Volume calculator
Aquarium volume calculator
Turn three measurements into gallons and litres — the number you need for heaters, filters, dosing and water treatments.
Volume calculator
How many gallons does it hold?
Interior glass-to-glass measurements. Use the filled height, not the rim of the tank.
Result
of water the tank holds
- Litres
- 0 L
- Water weight
- 0 lb
- Internal volume
- 0 in³
- Footprint
- 0 in²
Guide
Why the volume on the box is wrong
The gallon figure printed on an aquarium box is the manufacturer's maximum — glass to glass, filled to the rim, with nothing inside it. The moment you add substrate, driftwood and rocks, the water volume drops, sometimes by fifteen percent or more. That matters because every dose on a bottle of water conditioner, every heater wattage chart and every fertilizer schedule is written against actual water volume, not the number on the carton.
The calculation itself is straightforward: length × width × height in inches gives cubic inches, and 231 cubic inches make one US gallon. Multiply the gallons by 8.34 to get the weight of the water in pounds, which is worth knowing when you are checking whether a stand or a second-floor floor can take the load.
Subtract what takes up space
Start from the volume above, then take off the space youraquarium substrate occupies. A two-inch gravel bed in a 55 gallon removes roughly 5 gallons of water volume on its own, and a deep planted layout with hardscape can remove considerably more. If you are dosing by the bottle, measure the displacement rather than trusting the label.
Standard tank sizes in practice
A "55 gallon" tank has a 48 × 13 × 21 inch interior, but the true filled volume is closer to 52 gallons once the rim gap and the substrate are accounted for. The same applies to the 20 gallon long at 30 × 12 × 12 inches and the 40 gallon breeder at 36 × 18 × 16. Selecting a preset in the calculator above fills in those interior dimensions so you are working from real measurements rather than marketing names.
Once you know the volume, the next question is what goes on the bottom — thegravel calculator,sand calculator andsoil calculator all take the same footprint and give you a shopping list.