A tourist filling up in the Cayman Islands looks at the price board, sees the figure per gallon, and panics: fuel appears to be a fifth dearer than in Florida. Except he is buying a different gallon. Same word, same gal symbol, two different volumes — 3.785 litres in the United States, 4.546 litres in the British imperial system. The gap reaches 20%, and it is one of the costliest souvenirs of the fact that two countries speaking the same language never agreed on how much liquid fits in a barrel.
Three gallons in one kingdom
Before the nineteenth century there was no "gallon" in the singular in Britain. There were gallons — one per commodity, because every commodity was sold differently and taxed differently. Three of them mattered commercially:
- The wine gallon — 231 cubic inches. Sealed by a statute of 1706 that took effect in 1707, hence its nickname: Queen Anne's gallon.
- The Winchester or corn gallon — 268.8025 cubic inches, for dry goods: grain, flour, fruit.
- The ale gallon — 282 cubic inches, the largest of the three.
This was not sloppiness but fiscal policy. Duty was levied on volume, so each trade won itself a convenient standard — and the Crown tolerated it as long as the revenue held up. The side effect: a merchant selling both wine and barley had to keep two systems in his head.
1824: Britain tidies up, America stays with wine
The American colonies inherited that mess in its pre-reform state, and after 1776 Congress — despite the constitutional power to fix weights and measures — simply declined to use it. The wine gallon remained the liquid standard. It was pinned down only in the 1830s, when Ferdinand Hassler's Treasury office adopted 231 cubic inches as the federal standard and Congress distributed copies to the states in 1836.
Britain went the other way at exactly the same moment. The Weights and Measures Act 1824, in force from 1826, abolished all three gallons and introduced a single imperial gallon, defined physically: the volume of 10 pounds (4.5359 kg) of distilled water, weighed in air at 62 °F (16.7 °C) under 30 inches of mercury. That worked out to 277.274 cubic inches, or 4.543706784 litres — roughly the average of the old corn and ale gallons. The wine gallon, the smallest of the three, was dropped entirely.
And there is the whole divergence: the Americans stayed with the gallon of wine, the British moved to the gallon of water. Later harmonisations changed nothing. When the inch and the pound were standardised internationally in 1959, volume units were deliberately left alone — they were too deeply embedded in both countries' industry and retail. Today both gallons are defined directly against the litre, and exactly so:
- US liquid gallon = 231 cubic inches = 3.785411784 L
- Imperial (UK) gallon = 4.54609 L (≈ 277.41943 cubic inches)
- US dry gallon — a relic, used only for agricultural produce = 268.8025 cubic inches = 4.40488377086 L
Note that the modern imperial gallon is slightly larger than the 1824 one (277.419 rather than 277.274 cubic inches). Defining a volume through water proved less precise than its authors believed — a round number of litres simply replaced it.
The anatomy of twenty percent
The whole subordinate system — quarts, pints, fluid ounces — inherits the gallon's divergence. Almost all of it.
| Unit | US customary | Imperial | Ratio UK / US |
|---|---|---|---|
| Gallon | 3.785411784 L | 4.54609 L | 1.20095 |
| Quart (¼ gallon) | 0.946352946 L | 1.1365225 L | 1.20095 |
| Pint | 473.176473 mL | 568.26125 mL | 1.20095 |
| Fluid ounce | 29.5735295625 mL | 28.4130625 mL | 0.96076 |
| Fluid ounces per gallon | 128 | 160 | 1.25 |
The fluid-ounce row looks like a typo and is in fact the heart of the matter. The fluid ounce is the one unit in this family that is smaller in the imperial system, not larger. The reason: the imperial gallon splits into 160 ounces, the American one into 128. Cut a bigger gallon into more slices and the slices come out thinner.
That division is not arbitrary. The imperial gallon is by definition 10 pounds of water, and a pound holds 16 ounces — 160 ounces of weight. The framers of 1824 made sure that one imperial fluid ounce corresponds to the volume of one avoirdupois ounce of water. Elegant. The American fluid ounce has no such property: it is simply 1/128 of a wine gallon, a number taken from arithmetic rather than physics.
Hence two contradictory mnemonics. In the US the saying is "the pint is a pound the world around" — an American pint of water (473 g) weighs roughly a pound. In Britain it is "a pint of water weighs a pound and a quarter," because the imperial pint holds 20 ounces, weighs 568 g, and comes out at exactly a pound and a quarter.
It is also worth defusing the apparent contradiction you will meet in every discussion: sometimes the imperial gallon is said to be 20% larger, sometimes the American one 17% smaller. Both statements are true. Only the reference point differs: 4.546 / 3.785 = 1.2010, but 3.785 / 4.546 = 0.8327. That asymmetry is a plain property of percentages, not an error.
In practice one operation matters, and so does its direction:
- 20 L ÷ 3.785411784 ≈ 5.283 gal (US)
- 20 L ÷ 4.54609 ≈ 4.399 gal (UK)
- 12 gal (US) × 3.785411784 ≈ 45.42 L
- 12 gal (UK) × 4.54609 ≈ 54.55 L
Nine litres of discrepancy in a single tank. (A practical note: in the NebulaMath volume converter a "gallon" always means the US gallon, 3.785411784 L — because that is the one that dominates the kitchens, cars and recipes most readers actually encounter.)
Where fuel is still sold by the gallon
Metrication has squeezed the gallon into a narrow band of the world, but it has not finished it off.
The US gallon zone is the United States and its dependent territories (Guam, the Northern Mariana Islands, American Samoa, the US Virgin Islands), and beyond them — thanks to imported American metering equipment — Colombia, Ecuador, Peru, Guatemala, Haiti, Liberia, and the Pacific states in free association with the US: the Marshall Islands, Micronesia and Palau.
