Article — Aquarium Volume Calculator
Aquarium Volume Calculator: Gallons, Liters, and Water Weight
A rectangular aquarium's volume in US gallons equals length × width × height (in inches) divided by 231. A 30 × 12 × 18 inch tank holds 28.0 US gallons, or 106 liters. Filled with fresh water, that same tank weighs roughly 230 pounds (104 kg) before substrate.
Getting the volume right matters for three reasons. Medication dosing is per-gallon. Filter and heater ratings are per-gallon. Stocking calculations (how many fish you can keep) are per-gallon. A 5% error in volume means a 5% under- or over-dose of every medication you add for the life of the tank.
Aquarium volume basics
Volume is the product of three inside dimensions, converted into your preferred liquid measure. The conversion is exact: 231 cubic inches equals exactly 1 US liquid gallon, by definition since 1707. One liter is exactly 1,000 cubic centimeters, so a metric tank converts directly: cm × cm × cm divided by 1,000 gives liters.
Always measure inside the glass. Aquarium glass typically runs 0.25 inches (6 mm) thick on a 20-gallon tank and 0.5 inches (13 mm) thick on a 150-gallon tank. Outside-dimension volume overstates capacity by 3-5%. Trim, bracing, and the unused space at the top of the tank reduce effective volume by another 5-10%.
The largest single-piece acrylic aquarium panel ever built — the front window of the Dubai Aquarium at the Dubai Mall — measures 32.88 m long by 8.3 m tall and weighs 245 metric tons. The aquarium behind it holds 10 million liters of water.
Aquarium volume by shape
Rectangular tanks use length × width × height. Cylindrical tanks use π × radius² × height. Bow-front tanks approximate as a rectangle plus a half-elliptical bulge at the front. The calculator above swaps inputs based on the shape you pick, so you only enter the measurements that matter.
Hexagonal tanks (six-sided columns) and pentagonal corner tanks are more involved. A regular hexagonal tank uses V = (3√3 / 2) × s² × h, where s is the length of one side. A corner tank can usually be approximated as one-quarter of a cylinder, which gives a close-enough estimate without solving for the exact geometry.
Rectangle L × W × HCylinder π × r² × HBow-front LWH + 0.5π(L/2)(B)(H)Hexagon 2.598 × s² × Hin³ → US gal ÷ 231cm³ → liters ÷ 1000US vs. UK gallon aquariums
This is the single biggest source of confusion in aquarium sizing. A US liquid gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters — about 20% larger. A tank advertised as "30 gallon" in the UK actually holds 36 US gallons. Stocking and dosing calculations done from a US source will be 20% off if you plug in imperial gallons.
The simplest fix is to convert everything to liters. Liters are unambiguous worldwide. The calculator above gives you all three: US gallons, UK gallons, and liters, so you can match whatever your medication bottle or stocking guide uses.
Water weight and stand load
Fresh water weighs 8.345 pounds per US gallon at 60°F, or exactly 1.000 kilograms per liter at 4°C. A 75-gallon tank holds 626 pounds of water alone. Add 65 pounds of glass and trim, plus 100-150 pounds of substrate and decorations, and the full setup hits 800-850 pounds (363-385 kg) sitting on a footprint of about 6 square feet.
That weight load matters for two reasons. First, aquarium stands must be rated for the gross weight. A 75-gallon tank on an under-rated stand is a structural failure waiting to happen. Second, the floor underneath must support point loads of around 140 lb per square foot for a 75-gallon footprint — within residential code, but worth checking on older homes with weak joists or on upper floors.
Substrate and net aquarium volume
A 2-inch (5 cm) sand or gravel bed displaces roughly 5-10% of total tank volume. A 4-inch (10 cm) planted-tank substrate can displace up to 20%. On a 55-gallon tank, that is the difference between 50 gallons of water and 44 gallons of water — enough to throw off medication dosing.
To compute net water volume: subtract the substrate's footprint volume (L × W × substrate depth, in inches) from total tank volume, then convert. A 48 × 13 × 21 inch 55-gallon tank with a 2-inch sand bed loses 48 × 13 × 2 = 1,248 cubic inches, or about 5.4 US gallons. Net water volume becomes 49.6 US gallons.
Stocking rules by volume
The traditional rule is "one inch of adult fish per US gallon of water." It works for small slim-bodied fish (neons, guppies, tetras) but breaks down for bigger or wider species. Goldfish need 2-3 gallons per inch because of bioload. Cichlids need similar ratios plus territory considerations. Saltwater stocking is even more constrained — a marine tank typically supports half the inches-per-gallon of a comparable freshwater tank.
A more honest framework is bioload: total weight of fish that filtration and water changes can process. A well-filtered 75-gallon freshwater tank can support 6-8 medium tetra-sized fish per 10 gallons, or 2-3 small cichlids per 20 gallons. Always check species-specific guides because some fish (oscars, plecos, goldfish) produce far more waste per inch of body length than others.
- 1 US gallon = 3.785 liters = 231 cubic inches (exact)
- 1 UK gallon = 4.546 liters = 1.20095 US gallons (20% larger)
- 1 liter water = 1.000 kg (fresh) or 1.025 kg (saltwater)
- 1 US gallon water = 8.345 lb (fresh) or 8.554 lb (saltwater)
- Substrate displacement = 5-20% of total volume
- Top trim and unused space = another 5% reduction
- 55 gal tank filled = approximately 625 lb total
- 180 gal tank filled = approximately 2,000 lb total
Standard aquarium sizes reference
US aquarium manufacturers (Aqueon, Marineland, Tetra) standardize a handful of dimensions. The 20-gallon "high" is 24 × 12 × 16 inches; the 20-gallon "long" is 30 × 12 × 12 inches — same volume, very different layout. A 75-gallon tank shares the 48-inch length of a 55-gallon but is 5 inches deeper front-to-back, which doubles the perceived display area.
The 55-gallon and 75-gallon tanks are the two most common "step-up" sizes for hobbyists. Both fit standard 48-inch lighting and stands. The 75-gallon is generally the better long-term choice — more depth makes aquascaping easier and the wider footprint supports a richer ecosystem.
Drain the last 10% of a tank during major water changes rather than the typical 25% — you reset trace mineral levels without shocking the biological filter, which lives mostly in the bottom 90% of substrate and filter media.
Common aquarium volume mistakes
The first and most common error is using outside dimensions. A "30 gallon" tank measured outside the glass is actually about 27-28 gallons inside, and only 25-26 gallons of water after substrate and trim. The second error is using the manufacturer's nominal volume as the actual volume — those are marketing numbers, not measurements.
The third error is forgetting US versus UK gallons when buying from international suppliers. The fourth is over-stocking based on the 1-inch-per-gallon rule for species (goldfish, plecos) that need much more space. The fifth is under-rating the stand: a "30-gallon stand" must support at least 330 pounds, plus a generous safety margin.
Most aquarium medications dose per US gallon of actual water volume. Overdosing because you assumed nominal tank size can kill sensitive species (catfish, loaches, inverts). Compute net volume after substrate and add medication for that net figure.