Aquarium Volume Calculator

The number on the box is the outside dimensions multiplied together, which is never what goes in it. Glass takes room, gravel takes more, and the water line sits below the rim — a tank sold as 64 litres holds nearer 50 once it is set up. That matters, because doses and water changes are worked out per litre.

Shape

Length in centimetres

Outside measurement.

Width in centimetres

Front to back, outside.

Height in centimetres

Outside, base to rim.

Glass thickness in millimetres

Each pane. Six is typical for a small tank.

Substrate depth in centimetres

Gravel or sand on the bottom.

Water sits this far below the rim

Centimetres of freeboard.

Substrate porosity

Fraction of the gravel that is water in the gaps. 0.35 for gravel.

Water change, per cent

Of the water in the tank, not of the box figure.

Bucket size in litres

How much you carry at a time.

49.4 litres of water

13 US gallons, 10.9 imperial. The outside measurements would give 64.8 litres — the difference is 10.6 litres of gravel and air, and 4.9 litres of glass.

A 25% change is 12.3 litres — 2 buckets.

✓ The water is what is left after the glass, gravel and air — 59.9L inside the glass, less 10.6L of gravel and air, leaves 49.4L of water
✓ The inside is smaller than the outside — 64.8L from the outside measurements against 59.9L inside — 4.9L of that is glass
✓ The gallons convert back to the litres — 13 US or 10.9 imperial gallons, both converting back to 49.4L
✓ A water change is that percentage of the water, not of the tank — 25% of 49.4L is 12.3L — 2 buckets of 10L
✓ The buckets hold at least the change — 20L of bucket for a 12.3L change

What this generator does

Takes the glass off each dimension to get the inside volume, subtracts the substrate that is not water and the air above the water line, and converts what is left into gallons and buckets.

How to use this tool

Choose the shape and type the outside measurements.
Add the glass thickness, the substrate depth and how far below the rim the water sits.
Read the real volume in litres and both kinds of gallon.
Set a water change percentage to see the litres and the buckets.

Understanding the controls

Shape: Rectangular, a cube from a single measurement, a cylinder by diameter, or a bow front with a curved front panel.
Length, width and height: The outside measurements, as a shop would quote them. The glass is taken off inside.
How far the front bows out: For a bow front: how far beyond the flat the curve reaches at its deepest. The extra volume is the usual approximation for a shallow curve, about two thirds of the bounding rectangle.
Glass thickness: Each pane, in millimetres. It comes off both sides of the length and width and once from the height, which is why a small tank loses proportionally more.
Substrate depth: How deep the gravel or sand is. Only the solid part displaces water — the rest of it is gaps.
Water sits this far below the rim: The freeboard. A few centimetres of air is normal and is not water.
Substrate porosity: What fraction of the substrate volume is water in the gaps between grains. Around 0.35 for gravel, less for fine sand.
Water change, per cent: Taken as a percentage of the water actually in the tank, which is the point of working the real volume out first.
Bucket size: How much you carry at a time, to turn the change into a number of trips.

Common use cases

Dosing anything by the litre, where the box figure would be twenty per cent out
Working out how many buckets a weekly water change actually takes
Comparing tanks by what they hold rather than by their footprint
Converting between litres and US or imperial gallons without a decimal point going astray
Seeing how much volume a deep substrate costs you

How this generator works

The inside dimensions are the outside ones less a pane of glass on each side, and the inside volume follows from the shape. Two things are then taken off: the solid part of the substrate, which is its volume less the water held in the gaps, and the air above the water line. The checks prove the relationship rather than restating it — the inside volume less what is displaced must equal the water, the inside must be smaller than the outside, both gallon conversions must convert back to the litres they came from, and the water change must be its percentage of the water rather than of the box figure.

Randomness and fairness

Nothing is random here at all. The same measurements always give the same volume, down to the last tenth of a litre.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

The bow front figure is the usual approximation for a shallow curve, not an exact integral of the glass.
Rocks, wood and equipment displace water too and are not counted here.
Porosity varies with grain size; fine sand holds less water in its gaps than coarse gravel.
This is geometry, not fishkeeping advice — stocking and dosing are somebody else's field.

Privacy and your data

The volumes are worked out in your browser. No tank measurements are transmitted or stored.