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Aquarium Volume Calculator
Calculate nowQuick answer
For a rectangular tank, multiply length × width × water height in inches and divide by 231 to get US gallons. For a cylinder, the base is π × radius². For freshwater stocking, the Merck Veterinary Manual gives about 1 inch of fish for every 12 square inches of water surface, so a 30 × 12 inch tank carries about 30 inches of adult fish.
Guide · $9Aquarium Cycling ManualHow the math works
- Surface area: rectangular = length × width; cylinder = π × (diameter ÷ 2)²; bow front = length × depth at the sides + ⅔ × length × (depth at the center − depth at the sides).
- Volume = surface area × water height ÷ 231, in US gallons.
- Stocking = surface area (sq in) ÷ 12, in inches of adult fish.
Measure inside the glass. The bow is approximated as a rectangle plus a parabolic segment, which fits common curved-glass tanks well.
Where the rule comes from
The Merck Veterinary Manual recommends 1 inch of fish per 12 square inches of surface for freshwater, less for cold water. Count the adult length of each fish, not its size at the store, and add fish one or two at a time.
Before you add fish
A new tank needs weeks to grow filter bacteria. Read aquarium cycling and follow the Aquarium Cycling Manual for a day-by-day testing plan. For equipment, see freshwater aquarium for beginners, and for species, goldfish care and betta fish care.
Frequently asked questions
Is this the real amount of water in my tank?
Not quite. It is the gross volume up to the waterline. Gravel, rock, wood and the filter displace some of it, so the actual water is lower. Use gross volume to size the filter and heater, and the actual volume to dose medication and conditioner.
Why does stocking go by surface area instead of gallons?
Oxygen exchange happens at the surface. Two tanks with the same gallons, one tall and narrow and one long and shallow, can hold different amounts of fish.
Does the rule work for saltwater?
No. For saltwater, Merck uses a volume rule instead: about 1 inch of fish for every 5 gallons. Cold water tanks, like goldfish setups, should also be stocked more lightly.
Sources
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