US Gallons to imperial gallons
1 US gal = 0.832674 imp gal, derived from exact unit definitions.
| US gal | imp gal |
|---|---|
| 1 US gal | 0.8327 imp gal |
| 5 US gal | 4.1634 imp gal |
| 10 US gal | 8.3267 imp gal |
| 55 US gal | 45.7971 imp gal |
| 100 US gal | 83.2674 imp gal |
| 275 US gal | 228.9854 imp gal |
| 500 US gal | 416.3371 imp gal |
| 1,000 US gal | 832.6742 imp gal |
imperial gallons = US gallons × 3.785411784 / 4.54609
= US gallons × 0.8326741846289… (non-terminating)Both gallons are defined exactly — but in different systems, so their ratio is an exact rational number with a non-terminating decimal. Divide litres by 4.54609 rather than multiplying by a truncated ratio.How to convert US gallons to imperial gallons
An imperial gallon is about 20 % larger than a US gallon. That gap is big enough to be obvious once you know to look for it, and small enough to slip through unnoticed on a spec sheet — which is exactly why it causes trouble on UK, Irish and older Canadian tank documentation.
- Multiply US gallons by 3.785411784 to get litres.
- Divide by 4.54609 to get imperial gallons.
- Check the magnitude: the imperial figure should be about five-sixths of the US figure. If your answer went up, you converted the wrong way.
A 275 US gallon oil tank, quoted in imperial
- 275 × 3.785411784 = 1,040.99 L
- 1040.99 ÷ 4.54609 = 228.99 imp gal
275 US gal = 229.0 imperial gallons
A UK reader seeing “275 gallon” and assuming imperial would expect 1,250 litres — 20 % more oil than the tank actually holds.
A 500 US gallon tank
- 500 × 0.8326741846 = 416.34 imp gal
- = 1,892.7 L
500 US gal = 416.3 imperial gallons
Fuel economy, the classic trap
- A car rated 30 miles per US gallon
- 30 ÷ 0.8326741846 = 36.0 miles per imperial gallon
30 US mpg = 36.0 imperial mpg
Note the direction reverses for rates. Larger gallon, more miles per gallon — so mpg figures divide by the factor where volumes multiply by it. This is why UK and US fuel-economy numbers are never directly comparable.
Where the factor comes from
Two independent legal definitions, in two different systems, that were never reconciled.
US gallon ≡ 231 in³ = 3.785411784 L (NIST HB 44)
Imp gallon ≡ 4.54609 L (UK W&M Act 1985)
ratio = 3.785411784 / 4.54609
= 0.83267418462898886… non-terminatingNIST SP 811 prints the imperial gallon → litre factor as 4.546 09 E+00 in boldface, marking it exact, because it is a legal definition rather than a measurement.Why two gallons exist
The US gallon is the older of the two. It descends from the English wine gallon of 231 cubic inches, which the United States kept when it inherited English measures. Britain then abolished its patchwork of wine, ale and corn gallons in 1824 and replaced them with a single imperial gallon — originally the volume of ten pounds of water at 62 °F, later redefined outright as 4.54609 litres. The US never adopted the replacement, so the two systems have been about 20 % apart ever since.
FAQ
How many imperial gallons in a US gallon?
0.8326741846… imperial gallons. A US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres, so the US gallon is about 83 % of the imperial one.
Why is the imperial gallon bigger?
Because Britain replaced its old gallons in 1824 with a new one based on ten pounds of water, while the United States kept the older 231-cubic-inch wine gallon. Two separate legal definitions, never reconciled.
Is the imperial gallon still legal?
It remains defined in UK statute — Weights and Measures Act 1985, Schedule 1 — but its use for trade is heavily restricted, and fuel and heating oil are sold in litres. Treat it as a legacy unit you will meet in documents rather than one you should quote in.
How do I convert mpg between the two?
Divide US mpg by 0.8326741846 to get imperial mpg — 30 US mpg is 36.0 imperial mpg. Rates invert relative to volumes, which is the step most people get backwards.
Which gallon does Canada use?
Legally, litres. Fuel and most trade are metric. Imperial gallons persist in older tank and equipment literature, so a Canadian document quoting gallons is more likely imperial than US — but check the litre equivalent rather than guessing.
Sources and standards
Every factor on this page is traced to the document that defines it. Where a value is exact, it is exact because the cited standard makes it so — not because we rounded it well.
- Weights and Measures Act 1985, Schedule 1, Part IV — Measurement of CapacityUK Parliament; published by The National Archiveshttps://www.legislation.gov.uk/ukpga/1985/72/schedule/1Primary UK statute. Defines the imperial gallon as 4.54609 cubic decimetres — a flat legal definition, not a measurement, which is why the imperial gallon converts to litres exactly and to U.S. gallons only approximately.
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cThe legal-metrology reference for U.S. customary units. Fixes the U.S. liquid gallon at exactly 231 cubic inches and tabulates the full liquid-volume chain (minim → fluid dram → fluid ounce → gill → pint → quart → gallon).
- Special Publication 811, Appendix B.8 — Factors for Units Listed AlphabeticallyNational Institute of Standards and Technologyhttps://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8NIST's master table of conversions into SI. Factors printed in boldface are exact by definition rather than measured or rounded — which is how you tell a defined unit from an approximated one.
- The International System of Units (SI), 9th edition (SI Brochure)Bureau International des Poids et Mesures (BIPM)https://www.bipm.org/en/si-brochure-9The defining document for the SI, maintained by the treaty organisation that owns the definitions. The litre is exactly one cubic decimetre — 10⁻³ m³ — and is a unit accepted for use with the SI rather than an SI unit itself.
Every linked source above was retrieved and checked on 2026-09-02. Factors quoted as exact are exact by definition in the cited document, not rounded by us.