Liters to Gallons

Your Tank Is Rated in Litres, the Fishkeeping Advice Is in Gallons

The box the aquarium came in said 180 litres. Then the stocking article you are reading counts fish per gallon, the bottle of plant fertiliser gives a cap per ten gallons, and the canister filter on your shortlist is rated in gallons per hour. None of those numbers can be judged until the tank has a gallon figure attached to it.

It runs the other way too. A US keeper sizing a 55-gallon setup for a European stand, or reading a dose written per 100 litres on a European-made medication, needs the litre value before anything can be measured out. Getting the two scales lined up once, at the start, saves guessing at the water-change bucket later.

Conversion factor: 1 L = 0.264172052 US gal, and 1 US gal = 3.785411784 L. So a 180 L aquarium is 47.55 US gallons of nominal capacity, which lands between the US hobby's 40-gallon breeder and its 55-gallon standard rather than matching either of them.

Where the Gallon Keeps Turning Up in Fishkeeping

Stocking guidance counts by the gallon

Most of the widely-copied advice on how many fish a tank can carry grew up in the United States, so the thresholds are all written as 10, 20, 29, 55 and 75 gallons.

Dosing caps are sized per ten gallons

Conditioners, fertilisers and medications from US brands print a cap or millilitre figure per ten US gallons, which is 37.85 litres — not a round metric amount.

Pump output is quoted in gallons per hour

Filter and powerhead boxes often carry a GPH rating next to an LPH one, and the two rarely agree with the marketing headline, so a conversion settles which figure to trust.

Nominal litres are not water litres

The rating on the box is the glass box filled to the brim. Substrate, rock, wood and the gap you leave below the trim all take a share before a single fish goes in.

Getting a Usable Gallon Figure Out of a Tank Rating

Do this once when the tank arrives, write both numbers on a label inside the cabinet door, and every dose and stocking decision after that becomes arithmetic you have already done.

Enter the litre rating on the left

Type the number from the box or the manufacturer's page. Decimals may use a comma or a dot and spaces are ignored, so a figure copied as 1 200 from a spec table still parses.

Read the gallon value as it forms

Nothing needs pressing — the right-hand box tracks every keystroke. Results carry up to eight decimals, so small volumes such as a 12 L quarantine tub stay exact rather than collapsing to a round number.

Knock off the volume you cannot dose

Retype the litre figure with substrate and hardscape deducted to get the honest number. Both boxes are editable and the ↔ button reverses the pair, so a gallon-based instruction can be worked back into litres just as easily.

Copy the value into your maintenance notes

The copy button on either field yields the number with no unit and no thousands spacing, which drops cleanly into a tank log, a spreadsheet or a dosing app. Ctrl + C inside a field does the same thing.

Doing it by hand is no drama either: divide litres by 3.785411784, or multiply by 0.2642 if you only need two decimals. A 240 L tank works out at 63.4 US gallons.

Standard Aquarium Sizes in Litres and US Gallons

Footprints are the common European sizes and vary by brand. The last column applies a rough 15% deduction for substrate, hardscape and the gap under the trim, which is the figure you should actually dose and stock against.

Tank	Nominal litres	US gallons	Typical footprint	Water really in it
Nano cube	30 L	7.93 gal	≈ 30 × 30 × 35 cm	≈ 6.7 gal
Two-foot starter	60 L	15.85 gal	≈ 60 × 30 × 30 cm	≈ 13.5 gal
Compact community	100 L	26.42 gal	≈ 80 × 35 × 40 cm	≈ 22.4 gal
Three-foot planted	125 L	33.02 gal	≈ 100 × 30 × 40 cm	≈ 28.1 gal
Four-foot community	180 L	47.55 gal	≈ 120 × 40 × 40 cm	≈ 40.4 gal
Deep four-foot	240 L	63.40 gal	≈ 120 × 50 × 45 cm	≈ 53.9 gal
Five-foot display	350 L	92.46 gal	≈ 150 × 50 × 50 cm	≈ 78.6 gal
Six-foot display	450 L	118.88 gal	≈ 180 × 50 × 50 cm	≈ 101 gal

If you would rather measure than estimate, fill the empty tank with a marked bucket and count, or work the internal dimensions in centimetres: length × width × water depth ÷ 1000 gives litres directly, and the converter turns that into gallons.

Dose against the real volume. Treating a furnished 180 L tank as though it still holds 180 litres overshoots most medications by something like a sixth. With copper-based or invertebrate-sensitive treatments that margin is the difference between a cure and a wipe-out.

What Makes Tank Arithmetic Less Painful

Answers arrive as you type

Nothing to submit — adjust the litre figure for displacement and the gallon value follows immediately, so trying two or three estimates costs nothing.

Works from a gallon instruction backwards

Type into the right-hand box or press ↔ to reverse the pair when a label tells you what to add per gallon and your bucket is marked in litres.

Clean figures for a tank log

Each field copies the bare number, no unit and no spacing, ready to paste beside a date in whatever record you keep of doses and changes.

Millilitres and quarts in the same list

Both dropdowns are searchable and cover all 28 volume units, so a dosing amount in millilitres or a US quart of prepared saltwater is one selection away.

Aquarium Volume and Stocking Questions

Why does a 200 litre tank hold less than 200 litres of water?

The rating describes the glass box filled level with the top edge, which is not a state any running aquarium is ever in. You leave two to four centimetres below the trim, and then substrate takes its share — a 5 cm bed across a 100 × 40 cm base is 20 litres on its own — before rock, wood and equipment displace more. A furnished freshwater setup commonly ends up holding 10 to 20 per cent less than the label claims.

How do I apply a dose written per 10 US gallons to a litre tank?

Ten US gallons is 37.854 litres, so divide your real water volume by 37.854 to find how many doses to give. A 180 L tank holding about 153 litres once furnished comes to 153 ÷ 37.854 = 4.04, meaning four capfuls rather than the 4.76 you would get by dosing the nominal rating. Rounding down is the safer habit with anything that can be topped up later, and essential with treatments that have a narrow margin.

How much water is a 25% change on my tank?

Take a quarter of the water the tank actually holds, not a quarter of the box rating. On that same 153 litre working volume it is roughly 38 litres, which is 10.1 US gallons, or four buckets if yours takes 10 litres. Matching the replacement is where the gallon figure earns its keep: a conditioner dosed per gallon has to cover the new water only, so convert the change volume as well as the tank volume and treat them separately.

Is the one-inch-per-gallon stocking rule worth following?

Only as a first sanity check on small community fish, and only with adult sizes rather than what is in the bag today. The rule ignores body mass entirely: a 10 cm fish is far heavier and messier than five 2 cm ones, so it scales badly upward. It also says nothing about territory, shoal size, swimming style or the filtration you have. Convert your litres to gallons by all means, then judge the result against species requirements rather than against a single length figure.

Do aquarium products mean US gallons or imperial gallons?

Almost always US gallons, because the brands and the hobby writing that use gallons at all are largely American. The gap is not trivial: one imperial gallon is 1.20095 US gallons, so reading a "gallon" as imperial when the bottle meant US under-doses by about 17 per cent. Older British books and some Commonwealth sources do mean the imperial gallon of 4.54609 litres. If a product is silent on which, the safe assumption for anything sold in the US market is the US gallon of 3.785 litres.