Aquarium Volume Calculator
Get the true volume of a rectangular aquarium from its length, width, and height — in inches or centimeters — plus liters and the weight of the water once filled. Fill options cover brim-full, 90%, and an 85% real-world fill with substrate.
Example: with Length 24 · Width (front to back) 12 · Height 16 · Units inches · Fill level To the brim (rated size) → Volume (US gallons): 19.9 US gallons.
- Volume (liters)75.5 liters
- Water weight when filledabout 166 lb (76 kg) of water — plus tank, stand, and substrate
Computed by the calculator below using its default values. Change any input to see your own numbers.
Gallons = length × width × height in inches ÷ 231. Filled, water alone weighs about 8.34 lb per gallon — before glass, stand, and gravel.
Nominal size versus real water
A "55-gallon" tank is a trade name, not a measurement. Compute 48 × 13 × 21 inches and you get 56.7 gallons brim-full — but once you leave an inch of air at the top and add gravel, rock, and equipment, the actual water is typically 85–90% of rated volume, call it 48 gallons. That difference matters most when you dose medication or salt by volume: dosing the rated size into the real water overdoses by 10–15%.
The 231 in the formula is exact — a US gallon is defined as 231 cubic inches. In metric mode the math is even cleaner: liters are just cubic centimeters divided by 1,000.
Weight is the number that bites
Water runs 8.34 lb per gallon, so even a modest 20-gallon high carries about 166 lb of water — and a running 55-gallon setup, with glass, stand, and substrate, lands around 600 lb on a couple of square feet of floor. The aquarist rule of thumb is 10 lb per gallon for a full freshwater system. Saltwater is about 2.5% heavier (8.55 lb/gal at a specific gravity of 1.025).
How it’s calculated
US gallons = length × width × height in inches ÷ 231 (a US gallon is exactly 231 cubic inches); liters = gallons × 3.785411784, or cm³ ÷ 1,000 in metric mode. The fill option scales volume by 100%, 90%, or 85% for the waterline gap and substrate. Water weight uses 8.34 lb per US gallon (1 kg per liter) for freshwater.
Uses outside dimensions and square corners — glass thickness, bracing, bowed fronts, and built-in overflows shave off a bit more of the true water volume.
Standard US tank sizes: rated vs computed
| Rated size | Dimensions (inches) | Computed brim-full volume |
|---|---|---|
| 10 gallon | 20 × 10 × 12 | 10.4 gal |
| 20 gallon high | 24 × 12 × 16 | 19.9 gal |
| 29 gallon | 30 × 12 × 18 | 28.1 gal |
| 55 gallon | 48 × 13 × 21 | 56.7 gal |
| 75 gallon | 48 × 18 × 21 | 78.5 gal |
Common US trade dimensions; volume computed as L × W × H ÷ 231 and rounded to 0.1 gal.
Common mistakes
- Measuring in inches but dividing by 1,000 (the metric rule) — or in cm while dividing by 231; the unit selector matters.
- Dosing medication or salt by the rated gallons: substrate, decor, and the waterline gap mean the real water is usually 10–15% less.
- Mixing inside and outside dimensions — on big tanks, half-inch glass all around removes a couple of gallons by itself.
- Sizing the stand and floor for the empty tank: budget roughly 10 lb per gallon for a running freshwater setup.
Frequently asked questions
How do I calculate aquarium gallons?
Multiply length × width × height in inches and divide by 231 — a US gallon is exactly 231 cubic inches. A 24 × 12 × 16 inch tank is 4,608 in³ ÷ 231 = 19.9 gallons, the standard 20-gallon high.
How much does a filled aquarium weigh?
Water alone is 8.34 lb per gallon (8.55 for saltwater). Add the glass, stand, substrate, and rock and a good planning figure is about 10 lb per gallon of rated size — 550 lb for a 55.
How many fish can my tank hold?
The old inch-of-fish-per-gallon rule is only a rough ceiling for small, slim-bodied fish. Adult size, waste load, filtration, and territory matter more — research each species before stocking.
Why does my 55-gallon tank hold less than 55 gallons of water?
Rated sizes are brim-full trade names. After the waterline gap, gravel, and decor, actual water volume is typically 85–90% of the rating — use the fill-level option to estimate it.
Does the calculator work for bowfront or hexagon tanks?
Not directly — it assumes a rectangular box. A rough workaround for a bowfront is to average the front-to-back depth at the ends and the middle, then enter that as the width.