Aquarium volume calculator
How many gallons is your tank really? Enter the inside dimensions for the volume in gallons and litres, and how much the water weighs — the number every other sizing decision starts from.
Measure the inside of the glass, and the real water line sits about an inch below the rim, so actual filled volume runs a little under this. Substrate and rock displace more.
The formula
For a rectangular tank, volume in US gallons is simply length × width × height (in inches) ÷ 231, because there are 231 cubic inches in a US gallon. Water weighs about 8.34 pounds per gallon, so a filled tank is heavy fast: a standard 40-gallon tank holds over 330 pounds of water before you add glass, substrate and rock.
Why the real number is a little lower
Two things pull actual water volume below the box math. The water line sits an inch or so below the rim, and substrate, rocks and hardscape displace water. For stocking and dosing, err toward the lower real volume; for checking whether a stand can take the load, use the full weight above and then some.
Why volume drives everything else
Volume is the input to nearly every other decision: heater wattage, filter turnover, dose size and how many fish the tank can hold. Get it right once and the rest follows. Size the gear next with the heater wattage calculator, the filter GPH calculator and the conditioner dosing calculator.
Source
- US gallon = 231 cubic inches; fresh water ≈ 8.34 lb/gal (standard reference values).
Common questions
How many gallons is a 24x12x16 inch tank?
About 20 US gallons. Multiply length by width by height in inches and divide by 231 (there are 231 cubic inches in a US gallon): 24 x 12 x 16 / 231 is roughly 20. The real filled volume is a little less once you subtract the space above the water line and what substrate and rock displace.
How do I calculate aquarium volume in litres?
Work out US gallons (length x width x height in inches, divided by 231) and multiply by 3.785. The calculator shows both gallons and litres at once.
Why is my tank's real volume less than the label?
The water sits about an inch below the rim, and substrate, rocks and hardscape displace water, so the real volume runs below the box figure. Use the lower number for stocking and dosing, and the full figure for checking stand weight.
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