The imperial gallon zone has shrunk to the Caribbean: four British Overseas Territories (Anguilla, the British Virgin Islands, the Cayman Islands, Montserrat) and a handful of independent Commonwealth states — Antigua and Barbuda, Grenada, Saint Lucia, and Saint Vincent and the Grenadines.
The most interesting places are those where both systems circulate at once: the Bahamas and the Turks and Caicos Islands. On the latter archipelago the divergence was exploited with surgical precision. The government changed the reference unit for fuel duty — from the imperial gallon (4.55 L) to the American one (3.79 L) — while keeping the same flat rate "per gallon." Nominally, no tax went up. In reality the burden on every litre of fuel jumped by a fifth. A rise without a rise, hidden inside the definition of a unit.
The rest of the world went the opposite way. Canadian pumps switched to litres in 1979, the United Arab Emirates dropped the imperial gallon from fuel sales in 2010, Guyana in 2013, Myanmar in 2014. In March 2025 Dubai's DEWA even moved water billing from gallons to cubic metres. Antigua and Barbuda has been debating the litre since 2015.
Marketing traces of the old system survive. In the UAE and Bahrain the standard water-cooler bottle holds 18.9 litres and is sold as a "five-gallon" bottle — because 5 × 3.785 L ≈ 18.93 L. The American gallon, in a country that never officially used it.
Why a British car "burns less"
The gallon gap deforms the most popular economy figure of all — miles per gallon. The same car, burning exactly the same fuel, posts a higher MPG in Britain, because the bigger gallon carries it further:
MPG (UK) ≈ MPG (US) × 1.20095
Take a car doing 10 litres per 100 km. In American notation that is roughly 23.5 MPG (US). The same car, the same tank, British notation: roughly 28.2 MPG (UK). Nothing changed but the denominator.
Comparing US and UK economy tests without converting is therefore a methodological error — and that is only the first layer. The second: the American EPA cycles and the European WLTP measure different things over different driving profiles. Even once the units share a denominator, the numbers are not fully comparable.
If you are renting a car abroad, the trip computer can usually be switched over: the infotainment menu, roughly Settings → General → Units → Fuel Consumption, sometimes via the steering-wheel buttons.
Not just volume: 87 octane is not worse fuel
A European driver looks at an American pump — 87, 89, 93 — and concludes the fuel is worse than our 95. That is a misunderstanding rooted in a different measurement method, not a different quality.
Europe quotes RON (Research Octane Number), measured under light load on a test engine at 600 rpm. The United States, Canada and Mexico quote AKI (Anti-Knock Index, also called PON) — the arithmetic mean of RON and MON (Motor Octane Number), which is measured more harshly: 900 rpm and a preheated mixture.
AKI = (RON + MON) / 2
The RON − MON difference, known as the fuel's sensitivity, usually runs 8–12 points. That is why the number on an American pump is roughly 4–6 points lower than the European one for the very same fuel.
| RON (Europe) | MON | AKI (US, Canada) | US trade name |
|---|---|---|---|
| 91 | 83 | 87 | Regular |
| 95 | 87 | 91 | Premium |
| 98 | 90 | 94 | Super Premium |
American "91 AKI" is therefore the equivalent of European 95. Looking for the counterpart of ordinary unleaded, you reach for premium, not regular.
Nozzle colour guarantees nothing
The last trap has nothing to do with arithmetic. In Europe nozzle colours are settled in practice: black is diesel, green is unleaded petrol. In the United States no federal colour regulation exists — and most chains use green for diesel. A reflex imported from Europe leads straight to putting diesel into a petrol engine. The reverse happens too: some chains use green as a brand colour, for petrol.
The only reliable information is the text label on the pump. In Europe the EN 16942 standard adds geometric markings: a circle for petrol grades (E5, E10), a square for diesel (B7), a rhombus for gaseous fuels. In the US only the EPA-mandated wording applies.
The mechanical safeguard works in one direction only. A diesel nozzle spout is wider (on the order of 24–25 mm) than a petrol one (about 21 mm), so it will not enter the narrow filler neck of a petrol car. The other way round, the narrower petrol nozzle slips easily into a diesel's wide neck. It is precisely that direction of mistake that destroys injection systems and generates bills in the thousands.
What this all means
Three things are worth taking on holiday.
A price per gallon means nothing until you know which gallon. Filling up in the Caymans you buy 20% more liquid than in Colombia for nominally the same unit. The only honest denominator is the litre — reduce both prices to it before concluding that a country is "expensive."
MPG figures in the British press are a fifth more generous than American ones. Not because the engines there are better.
The label beats the colour. And if you are driving a diesel, check twice what you are holding.
The gallon survived because measures rarely die by decree. They die when they stop being convenient — and on an island where everyone has counted in gallons for generations, convenience and precision are two different things. Which is why it pays to keep a calculator that knows 3.785411784 is not the same as 4.54609.
Further reading
- Weights and Measures Act 1824 (UK) — the act that abolished the wine, corn and ale gallons in favour of a single imperial one.
- NIST, Special Publication 811: Guide for the Use of the International System of Units — the official, exact conversion factors for US customary units.
- NIST, Handbook 44 — metrological requirements for commercial devices, fuel dispensers included.
- U.S. Energy Information Administration, Gasoline explained — octane in depth — RON, MON and AKI at the source.
- EN 16942 — the harmonised fuel markings on European pumps and vehicle filler flaps.
- Encyclopædia Britannica, entry Imperial units — a concise history of the imperial system.
- Wikipedia, Comparison of the imperial and US customary measurement systems — the fullest catalogue of the divergences, with links to primary sources.