Aquarium Volume & Weight Calculator

Understanding the aquarium volume calculator — concept and principle

Before you buy a filter, heater, water conditioner or medication, you need to know how much water your tank actually holds. The same number also tells you how heavy the filled tank will be, which matters for the stand, the shelf and the floor. This calculator takes the inside dimensions of a rectangular tank in centimetres and a fill level, and returns the gross tank volume, the water volume in litres and US gallons, and the water weight.

How it is calculated

• Gross volume (L) = length × width × height (cm) ÷ 1,000
• Water volume (L) = gross volume × fill level (%) ÷ 100
• US gallons = water volume ÷ 3.785411784
• Water weight (kg) ≈ water volume (L), because fresh water weighs about 1 kg per litre
• Rough total with substrate and decor = water weight × 1.1

The principle

A rectangular box holds length × width × height of space. One litre is defined as one cubic decimetre, which is 10 × 10 × 10 = 1,000 cm³, so dividing cm³ by 1,000 gives litres. Tanks are rarely filled to the brim: the water usually stops a few centimetres below the rim, so the fill percentage scales the gross figure down. The weight follows from density. Fresh water is about 1.00 kg/L (slightly less when warm); sea water is about 1.02–1.03 kg/L, so a marine tank is a few percent heavier.

Worked example

Default inputs: 60 × 30 × 36 cm, filled to 90%.

1. 60 × 30 × 36 = 64,800 cm³, so the gross volume is 64,800 ÷ 1,000 = 64.8 L.
2. Water at 90%: 64.8 × 0.90 = 58.32 L, shown as 58.3 L.
3. In US gallons: 58.32 ÷ 3.785 ≈ 15.4 gal.
4. Water weight ≈ 58.3 kg; with roughly 10% extra for gravel and decor, 58.32 × 1.1 ≈ 64 kg.

How to read the result

Use the water volume, not the gross volume, for dosing. Gravel, rocks and wood displace water, so the true water volume in a decorated tank is often 10–20% lower than the figure here. For the load, add the glass, the lid, lights and the stand. A common 60 cm tank therefore ends up around 70–80 kg in total, concentrated on a small footprint.

Common mistakes

• Measuring the outside of the tank: glass thickness makes the inside smaller.
• Dosing medication by gross volume and overdosing a decorated tank, or underdosing when a sump adds extra water.
• Putting a heavy tank on furniture that is not rated for the load.

Frequently asked questions

How many litres is a 60 cm tank?

A typical 60 × 30 × 36 cm tank is 64.8 L gross, or about 58 L of water at 90% fill.

How much does 1 litre of aquarium water weigh?

Fresh water is about 1 kg per litre; sea water about 1.02–1.03 kg per litre.

How do I convert litres to US gallons?

Divide by 3.785. UK (imperial) gallons are larger: divide by 4.546.

Does gravel reduce the water volume?

Yes. A 5 cm layer of gravel in a 60 × 30 cm tank takes up about 9 L of space, part of which is filled by water between the grains.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 (definition of the litre) · NIST Special Publication 811 (2008), Appendix B (gallon conversion)