Aquarium volume and stocking calculator
Enter your tank's inside measurements to see how much water it really holds, what it weighs, and the heater, filter, and substrate it needs.
Your tank
Total volume
18.7US gallons
70.8 L / 15.6 UK gal
- Actual water (about)
- 15.9 gal / 60 L
- After substrate and decor, roughly 85% of the total
- Water weight
- 133 lb / 60 kg
- Plus the tank, stand, and rock: use a stand rated for it
- Heater size
- 50-80 W
- Tropical fish in a normal room; go higher for cold rooms
- Filter flow
- 70-110 GPH
- 4 to 6 turnovers an hour; more for messy fish like goldfish
- Substrate for 2.0 in
- 11.8 L / about 42 lb
- Gravel or sand at about 1.6 kg per liter
- Stocking (rule of thumb)
- Up to 15 in (38 cm) of fish
- Total adult length of small, slim community fish only
How the calculator works
Volume is the tank's footprint multiplied by its water height. For a rectangle the footprint is length times depth. A cylinder uses pi times the radius squared. A hexagon is measured across its flat sides, and its footprint is that width squared times 0.866. A bow-front is treated as a rectangle at the side depth plus the curved bow, which bulges like a parabola and adds two-thirds of its bounding rectangle.
Cubic inches convert to US gallons by dividing by 231, and cubic centimeters convert to liters by dividing by 1,000. One US gallon is 3.785 liters and one UK (imperial) gallon is 4.546 liters, which is why the same tank shows a smaller number in UK gallons.
Why actual water volume is lower
Glass thickness, substrate, rock, wood, and the air gap below the rim all take space, so the calculator also shows an estimate of about 85% of the total. Use that lower figure for anything dosed per gallon, such as water conditioner, medication, salt, or fertilizer. If you need it exact, count the buckets during your next water change.
Reading the equipment numbers
Heater size assumes tropical fish in a room kept in the high 60s to low 70s°F. Colder rooms or a tank by an outside wall need the top of the range, or two smaller heaters at each end so one failing can't cook or chill the tank. Filter flow is shown as four to six turnovers an hour, which suits most community tanks; goldfish, cichlids, and other heavy feeders do better at eight or more.
The stocking line is the old one-inch-per-gallon rule applied to the actual water volume. Treat it as a ceiling for small, slim fish only, never as a target, and check each species' adult size and temperament before buying.
Guides that use these numbers
Frequently asked questions
How do I calculate how many gallons my fish tank holds?
Multiply the inside length, depth, and height in inches, then divide by 231 (the number of cubic inches in a US gallon). A tank measuring 24 x 12 x 16 inches holds 4,608 cubic inches, which is about 19.9 US gallons or 75.5 liters.
How do I calculate aquarium volume in liters?
Measure in centimeters and multiply length, depth, and height, then divide by 1,000. A 60 x 30 x 36 cm tank is 64,800 cubic centimeters, or 64.8 liters.
Why does my tank hold less water than its rated size?
Rated sizes use outside dimensions, and in a running tank substrate, rock, wood, and the gap below the rim take up space. Most set-up tanks hold roughly 10-20% less water than the nominal figure, which matters when you dose medication or fertilizer.
How heavy is a full aquarium?
Fresh water weighs about 8.34 pounds per US gallon (1 kg per liter). Add the glass, substrate, and rock and a set-up tank usually weighs around 10 pounds per gallon, so a 55-gallon tank is roughly 550 pounds and needs a purpose-built stand on a level, solid floor.
Is the one inch of fish per gallon rule accurate?
Only as a rough ceiling for small, slim fish such as tetras, rasboras, and livebearers. It ignores body bulk, waste output, adult size, and swimming room, so it badly overestimates capacity for goldfish, cichlids, and plecos. Filtration, water changes, and each species' adult size matter more.