How this instrument works
One litre is one cubic decimetre — a cube 10 cm along each edge — and has been exactly that since 1964, when the 12th CGPM restored its original 1795 definition. For sixty-three years before then it meant something subtly different: volume occupied by one kilogram of pure water at maximum density, which measurement later showed to be 1.000028 dm³. That 28-parts-per-million offset still lurks in pre-1964 chemistry literature, where careful authors wrote millilitre and cubic centimetre as distinct things.
A US liquid gallon is fixed at 231 cubic inches. That odd number descends from an English wine gallon standardised under Queen Anne in 1707, taken as a cylinder 7 inches across and 6 inches deep; π/4 × 7² × 6 comes to 230.907 cubic inches, and Parliament rounded up for tidiness. Britain scrapped that measure in 1824 for an imperial gallon built on 10 pounds of water, but American merchants had already priced their trade in wine gallons and kept it — which is why US gallons run roughly one fifth smaller than British ones.
Because 1959 pinned an inch to 25.4 mm exactly, 231 cubic inches works out to 3.785411784 litres with no residual uncertainty whatever. Invert it and you get 0.264172052358148…, an endless decimal shown here to twelve places. American fuel pumps, milk jugs, paint tins, aquarium labels and 42-gallon oil barrels all count in gallons; nearly every other country meters fuel and sells drinks by litre, so this pair gets crossed daily by travellers, importers, boat owners and homebrewers.
- Type your volume into Litres (l). It opens at 10 — jerry-can and carboy territory.
- Read US gallons (gal) beneath it — recomputed on every keystroke, no button to press.
- Working backwards from gallons? Multiply your gallon figure by 3.785411784 to get litres; that factor is exact.
- Negative entries are refused, since no vessel holds less than nothing.
Worked example — a 10-litre fermenter
A homebrewer in Ohio orders a 10 L conical fermenter from a German supplier and wants to know whether a three-gallon batch will fit. Entering 10 into Litres (l) returns 2.64172052358 US gallons (gal) — barely over two and a half. Three gallons would spill, so she pitches a 2.5 gal batch and keeps 0.14 gal of headspace for krausen.
Larger volumes obey precisely the same factor: one 10,000 L road tanker holds 2,641.72 gal, while a 42 gal petroleum barrel comes back as 158.987294928 L. Digits past about the sixth are decorative for anything you can physically pour — retail fuel dispensers in America are held to roughly one hundredth of a gallon under NIST Handbook 44, and kitchen jugs to far less.
Questions
Is a US gallon the same as an imperial gallon?
No, and mixing them up is by far the costliest error in this conversion. A US liquid gallon is 3.785411784 L; a UK imperial gallon is 4.54609 L, about 20 per cent larger. So 40 mpg quoted by a British magazine is only about 33 mpg in American terms, and any boat's stated 50-gallon tank differs by nearly 40 litres depending on which side of the Atlantic wrote the brochure. Check provenance before converting anything from Commonwealth sources or older Canadian manuals.
Is 0.264172052358 an exact factor?
Almost, though exactness actually lives in the opposite direction. One US gallon is exactly 231 cubic inches and therefore exactly 3.785411784 litres, both definitions rather than measurements. Dividing 1 by that value gives 0.2641720523581484…, which never terminates, so any written decimal must stop somewhere. Twelve places leaves a relative error near six parts in ten trillion; spread over an Olympic swimming pool, that comes to roughly one millionth of a litre.
Why is a gallon 231 cubic inches?
It started life as a rounded cylinder. An English wine gallon of 1707 was described as a vessel 7 inches in diameter and 6 inches deep, whose true capacity is 230.907 cubic inches; legislators rounded to 231 and that flat integer stuck. When Britain replaced its wine gallon in 1824, American commerce had already been built on it for half a century, so no switch happened.
Does a US dry gallon exist too?
Yes, although you will rarely meet it by name. A US dry gallon is one eighth of a Winchester bushel: 268.8025 cubic inches, about 4.404884 L, some 16 per cent roomier than its liquid cousin. It survives mostly in agricultural produce measures, such as dry quarts of berries or peck baskets. This sheet handles liquid gallons only, which is what fuel, milk, paint and beverages are legally sold in.
Do US and imperial fluid ounces match?
They do not, and their relationship inverts what you might expect. A US fluid ounce is 29.5735295625 mL while an imperial fluid ounce is 28.4130625 mL — so smaller gallons carry larger ounces. That happens because 128 fluid ounces make a US gallon against 160 for an imperial one. Divide litres by 0.0295735295625 for US fluid ounces, and never assume a recipe's ounce matches your jug's markings.
How do I turn L/100 km into miles per gallon?
Divide 235.215 by your L/100 km figure, then divide that same constant by mpg to come back. That number is no accident: it equals 100 × 3.785411784 ÷ 1.609344, gallon-in-litres over mile-in-kilometres. A car rated at 7.5 L/100 km is doing roughly 31.4 US mpg. Quote it as 37.7 mpg and you have silently switched to imperial gallons, which is how identical cars appear thriftier in British road tests.