Why use net volume instead of gross?

Substrate, rocks, driftwood, and equipment all take up space that would otherwise hold water. Dosing medications and fertilisers by gross volume overdoses the tank. Net volume, after subtracting displacement, is the true amount of water your livestock actually live in.

How much volume does substrate displace?

It varies, but a typical planted tank with a few centimetres of substrate plus rocks and wood displaces roughly 10 to 15 percent of the gross volume. A bare tank displaces almost nothing. The tool lets you set this percentage to match your aquascape.

How do I convert liters to US gallons?

One US gallon is 3.78541 liters, so divide liters by 3.78541 to get gallons, or multiply gallons by 3.78541 to get liters. The tool shows both at once so you can use whichever your products are labelled in.

Why fill height rather than glass height?

Aquariums are filled to a centimetre or two below the rim to leave room for the lid and to avoid overflow. Calculating with the actual water height gives the real volume; using the full glass height overstates it and skews your dosing.

What is the formula for a hexagon tank?

A regular hexagon's area is 3 times the square root of 3 over 2, times the side length squared. Multiplying that cross-sectional area by the water height gives the volume, which the tool then converts to gallons and liters.

Aquarium Tank Volume Calculator

Knowing your aquarium’s true water volume is the foundation of safe dosing, stocking, and water changes. This calculator handles the common tank shapes — rectangle, cube, cylinder, and hexagon — and reports both gross volume and the net water volume after substrate and decor displacement.

How it works

Each shape uses its standard volume formula, computed from the water height rather than the glass height:

rectangle / cube : length × width × height
cylinder         : π × radius² × height
hexagon (side s) : (3√3 / 2) × s² × height

The raw result is in cubic units, converted to liters and then to US gallons (1 gallon = 3.78541 L). Multiplying by (1 − displacement%) gives the net water volume, which is the number you should use for medication and fertiliser dosing.

Example and tips

A standard 60 by 30 by 36 cm rectangle tank holds about 64.8 liters (17.1 gallons) gross. With a planted-tank displacement of 12 percent, the net water volume drops to roughly 57 liters (15 gallons) — a meaningful difference when dosing a treatment by the liter. Always dose by net volume, fill to your normal water line when measuring, and re-check the figure if you add a large rock or piece of driftwood later.