Convert fish tank gallons into cubic feet, liters, and water weight. Use the adjustment field to estimate real water volume after substrate, hardscape, equipment, and the top air gap.
Tank
55 gal
Adjust
10%
Water
49.5 gal
Example practical volume
6.62 ft³
A 55 gallon aquarium with a 10% allowance holds about 49.5 gallons, or 6.62 cubic feet of water.
Convert tank gallons into water volume and weight.
Use 5-15% for typical substrate, hardscape, and fill-line space. Use 0% for pure nameplate conversion.
Common aquarium presets
Water volume
Formula: US gallons × 0.133681 = cubic feet. The adjustment field estimates the real water volume after substrate, rocks, decor, and the top air gap.
Water gallons
49.5 gal
Liters
187.38 L
Cubic meters
0.19 m³
Freshwater weight
413.08 lb
Weight planning note
423.4 lb saltwater
Water is only part of total aquarium load. Add tank glass, stand, substrate, rock, equipment, canopy, and sump volume when checking floor support.
For US aquarium listings, multiply gallons by 0.133681. If you want the practical water volume, reduce the listed capacity for substrate, rock, decor, overflow space, and the normal fill line.
Aquarium ft³ = aquarium gallons × 0.133681 × (1 - displacement %)
For UK Imperial gallons, multiply by 0.160544 instead. If your aquarium is listed in liters, convert liters to gallons or use a liters calculator before comparing with US equipment specs.
Freshwater weighs about 8.345 lb per US gallon. A filled aquarium system is much heavier after glass, stand, substrate, rock, filters, and sump water are included.
Reference points for quick tank planning, water changes, and system-weight checks.
20 in × 10 in × 12 in
1.2 ft³
9 gal water · 75.11 lb
A small freshwater tank holds about 1.20 ft³ of water after a 10% adjustment.
48 in × 13 in × 21 in
6.62 ft³
49.5 gal water · 413.08 lb
A popular display tank with about 6.62 ft³ of practical water volume after decor allowance.
72 in × 18 in × 22 in
15.04 ft³
112.5 gal water · 938.81 lb
Large aquariums quickly become structural-load projects once stand, rock, and water are included.
Convert nameplate gallons into cubic feet when comparing tank sizes, sump capacity, display volume, and total system water volume.
Gravel, sand, driftwood, reef rock, and not filling to the rim all reduce actual water volume. A 5-15% adjustment is a useful planning range.
Cubic feet helps translate gallons into water weight. The final load also includes glass, stand, canopy, substrate, rock, filters, and sump water.
Real water volume matters for medication, fertilizer, reef dosing, and water-change percentages more than the nominal tank size printed on a listing.
Practical notes for converting aquarium gallons without overestimating water volume.
For US gallons, multiply gallons by 0.133681. For example, a 55 gallon aquarium is 55 × 0.133681 = 7.35 cubic feet before any substrate or air-gap adjustment.
In the United States, aquarium listings usually use US gallons. UK listings may use Imperial gallons or liters, so choose the gallon type that matches the source before converting.
The listed tank size is a nominal container capacity. Sand, gravel, rock, driftwood, equipment, a lower fill line, and overflow spacing reduce the actual amount of water in the system.
Freshwater weighs about 8.345 pounds per US gallon. Saltwater is slightly heavier, often estimated around 2.5% more, before adding tank glass, stand, rock, and equipment.
From cubic meters to cubic feet, compare volumes quickly with clear formulas and fast results.