Convert gal to l

A US gallon is defined as 231 cubic inches, exactly — a definition that comes from an English wine gallon of 1707. The imperial gallon is 4.55 litres, about 20% larger.

A litre is a thousand cubic centimetres. It is not strictly an SI unit — the SI unit of volume is the cubic metre — but it is accepted for use with SI and is what fuel, drinks and engines are measured in.

The factor is 3.785412, and almost nobody carries that around. Rounded to 3.8 it is off by 0.39 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 gal.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The US gallon is two different sizes wearing one name. This page uses the US one, which is 3.78541 l; the imperial one still used in the UK and Ireland is 4.54609 l — larger by 20.1 %.

That gap is the awkward size: far too big to ignore in a recipe, and far too small to spot from the answer alone. If your figure came from a British source, take the second number, and note that the fluid ounce is the one unit here where the imperial measure is the smaller of the two.

The fuel-economy figure is where this conversion actually costs somebody something. A car advertised in the United States at 30 mpg is, on the identical fuel and the identical distance, 36 mpg when a British magazine quotes it — because an imperial gallon holds 4.54609 litres against the US gallon’s 3.785411784, and a bigger gallon takes the car further. Both numbers are written "mpg" and neither says which gallon it divided by.

The correction is a multiplication by 1.201 going from US to imperial and a division by the same going back. It is worth doing before any comparison between an American road test and a British one, because a 20% difference sits exactly in the range where a car looks like a genuinely better buy rather than like a units error. The metric figure is immune to this, which is part of why Europe stopped quoting economy as distance per volume at all.

Miles per gallon is distance divided by fuel; litres per 100 km is fuel divided by distance. They are reciprocals, so they run in opposite directions — a bigger mpg is a better car and a bigger litres-per-100-km is a worse one — and no factor converts one into the other. What converts them is a constant divided by the figure: 235.215 ÷ mpg gives L/100 km, and 235.215 ÷ L/100 km gives the mpg back.

So 30 mpg is 7.84 L/100 km, 20 mpg is 11.76, and 40 mpg is 5.88. The reciprocal also explains something that looks unfair about mpg: going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50 does, even though the second improvement is the larger number of miles. Litres per 100 km shows that directly, which is the argument for it as a measure.

Dividing by 3.785 turns one into the other. Fuel at $3.50 a US gallon is $0.92 a litre; at $4.20 a gallon it is $1.11. Going the other way, €1.75 a litre is €6.62 a US gallon, which is the number that makes an American reader understand a European fuel price rather than merely see it.

The comparison is honest about the price and dishonest about the cost. Most of the difference between a US and a European pump price is excise duty and VAT rather than the fuel or the refining, so the converted figure tells a traveller what they will hand over and tells nobody anything about the underlying commodity. When a price is quoted per imperial gallon — which still happens in older British and Canadian sources — divide by 4.546 instead.

The US liquid gallon is defined as 231 cubic inches exactly, a number inherited from the English wine gallon of 1707 rather than chosen for any property it has. An inch is exactly 25.4 mm by the 1959 international agreement, and a litre has been exactly one cubic decimetre since the CGPM redefined it in 1964. Chain the three and 3.785411784 falls out — long, exact, and never going to be refined.

The litre’s own history has one wrinkle worth knowing if you ever read an old figure. Between 1901 and 1964 the litre was defined as the volume of a kilogram of pure water at its densest, which turned out to be 1.000028 cubic decimetres rather than exactly one. Anything measured to better than about 30 parts per million in that period carries the difference; anything to do with a fuel tank does not.

Turning 30 mpg into 7.84 L/100 km makes the units comparable and does not make the tests comparable. American ratings come from the EPA’s drive cycles and European ones from WLTP, and the two procedures differ in how long they run, how hard they accelerate and how much of the distance is at motorway speed. The same car therefore carries two official figures that disagree after any amount of correct arithmetic.

The practical consequence is that a converted EPA number should be treated as an approximation of a European rating, not as a translation of it. Where a real comparison matters — an import, a fleet decision, a claim on a listing — take both official figures rather than converting one and quoting it as the other. Where the reader is filling a tank on holiday, the arithmetic above is all the precision the question has.

American technical documents quote a lot in gallons that has nothing to do with driving: paint coverage in square feet per gallon, pool chemistry in ounces per 10,000 gallons, herbicide and fertiliser rates in gallons per acre, domestic water use in gallons per person per day. All of them use the same 3.785411784, so the conversion never changes.

What changes is how much rounding the figure can stand. Calling a gallon 3.8 litres is a 0.4% error, invisible in a tank of fuel and visible in a dosing rate applied across a field or a pool. Where a spec sheet gives a concentration rather than a capacity, carry the full factor; where it gives a tank size, round freely. And check whether an older British or Canadian source means the imperial gallon, which would make the same figure 20% larger.

Multiply by four and take five per cent off. A 15-gallon tank is 60 less 3, so about 57 litres — the exact figure is 56.8. A 20-gallon tank is 80 less 4, so 76 against 75.7. The five per cent works because 3.785 is 4 × 0.946, and losing about a twentieth is easy to do in your head at a forecourt.

Backwards, quarter it and add five per cent: 50 litres is 12.5 plus 0.6, about 13.1 gallons, against 13.2 exactly. Both shortcuts sit inside a per cent, which is far better than the resolution of a fuel gauge and good enough to know whether a tank will take the fill you are about to pay for. The field above carries the exact factor when the number has to be right.