How is this calculated?
The gross volume comes from simply multiplying length × width × height. This is the number often printed on a tank's spec sheet — but it describes the tank filled to the very top rim, which in practice never matches the actual amount of water in the tank.
For the net volume, we subtract realistic factors: the actual water level (almost nobody fills to the rim), the volume taken up by substrate, and a rough estimate for decor, driftwood, rocks, and equipment inside the tank. The result comes much closer to the amount of water you actually move during a water change.
Common standard tank sizes
| Size (L×W×H) | Gross volume | Type |
|---|---|---|
| 24 × 12 × 12 in | ~14 gal | Beginner nano |
| 30 × 12 × 12 in | ~18 gal | Classic small |
| 36 × 18 × 16 in | ~40 gal (breeder) | Popular mid-size |
| 48 × 13 × 20 in | ~55 gal | Community standard |
| 72 × 18 × 21 in | ~125 gal | Large tank |
Click a row to load those dimensions directly into the calculator above.
Why net volume matters for water parameter care
Many dosing recommendations — for fertilizer, medications, or water conditioners — are based on the actual water volume, not the tank's gross volume. Using the gross figure often leads to slight overdosing, which can matter especially for sensitive parameters like copper or CO2.