Definition
A unit of liquid volume used in the United States, equal to 231 cubic inches, 128 fluid ounces, 4 quarts, or approximately 3.785 liters. One US gallon of aviation gasoline (avgas) weighs about 6.0 pounds at standard temperature; one US gallon of jet fuel weighs about 6.7 pounds.
Plain English
The standard American measure for liquids. Aircraft fuel quantities in the United States are given in US gallons, and pilots use this unit to calculate how much fuel they have, how much they need, and how much it weighs.
Context Anchor
Seen in fuel capacity, fuel burn, oil quantity, refueling, and weight-and-balance calculations for U.S. aircraft.
Derivation
The US gallon descends from the old English wine gallon of 231 cubic inches, which the United States kept after independence. Britain later replaced its wine gallon with the larger Imperial gallon (about 4.546 liters), so today a US gallon and a UK gallon are different sizes. This is why the word 'US' is attached — to distinguish it from the Imperial gallon still used in some other countries.
Why Pilots Care
Using the correct US gallon volume prevents fuel-quantity errors that can result in inadequate reserves or range miscalculations.
Intuition Check
Do not assume every gallon is the same size. In this context, U.S. gallon means the American liquid gallon: about 3.785 liters.
Example Sentence 1
The Cessna 172 holds 56 US gallons of usable fuel, which works out to about 336 pounds of avgas for weight and balance.
Example Sentence 2
After refueling, the tanks held 32 US gallons of avgas.