Last updated: 2026-06-16

Knowing your tank's water volume is the starting point for everything from dosing medications and conditioners to deciding how many fish to keep. The gross volume is what the glass holds at full fill; the net volume is what remains once substrate and decorations displace some of that water.

Volume formulas

For a rectangular tank the gross volume is straightforward:

$V = L \times W \times H$

where $L$, $W$, and $H$ are the interior length, width, and water height.

For a cylindrical tank with interior diameter $D$:

$V = \pi \left(\frac{D}{2}\right)^2 \times H$

Worked example

A common beginner tank measures 60 cm long, 30 cm wide, and is filled to 36 cm:

The 10% deduction for substrate and decor is a rule of thumb; a bare-bottom tank with no ornaments would retain close to the full gross volume, while a heavily aquascaped tank with thick gravel and large rocks might lose 15–20%.

The 1-inch-per-gallon guideline

The traditional stocking rule states that a tank can support roughly one inch of adult fish body length per US gallon of net water. For the 58-litre net volume above (about 15 US gallons), that corresponds to around 15 inches of fish — for example ten 1.5-inch neon tetras.

The rule has well-known limitations. It was developed for small, slender community fish and assumed adequate filtration. Large or heavy-bodied fish produce more waste per inch than slender ones. Goldfish are particularly demanding; a single common goldfish is generally recommended to have at least 75 litres of well-filtered water. Conversely, a heavily planted tank with an oversized filter can often support more fish than the rule suggests. Use the figure as an upper bound under good conditions, not a target.

For large-volume comparisons — pools, ponds, or water features — see the Swimming Pool Volume Calculator.