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The calculator applies the standard aquarium volume math: inside dimensions in inches divided by 231 cubic inches per US gallon, with a per-shape formula (rectangular, cube, cylinder, bow front, hexagon). It converts to liters, subtracts your displacement percentage for substrate, decor, and the unfilled rim to get the real water volume, and estimates filled weight at 8.34 pounds of water per gallon plus an 18% allowance for glass, substrate, and equipment. The stocking figure is the widely quoted 1 inch of small community fish per gallon of actual water, shown with the honest caveats it needs. Stocking is a rough guideline, not a guarantee: filtration, water changes, species temperament, and adult size matter more than volume alone.
Rectangular tank: gallons = length × width × height in inches / 231, because one US gallon holds 231 cubic inches. Metric: liters = length × width × height in cm / 1000. Cylinder: gallons = pi × (diameter / 2)² × height / 231. Bow front: rectangular base × 1.17 / 231, since the bow typically adds 10 to 25% over the base. Hexagon: 2.598 × side² × height / 231. The calculator then converts to liters (x 3.78541), subtracts your displacement percentage for the real water volume, and estimates filled weight at 8.34 pounds of water per gallon plus an 18% allowance for glass, substrate, and equipment.
A standard 24 × 12 × 16 inch tank: 24 × 12 × 16 = 4,608 cubic inches / 231 = 19.9 gallons, about 76 liters. With a typical 10% displacement for substrate and decor, the real water volume is about 18.0 gallons. Filled weight comes to roughly 196 pounds, so it needs a proper stand, not a bookshelf. The stocking guideline gives about 17 inches of small community fish for the actual water volume.
Rectangular is the workhorse: L × W × H / 231. Cube needs only one side: side³ / 231. Cylinder uses the diameter: pi × (d/2)² × height / 231. Bow front is a rectangular base plus the curved bow, approximated at 1.17x the base volume. Hexagon uses 2.598 × side² × height / 231. For odd or irregular tanks, the fallback everyone trusts is the jug method: fill with a known gallon container and count.
Common off-the-shelf sizes with approximate inside dimensions in inches:
5 gallon: 16 × 8 × 10 | 10 gallon: 20 × 10 × 12 | 20 gallon long: 30 × 12 × 12 | 29 gallon: 30 × 12 × 18 | 40 gallon breeder: 36 × 18 × 16 | 55 gallon: 48 × 13 × 21 | 75 gallon: 48 × 18 × 21 | 100 gallon: 72 × 18 × 19 | 125 gallon: 72 × 18 × 22.
Measure your own tank anyway: printed sizes are nominal, and inside dimensions are what hold water.
Three things shrink real water volume below the rated size. Substrate and rocks displace water, decorations and equipment displace more, and nobody fills to the rim, so the top inch or two is air. Typical setups hold 85 to 90% of the rated volume; heavily aquascaped tanks can drop to 75 to 80%. Set the displacement slider honestly, because dosing medication, salt, and fertilizer by the rated size instead of the real volume overdoses the tank.
Water weighs 8.34 pounds per gallon, and the tank, substrate, and stand add roughly 18% more. A 55-gallon setup lands around 550 to 700 pounds total, about the weight of a grand piano. Upper-floor rooms need a stand that spreads the load across joists, not a point load on one spot. Use the weight estimate on the result panel before you commit to a location.
The beginner rule says 1 inch of fish per gallon of water, and it is only a rough starting point for small, peaceful community fish. It breaks down for larger, messier, or more active species, so always plan with adult size, not the size at purchase. What actually sets safe stocking is filtration capacity, water change frequency, species temperament, waste production, and surface area for oxygen exchange. A common goldfish exception: about 20 gallons for the first fish and 10 more per additional fish. Treat the calculator's number as a ceiling for small community fish, not a target.
Size equipment from the actual water volume, not the sticker size. Heaters: 3 to 5 watts per gallon, so a 20-gallon tank wants a 75 to 100 watt heater. Filters: aim for turnover of 4 to 6 times the tank volume per hour for freshwater. These rules of thumb assume a normally stocked tank; heavily stocked or messy species want more filtration, not less.
US gallons and UK (imperial) gallons are different units: 1 US gallon = 231 cubic inches, 1 UK gallon = 277.42 cubic inches. A tank that holds 20 US gallons holds about 16.7 UK gallons. Convert with liters as the bridge: US gallons × 3.78541 = liters, liters / 4.54609 = UK gallons. Most online calculators, including this one, show US gallons unless stated otherwise.
Volume is the dosing math for everything you put in the water. Medication is labeled per gallon, so dosing by rated size instead of actual volume overdoses. Marine salt mixes at roughly half a cup per gallon for standard reef salinity. Water conditioner is dosed per gallon on water changes, and most keepers change 10 to 25% weekly. Planted tanks dose fertilizer and CO2 by volume too. Every one of these gets safer when the volume number is honest.
Pick the tank shape, then enter the inside dimensions in inches: measure inside the glass, not outside, and use the water height you actually fill to rather than the tank height. For a cylinder enter the diameter as the length; for a hexagon enter the side length. Set the displacement percentage for substrate, rock, and the unfilled rim (10% is typical). The result shows gallons, liters, real water volume, filled weight, and the rough stocking guideline with its caveats.
Bigger tanks are more forgiving: more water means slower chemistry swings, and that makes 20 to 55 gallons the sweet spot for beginners. Nano tanks under 10 gallons look easy and are actually harder, because a small mistake becomes a big swing fast. Whatever size you choose, cycle the tank before adding fish and add stock gradually.
Multiply the inside length, width, and water height in inches and divide by 231, since one US gallon is 231 cubic inches. A 24 × 12 × 16 inch tank is 4,608 / 231 = about 19.9 gallons. In metric, multiply the cm dimensions and divide by 1000 for liters.
Substrate, rocks, decorations, and equipment displace water, and the tank is never filled to the rim. Typical setups hold 85 to 90% of the rated volume, heavily aquascaped ones 75 to 80%. Measure inside dimensions and set a displacement percentage for the honest number.
It is a rough beginner guideline for small, peaceful community fish, not a law. It breaks down for larger, messier, or territorial species. Filtration capacity, water changes, adult fish size, and surface area matter more than raw volume, so treat the number as a starting ceiling, not a target.
Start from the guideline of about 1 inch of adult small community fish per gallon of actual water volume, then adjust down for messy or territorial species and up for strong filtration and frequent water changes. Research each species' adult size and temperament before buying.
Multiply US gallons by 3.78541 to get liters; divide liters by 3.78541 to get US gallons. A 20-gallon tank is about 76 liters. Note that UK gallons differ: divide cubic inches by 277.42 for UK gallons.
Size heaters at 3 to 5 watts per gallon of actual water volume. A 20-gallon tank wants a 75 to 100 watt heater. Undersized heaters run constantly and wear out; oversized ones can overshoot, so match the range to your room temperature.
Inside. Glass or acrylic thickness eats a surprising amount of volume, and the water height you actually fill to is what counts, not the tank's outer height. Outside measurements overstate volume by 5 to 15% on typical tanks.
20 to 55 gallons. More water buffers chemistry swings, which forgives beginner mistakes. Tanks under 10 gallons look simple but are harder because small errors become large swings fast.
Dose by the label, which is usually per gallon of actual water, and do the math on real water volume after displacement, not the rated size. Overdosing from a generous volume estimate is one of the most common beginner mistakes.
Medication labels assume a known volume, so an overstated volume means an overdose. Compute the real water volume with displacement subtracted, then dose to that number. When in doubt, dose slightly under and observe.