Calculate Your Aquarium's Real Volume and the Substrate You Need
This tool was built by Serge (MSc) using established science and government sources. My academic background covers plant biology, environmental biology, and biogeochemistry. My tools are for educational purposes and give practical guidance, not professional advice.
Knowing your tank's real volume matters for stocking, dosing, and water changes, and the number printed on the box is often rounded.
This calculator works out the volume of a rectangular aquarium from its measurements, and estimates how much substrate you need for a planted tank at your chosen depth.
It gives volume in litres, US gallons, and UK gallons.
Tank volume:
This is the full geometric volume. Real water volume is a little less once substrate, rocks, and equipment take up space.
How this works
Tank volume is length times width times water height.
Your measurements are converted to one unit, the volume is worked out, and the result is then converted to litres, US gallons, and UK gallons.
For substrate, the calculator uses the tank's floor area, length times width, times the substrate depth you enter.
That gives the volume of gravel or aquasoil to buy.
Understanding your result
The volume is what the tank holds filled to the water height you gave.
Your real water volume will be a little lower, since substrate, hardscape, and equipment take up room, so for dosing and stocking many aquarists work to about 90 percent of this figure.
For a planted tank, a substrate depth of around 5 to 8 centimetres gives roots room, while sloping it higher at the back looks good and aids planting.
Important limitations
This is for rectangular tanks.
Bow-front, cylindrical, and other shapes hold different amounts and are not covered here.
The volume is geometric, so it does not subtract glass thickness, substrate, or decor, all of which lower the true water volume.
Treat it as a close upper figure.
Scientific basis
The calculation uses standard geometric volume and unit conversions, and draws on public science resources for water measurement and volume references:
Keep Planning With These Calculators
I recommend these calculators alongside this one because a planted tank relies on the right volume, light, and feed working together.
Once the tank volume is known, the Fertiliser Dilution Calculator turns a label dose into the right amount of liquid feed.
Planted tanks need the right light, and the DLI Calculator turns a light-meter reading into a daily light integral.
For deep fills in larger setups, the Soil Volume Calculator works out the amount in litres, cubic feet, and bags.
Related reading
My guide on aquarium substrate covers choosing the right base for a planted tank.
My article on aquascaping tools helps you plant and shape it.
My guide on CO2 systems for planted aquariums pairs with a healthy substrate.
This calculator is for educational purposes and general guidance only. It is not professional aquarist advice. Confirm stocking and dosing against your specific setup and livestock needs.
