Definition
A unit of liquid volume used in the United Kingdom and some Commonwealth countries, equal to 4.546 litres, or approximately 1.201 U.S. gallons.
Plain English
The British version of a gallon. It is bigger than the American gallon — about one-fifth larger.
Context Anchor
Seen in fuel capacity, fuel planning, oil quantity, and weight-and-balance information in some older or non-U.S. aircraft documents.
Derivation
Imperial' refers to the British Imperial system of measurement, established by the Weights and Measures Act of 1824. The word comes from the Latin imperium, meaning 'authority' or 'rule' — these were the units set by the authority of the British Empire. Knowing this helps you remember that an Imperial gallon is the British standard, not the American one.
Why Pilots Care
Using the wrong gallon type in fuel calculations can produce incorrect weight or range figures.
Intuition Check
Do not assume every gallon is a U.S. gallon. If the document says Imperial gallon, convert it before using the number for fuel planning or weight calculations.
Example Sentence 1
The UK fuel receipt was written in Imperial gallons, so the pilot converted to U.S. gallons before updating the aircraft logbook.
Example Sentence 2
Converting fuel to pounds requires using 7.2 pounds per Imperial Gallon for avgas.