Gross vs actual volume
48 × 24 × 24 in multiplies out to 119.7 US gallons, but that box includes the glass and runs to the very top of the frame. Taking off 10 mm of glass on each side and the base, and filling to 1 inch below the rim, leaves about 107.3 gallons of water. Gravel, rock and wood take that down further; the volume calculator estimates it for your aquascape.
How much a filled 120 gallon weighs
| Water | ≈ 848 lb |
| Glass | ≈ 164 lb |
| Gravel (2 in) | ≈ 127 lb |
| Total | ≈ 1 138.1 lb |
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Water weighs 8.34 lb per US gallon, so it is most of the load. Put the tank on a stand rated for it, level, and supported under the whole frame. More on this in how much a fish tank weighs.
Heater and filter
Holding 78 °F in a 68 °F room with a lid takes about 2 × 200 W of heater, using the same heat-loss model as the heater calculator (compare sizes on the heater size comparison). For filtration, a common target is turning the water over 4–6 times an hour: 429–644 gallons per hour of rated flow.
Frequently asked questions
What are the dimensions of a 120 gallon tank?
A standard 120 gallon aquarium is 48 × 24 × 24 in (length × width × height), or about 122 × 61 × 61 cm. These are outside dimensions; brands differ by a fraction of an inch.
How many gallons does a 120 gallon tank actually hold?
Its outside dimensions work out to 119.7 US gallons (453 liters). Inside the glass, with the water about an inch below the rim, it holds roughly 107 gallons (406 liters), and less once gravel and rock go in.
How much does a 120 gallon aquarium weigh when full?
Roughly 1 138.1 lb (516 kg): about 848 lb of water, 164 lb of glass and 127 lb for a 2 inch gravel bed. Rock, decor and the stand add to that.
What size heater does a 120 gallon need?
About 2 × 200 W to hold 78 °F in a 68 °F room with a lid. A colder room or an open top needs more; the heater calculator works it out for your conditions.