check the water meter before and after, without any other consumption in the house during this time. One cubic meter on the meter corresponds to 1,000 liters.

An external filter also adds a little water to the circuit. This volume remains low in a small aquarium, but it counts more if you use a settling filter.

Other aquarium shapes

Non-rectangular tanks are calculated with a suitable formula. All measurements are in centimeters, and the results are gross volumes.

For a curved front, the curve is the difference between the width at the center and the width at the rim. For a sphere or a free-form aquarium, rely on the capacity indicated by the manufacturer: these tanks are not filled to the top, and a simple geometric formula would be misleading.

Which volume to use, and for what?

Both figures are useful, but not at the same time.

For our 63-liter tank, a filter rated for 40 to 120 liters is suitable. The HeatUp 100 from Oase, designed for up to 100 liters, leaves a margin, whereas a model limited to 50 liters would be too tight given the gross volume. To go further, read what filtration flow rate for an aquarium.

For a 20% water change, rely on the actual volume: 20% of 43 liters is 8.6 liters, not 12.6 liters. Our guide to a successful freshwater aquarium details the pace to follow, and how many fish in an aquarium helps you set the population.

For weight, keep the benchmark of 1 kg per liter of freshwater. A 200-liter aquarium therefore weighs well over 200 kg once equipped, with the glass, substrate, and décor. Install it on a cabinet designed for this purpose, perfectly level.

Common mistakes

• Measuring up to the rim of the tank instead of stopping at the water level.
• Mixing centimeters and meters in the same multiplication.
• Dosing a treatment according to the volume written on the label.
• Forgetting the weight of the substrate and décor when choosing the cabinet.
• Keeping the old figure after adding substrate or décor.

Your questions

How to calculate the volume of an aquarium in liters?

Multiply the length, width, and height in centimeters, then divide the result by 1,000. An 80 x 35 x 40 cm aquarium gives 80 x 35 x 40 / 1000, or 112 gross liters.

What is the volume of a 60 x 30 x 30 cm aquarium?

Its gross volume is 54 liters. The actual water volume is lower, because you must subtract the glass, the space below the rim, the substrate, and the décor.

Should you measure the aquarium inside or outside?

Outer dimensions are sufficient for the gross volume. For the actual volume, measure inside the glass panels and take the water height, not the height of the tank.

How to account for gravel and décor?

Subtract the substrate layer with the calculation inner surface x thickness / 1000. For a rock or a root, submerge it in a marked bucket and measure the water that needs to be added once the object is removed to return to the mark.

How much does a filled aquarium weigh?

Count about 1 kg per liter of freshwater, then add the weight of the glass, substrate, décor, and equipment. Check that the cabinet and the floor of the room can support this assembly.

How to calculate the volume of a cylindrical aquarium?

Use the formula 3.14 x radius x radius x height / 1000, with measurements in centimeters. A cylinder 40 cm in diameter, or 20 cm in radius, and 50 cm high contains approximately 62.8 gross liters.

Read also

• What aquarium to choose?
• When should you change the water in an aquarium?
• How to set up a nano aquarium?