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Real water volume after the rim gap and displacement.
Volume matters for three things: dosing, water-change percentage, and stocking judgement. Using the label volume instead of the real one makes every one of those slightly wrong — and in a nano tank, slightly wrong is enough to matter.
What the calculator subtracts
- The air gap below the rim — usually 2–3 cm, and it is not water.
- Substrate: roughly 1 kg per litre of gravel depth per 100 cm² of footprint; the tool takes a percentage instead.
- Hardscape displacement: driftwood and rock typically take 5–15% of the internal volume.
The result is the number to use for water change calculations and for logging tank volume in the app, so percentages stay meaningful as the tank matures.
