Imperial Gallons to US gallons
1 imp gal = 1.20095 US gal, derived from exact unit definitions.
| imp gal | US gal |
|---|---|
| 1 imp gal | 1.2009 US gal |
| 5 imp gal | 6.0047 US gal |
| 10 imp gal | 12.0095 US gal |
| 50 imp gal | 60.0475 US gal |
| 100 imp gal | 120.0950 US gal |
| 500 imp gal | 600.4750 US gal |
| 1,000 imp gal | 1,200.9499 US gal |
US gallons = imperial gallons × 4.54609 / 3.785411784
= imperial gallons × 1.2009499255… (non-terminating)Convert through litres: multiply by the exact 4.54609, then divide by the exact 3.785411784. The combined ratio does not terminate.How to convert imperial gallons to US gallons
Multiply by 1.2009499255. An imperial gallon is the larger unit, so the US figure always comes out higher — which is the sanity check that catches a wrong-direction conversion immediately.
- Multiply imperial gallons by 4.54609 to get litres.
- Divide by 3.785411784 to get US gallons.
- Check the magnitude: the US figure should be about 20 % higher.
A 500 imperial gallon UK heating oil tank
- 500 × 4.54609 = 2,273.05 L
- 2273.05 ÷ 3.785411784 = 600.47 US gal
500 imp gal = 600.5 US gallons = 2,273 L
Reading that tank as “500 gallons” in a US context understates it by a hundred gallons.
A nominal 1,000-litre UK bunded tank
- 1000 ÷ 4.54609 = 219.97 imp gal
- 1000 ÷ 3.785411784 = 264.17 US gal
1,000 L = 220 imp gal = 264.2 US gal
The same tank is “220 gallon” in older UK literature and “264 gallon” in US terms. Neither label is wrong; they are counting different gallons.
A 2,500-litre domestic tank, all three ways
- 2500 L ÷ 4.54609 = 549.9 imp gal
- 2500 L ÷ 3.785411784 = 660.4 US gal
- 2500 L = 2.5 m³ = 88.29 ft³
2,500 L = 550 imp gal = 660 US gal
Where the factor comes from
Imp gallon ≡ 4.54609 L exactly (UK W&M Act 1985, Sch. 1)
US gallon ≡ 3.785411784 L exactly (231 in³ × 2.54³)
ratio = 4.54609 / 3.785411784
= 1.20094992550485… non-terminatingBoth inputs are exact legal definitions. The awkward ratio is not uncertainty — it is simply what you get when two systems fix the same quantity independently.The imperial gallon's own history
The imperial gallon was created by the Weights and Measures Act 1824 as the volume of ten pounds of distilled water at 62 °F under a specified barometric pressure — a definition that depended on a physical measurement and so drifted slightly with better metrology. In 1976 it was redefined outright as 4.54609 litres, a flat number chosen to match the old value closely, and that definition is what stands in Schedule 1 of the Weights and Measures Act 1985 today.
FAQ
How many US gallons in an imperial gallon?
1.2009499255… US gallons. An imperial gallon is 4.54609 litres by UK statute; a US gallon is 3.785411784 litres.
How many litres in an imperial gallon?
Exactly 4.54609 litres. This is a flat legal definition in the Weights and Measures Act 1985, Schedule 1, Part IV — not a measured value.
How big is a 500 imperial gallon tank in US gallons?
600.5 US gallons, or 2,273 litres. The 20 % difference between the two gallons is the single most common source of tank-capacity confusion between the UK and the US.
Was the imperial gallon always 4.54609 litres?
No. From 1824 it was defined as the volume of ten pounds of water at 62 °F, which made it dependent on a physical measurement. It was redefined as exactly 4.54609 litres in 1976, a value very close to the old one.
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.