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Aquarium volume and water change

Works out the real water volume rather than the nominal capacity on the box, and converts a percentage water change into litres.

The question this answers: “How much water is actually in this tank, and how much is a 25% change?”

Roughly 5% for bare-bottom, 10–15% with substrate, more with heavy rock.

Result

Actual water volume
53.5 litres
Nominal capacity is 65 litres. Use this figure for dosing.
A 25% water change
13.4 litres
Before displacement
59.4 litres
Water height only, before substrate and hardscape are subtracted.

Worth knowing about this answer

  • Dose medications and treatments against the actual water volume, not the figure on the box. The error runs towards overdosing, which is the direction that kills fish.
  • The real volume is about 17% below the tank's nominal capacity of 65 litres. That gap is normal and it is why "a 60 litre tank" is rarely 60 litres of water.
Show the working
  1. Water height36cm − 3cm33cm
  2. Volume at that height60 × 30 × 33 ÷ 100059.4 L
  3. Less displacement59.4 × (1 − 0.1)53.5 L
  4. Water change53.5 × 25%13.4 L

Actual water volume: 53.5 litres. A 25% water change: 13.4 litres. Before displacement: 59.4 litres

How it works

Internal dimensions to the actual water line give a gross volume; displacement is then subtracted as a proportion. The nominal capacity is shown alongside so the gap is visible.

What it assumes

  • The tank is rectangular. Bow-front and curved tanks will read high.
  • Displacement is entered by the user as a proportion, because it depends entirely on how much substrate and rock is in the tank.

Where it stops being reliable

  • It gives a volume and nothing else. It deliberately does not suggest a stocking level: that depends on species, adult size, waste output, filtration and planting, and the familiar inch-per-gallon rules are wrong often enough to be harmful.
  • Volume is not the same as the load a floor carries. A modest tank is a substantial weight once filled and furnished, and whether a particular floor takes it is a building question rather than an arithmetic one.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on Fishkeeping.