🐠 Aquariums

Aquarium Water Volume

Work out how much water a rectangular or cylindrical tank actually holds after substrate and decor. Enter the dimensions, choose centimetres or inches, then press Convert.

Written and reviewed by SM Arijul Islam · Last reviewed 27 October 2026 · Editorial standards · Report an error

About the Aquarium Water Volume

Aquarium volume is usually quoted as the outside dimensions, but the water volume is smaller. Glass thickness, a substrate layer, rocks and equipment all displace water. This calculator subtracts a uniform substrate depth and reports the usable water volume, which is the figure that matters for stocking, medication and filter flow.

For a rectangular tank, volume is length × width × height. A cylindrical tank uses π × radius² × height. If your tank has a bow front or a tapered profile, measure the widest and narrowest points and use the rectangular result as an upper bound.

How the calculation works

Rectangular volume (L) = L × W × H ÷ 1,000 with dimensions in cm.
Cylindrical volume (L) = π × (diameter ÷ 2)² × H ÷ 1,000.
1 US gallon = 3.78541 L; 1 UK gallon = 4.54609 L.
1 L of fresh water weighs about 1 kg; salt water is about 1.025 kg/L.

Reference values

Typical aquarium sizes
Tank	Dimensions	Gross volume	Usable (3 cm substrate)
Nano	30 × 30 × 30 cm	27 L	24.3 L
20 long	60 × 30 × 30 cm	54 L	48.6 L
40 breeder	90 × 45 × 40 cm	162 L	149.9 L
75 gallon	120 × 45 × 50 cm	270 L	253.8 L
125 gallon	180 × 45 × 55 cm	445.5 L	421.2 L

Worked example: a 60 cm tank

A 60 × 30 × 30 cm tank is 54 L gross. With 3 cm of substrate, the usable depth is 27 cm: 48.6 L. At about 1 kg per litre that is 48.6 kg of water, plus the glass and hardscape—place it on a stand rated for the total. A 30 cm cube is 27 L gross and 24.3 L usable.

Use inside dimensions for water volume if you can measure them. Outside dimensions overestimate the volume by the glass thickness; the difference is small on a nano tank but grows on a large one. Stocking, medication and filter flow should all be based on the usable volume, not the box size.

Frequently asked questions

Should I use gross or usable volume?

Use usable water volume for stocking and dosing. Gross volume includes glass, substrate and equipment and can overstate the water by 10–20%.

How much substrate do I need?

A 3–5 cm layer is typical for planted tanks; 5–7 cm for deep-root plants. The calculator subtracts a uniform depth, so measure the average.

How many fish can I keep?

There is no single rule. Use adult size, activity level, filtration and water-change routine. The old 'one inch per gallon' rule is unreliable, especially for goldfish and cichlids.

Do I use inside or outside dimensions?

Use inside dimensions for water volume if you can measure them. Outside dimensions overestimate the volume by the glass thickness, and the difference grows on large tanks.