Aquarium Volume Calculator
Internal dimensions of 36 x 18 x 16 inches produce a brim-full rectangular volume of 44.88 US gallons, or 169.87 liters. This aquarium volume calculator also estimates the weight of freshwater alone from one kilogram per liter. Use inside length, inside width, and actual water height for a usable-water estimate. Outside measurements include glass or acrylic thickness, and full tank height includes the unfilled gap above the waterline. Substrate, rock, wood, equipment, and internal chambers displace water. The displayed weight excludes the empty tank, stand, decor, livestock, and equipment.
Quick answer
Volume is length x width x height, converted from cubic inches or centimeters to gallons and liters.
Volume
44.9 US gallons (169.9 L)
Liters
169.9
Water weight
374.6 lb (169.9 kg)
What this tells you
- •Volume is length x width x height, converted from cubic inches or centimeters to gallons and liters.
- •One US gallon is 231 cubic inches, and one liter is 1,000 cubic centimeters.
- •Water weighs about 8.34 lb per gallon (1 kg per liter), before glass, gravel, and rock.
- •Outside dimensions usually overstate internal water volume because panel thickness occupies space.
- •Using waterline height is more accurate than using full outside tank height.
- •Substrate, rock, wood, filters, and internal chambers displace water by amounts the calculator cannot infer.
- •The weight result treats one liter of freshwater as one kilogram and excludes every other component.
- •Nominal tank names may not equal geometric brim-full or operating volume.
- •Bowfront, cylindrical, hexagonal, and irregular tanks require different geometry.
How to Use
- 1Measure internal length and width between the inside faces of the panels.
- 2Measure height from the inside bottom to the intended waterline, or subtract substrate separately when estimating open water.
- 3Choose inches or centimeters and keep all three dimensions in that unit.
- 4Calculate to see geometric volume in US gallons and liters plus freshwater-only weight.
- 5Estimate displacement for substrate, rock, wood, equipment, and internal chambers when dosing or stocking depends on operating volume.
- 6Add the verified weights of the tank, stand, substrate, decor, equipment, and other contents for total installed load.
- 7Use manufacturer data and qualified structural guidance rather than this water-only estimate for stand or floor decisions.
How It Works
Formula
gallons = L x W x H (inches) / 231The implementation converts inches to centimeters with 2.54 cm per inch, multiplies length x width x height, and divides cubic centimeters by 1,000 for liters. It divides liters by 3.785411784 for US gallons. For 36 x 18 x 16 inches, volume is 169.869079 liters and 44.875325 gallons before rounding. Freshwater-only mass is treated as one kilogram per liter, then multiplied by 2.2046226218 for pounds.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Standard 40-gallon breeder
The inside-box calculation gives 169.869079 L and 44.875325 US gal, rounded to 169.87 and 44.88. Freshwater alone is about 169.9 kg or 374.5 lb.
Common 60 x 30 x 30 cm tank
60 x 30 x 30 / 1,000 gives exactly 54 L. Dividing by 3.785411784 gives 14.26 US gallons. Operating volume will be lower after displacement and waterline clearance.
10-gallon starter tank
The dimensions equal 2,400 in3, or 39.329 L. That converts to 10.39 US gallons and about 86.7 lb of freshwater.
Internal 90 x 45 x 40 cm tank
90 x 45 x 40 / 1,000 gives 162 L. Dividing by 3.785411784 gives 42.8 US gallons, with 162 kg or 357.1 lb of freshwater.
Same footprint with a lower waterline
Using 14 inches of actual water height instead of 16 reduces geometric volume to about 148.64 L or 39.27 US gallons before decor displacement.
Standard Aquarium Sizes
Common tank names against their calculated volume and water weight.
| Tank | Dimensions (in) | Volume | Water weight |
|---|---|---|---|
| 10 gallon | 20 x 10 x 12 | 10.4 gal | 87 lb |
| 20 gallon long | 30 x 12 x 12 | 18.7 gal | 156 lb |
| 40 breeder | 36 x 18 x 16 | 44.9 gal | 374 lb |
| 55 gallon | 48 x 13 x 21 | 56.7 gal | 473 lb |
| 75 gallon | 48 x 18 x 21 | 78.5 gal | 655 lb |
| 125 gallon | 72 x 18 x 21 | 117.8 gal | 982 lb |
Brim-full volume versus operating volume
Brim-full volume uses the complete internal box to its top edge. An operating aquarium stops below that edge, and substrate raises the effective bottom. Those two differences can remove a meaningful amount before decor is added.
Displacement depends on the submerged volume of rock, wood, equipment, filter chambers, and other objects. A fixed percentage is only a guess. Water added during an actual fill or removed during a measured water change can provide a better practical check.
Water-only weight is not total system weight. Glass or acrylic, the cabinet, substrate, stone, equipment, canopy, and stored supplies all add load. Structural suitability depends on load distribution and building details that this calculator does not evaluate.
Common mistakes
- Measuring outside the glass. Panels are 6 to 12 mm thick, and outside dimensions overstate a large tank by a gallon or more.
- Forgetting the tank never fills to the brim. Substrate, hardscape, and the waterline typically remove 10 to 15 percent.
- Treating freshwater-only weight as the complete installed load
- Using a generic percentage deduction for displacement without checking a heavily scaped or chambered setup
- Applying the rectangular formula to a bowfront, cylinder, hexagon, or tapered tank
- Assuming nominal model names are exact volume specifications
- Using geometric volume alone to make stocking or medication decisions without accounting for operating water volume
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