How this instrument works
The litre has an unusually wobbly past for a metric unit. France's 1795 decimal reform first tied it to the cubic decimetre, but in 1901 the international standards conference redefined it instead as the volume of exactly one kilogram of pure water at its temperature of maximum density — a physical, measurable quantity rather than a geometric one. That change made a litre very slightly larger than a cubic decimetre, by about 28 parts per million, a discrepancy fine chemistry glassware had to account for. The 12th General Conference on Weights and Measures reverted the litre to exactly one cubic decimetre in 1964, quietly closing a 63-year gap most unit tables never mention.
The US gallon used here is the old English wine gallon, 231 cubic inches, that American law kept after independence while Britain moved on. Britain's 1824 Weights and Measures Act replaced its own wine gallon with a new imperial gallon defined by the volume of ten pounds of water — 4.54609 litres, roughly 20 percent larger than the US version. That single decision is why a UK 'gallon' of fuel, milk, or beer has never matched a US one, and why any recipe, tank spec, or fuel figure crossing the Atlantic needs to say which gallon it means.
Fish keepers buy tanks in litres almost everywhere outside North America and then need US gallon figures to match American filtration, heater, and dosing charts, which are near-universally written per US gallon. Home brewers hit the same wall from the other side: a European recipe scaled for a 19-litre 'Cornelius' keg needs converting before it fits a US-market 5-gallon carboy or fermenter, and the roughly 0.02-gallon gap between the two is exactly the kind of rounding error that throws off a hop or priming-sugar calculation.
- Enter your volume into Litres (l) — 10 is preloaded, a common small aquarium size.
- Read US gallons (gal) beneath it; it recalculates on every keystroke to eleven decimal places.
- Dosing chemicals or medication sold per US gallon? Use the converted figure here, not a rounded tank-chart number.
- Working backwards from gallons? Multiply by 3.785411784 to recover litres exactly.
Worked example — a 10-litre nano aquarium
A fish keeper buys a 10-litre nano tank in Europe and wants the US gallon figure to match American dosing charts for water conditioner. Typing 10 into Litres (l) returns 2.64172052358 in US gallons (gal) — 10 × 0.264172052358, computed exactly rather than rounded to the 2.5 gal shorthand many size charts print.
That gap looks trivial until it's multiplied by a dosing rate: a conditioner rated 'one capful per gallon' applied at the rounded 2.5 gal figure under-treats the true 2.64172 gal tank by about 5 percent — usually harmless for a water conditioner, but the same rounding habit matters far more once fertilizers or medications with a narrow safety margin are involved.
Questions
Is the litre exactly one cubic decimetre?
Yes, since 1964. Between 1901 and 1964 the litre was instead defined as the volume of exactly one kilogram of pure water at its temperature of maximum density, making it about 28 parts per million larger than a true cubic decimetre. The 12th General Conference on Weights and Measures closed that gap in 1964, and every litre used in this calculator is the current, exact cubic-decimetre definition.
Why doesn't a UK gallon match a US gallon?
Because they were standardised from different reference liquids at different times. The US kept the old English wine gallon of 231 cubic inches, 3.785411784 litres, after independence. Britain's 1824 Weights and Measures Act replaced its own wine gallon with a new imperial gallon based on 10 pounds of water, 4.54609 litres — about 20 percent bigger. Both still carry the name 'gallon' today, which is the single biggest source of transatlantic volume confusion.
How many US gallons is a 19-litre Cornelius keg?
19 litres converts to 5.01926899480 US gallons — close enough to a nominal '5-gallon' fermenter or carboy that many home brewers treat the two as interchangeable, though the roughly 0.02-gallon gap is worth accounting for in a recipe calculated to the last gram of grain or hops.
Why do aquarium size charts often show rounder numbers than this calculator?
Manufacturers round tank volumes for marketing and for simple stocking-guide arithmetic — a '10-litre' tank sold as '2.5 gallons' is easier to print on a box than 2.64172052358. For casual stocking decisions that rounding rarely matters; for precise chemical or medication dosing, using the exact converted figure avoids compounding a 5-percent error into every treatment.
Is this the same gallon used for gasoline and fuel-economy figures in the US?
Yes. Every US fuel-economy figure, gas-pump reading, and EPA mileage rating uses the US liquid gallon this calculator converts to — the same 3.785411784-litre definition, not the larger UK imperial gallon still used in a few older fuel-economy contexts elsewhere.
How precise is the 0.264172052358 factor — is any of it rounded?
The relationship itself is exact: it's simply 1 litre divided by 3.785411784 litres per US gallon, both fixed legal definitions with no measurement uncertainty. The digits shown here are that division carried to twelve significant figures; the true decimal continues beyond that point, so only the display, not the underlying math, is ever rounded.