🥛A gallon isn’t always a gallon.

Same word, same spelling, two different standardised quantities — which is exactly the kind of trap that makes recipes and fuel figures travel badly.

Both gallons are exactly defined; they simply define different amounts. The U.S. liquid gallon is 3.785411784 L. The Imperial gallon is 4.54609 L. Neither is approximate, and neither is more correct — the label alone does not tell you which one is meant, so the source of the number matters.

The U.S. and Imperial systems also define different fluid ounces: a U.S. fluid ounce is about 29.57 mL and an Imperial fluid ounce about 28.41 mL. Oddly, the bigger gallon comes with the smaller fluid ounce — an Imperial gallon is 160 Imperial fluid ounces, a U.S. gallon 128 U.S. fluid ounces.

U.S. liquid gallon

3.785411784 L (exact)Source value

Imperial gallon

4.54609 L (exact)Source value

Ratio

4.54609 ÷ 3.785411784 ≈ 1.20Scaleidoscope calculation

Difference

The Imperial gallon is about 20% larger

Standard-defined — both values are exact definitions of different units.

Both gallons are exactly defined; they are simply different units.

• National Institute of Standards and Technology

  NIST Special Publication 811 — Guide for the Use of the International System of Units (SI)(opens external site)

• National Institute of Standards and Technology, Office of Weights and Measures

  NIST Handbook 44, Appendix C — General Tables of Units of Measurement(opens external site)

• United Kingdom Parliament

  Weights and Measures Act 1985, Schedule 1 — Definitions of units of measurement(opens external site)

• National Institute of Standards and Technology, Office of Weights and Measures

  NIST Handbook 44, Appendix C — General Tables of Units of Measurement(opens external site)

  U.S. fluid ounce and 128 US fl oz per U.S. gallon

• United Kingdom Parliament

  Weights and Measures Act 1985, Schedule 1 — Definitions of units of measurement(opens external site)

  Imperial fluid ounce and 160 fl oz per Imperial gallon

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