Aquarium Volume Calculator
The real water volume, not the box size.
Aquarium Volume Calculator
Net litres = length × width × (height − substrate) × (1 − displacement%) ÷ 1000
Calculated instantly
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What This Calculator Does
The Aquarium Volume Calculator works out how much water a tank actually holds, not just the outside dimensions. It subtracts the substrate layer and a displacement allowance for rock and decoration, then reports litres, US gallons, UK gallons, and the weight of the water.
How It Works
Gross volume is length times width times height, divided by 1,000 to convert cubic centimetres to litres. The substrate reduces the effective height, and the displacement percentage removes the space taken by hardscape. Water weight follows directly because fresh water is close to 1 kg per litre at normal aquarium temperatures.
Example
A 100 × 50 × 50 cm tank is 250 L gross. With 5 cm of substrate the water column is 45 cm, giving 225 L. Allowing 10% for rock and wood leaves about 202.5 L, or 53.5 US gallons and 44.5 UK gallons, weighing roughly 203 kg.
Formula
L × W × H ÷ 1000L × W × (H − substrate) × (1 − displacement) ÷ 1000litres ÷ 3.78541litres ÷ 4.54609Reading the Results
Net water volume
The volume you will actually fill, after substrate and hardscape. Use this figure for stocking, medication, and water-treatment dosing.
Water weight
The mass of the water alone. Add the tank, stand, substrate, and rock to judge whether the floor can carry the full setup.
Sources and References
NIST Special Publication 811 — Guide for the Use of the SI
Cubic centimetre, litre, and gallon conversions
The authoritative conversion factors used by the calculator.
USGS Water Science School — Water properties
Density of fresh water
Supports the approximation of 1 kg per litre used for the weight result.
NIST Handbook 44
US and Imperial gallon definitions
Explains why US and UK gallons differ by about 20%.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Why is the net volume lower than the tank’s advertised size?
Advertised sizes usually quote the outside dimensions or the gross volume. Substrate, rock, wood, and equipment all take up space, and the water line sits below the rim. A tank sold as 250 L commonly holds 200–220 L.
How much substrate should I allow?
About 5 cm is typical for a basic planted tank, and 7–10 cm for deep-rooted plants. Shallower is easier to clean; deeper gives more root space but displaces more water.
Does temperature change the volume?
Slightly. Water expands as it warms, but the change over the aquarium range is well under 1%, so it does not affect stocking or dosing decisions.
Is the water weight the same as the total weight?
No. The water is only part of it. Add the empty tank, stand, substrate, rock, and equipment. A large tank can weigh several hundred kilograms, which is why it should sit across floor joists rather than between them.