Aquarium Volume Calculator
Estimate how much water your tank really holds. Useful for sizing filters and heaters and for reading fertilizer labels.
Your estimate will appear here
The size printed on a tank is a nominal label; the water it actually holds is usually less. Measure inside the glass for a better estimate.
Why your tank holds less than its label
A tank sold as “20 gallons” is named for its approximate size. The glass has thickness, the water stops below the rim, and substrate, rock, wood, and equipment all take up space. Measuring inside the glass to the actual water line gives a more realistic number.
About the displacement allowance
Displacement is a rough percentage you choose, not something the calculator can know. If you need an accurate volume, the most reliable way is to measure the water as you fill or drain the tank.
Not for medication dosing
Use this for equipment planning and as a rough guide when reading fertilizer labels. For medications and anything where overdosing can harm livestock, follow the product’s label and measure the water you actually treat. Dosing for plants is covered in our fertilizer basics guide.
Other planning tools
- Substrate calculator: Liters and cubic feet of substrate for a flat or sloped bed, with an optional weight estimate from the maker's published density.
- Plant problem checklist: Match what you see to possible explanations and the safest first checks. It narrows things down; it doesn't diagnose.
- Maintenance checklist: A routine tailored to your filter, livestock, CO2, and fertilizers: what to glance at daily, do weekly, and check every few weeks.