Definition
A unit of liquid volume in the U.S. customary system equal to 1/128 of a U.S. gallon, or approximately 29.57 milliliters. There are 16 fluid ounces in a U.S. pint and 32 fluid ounces in a U.S. quart.
Plain English
A small unit used to measure how much space a liquid takes up. One fluid ounce is roughly two tablespoons.
Context Anchor
Seen when small amounts of aircraft liquids are measured, such as oil, additives, cleaning fluids, or sample containers.
Derivation
From Latin uncia, meaning 'a twelfth part.' The word 'fluid' was added to distinguish volume from the ounce used to measure weight, since the two are not the same thing.
Why Pilots Care
Precise fluid measurements prevent over- or under-servicing of engines, hydraulics, and fuel systems.
Intuition Check
Do not read fluid ounce as a weight measurement. If it says fluid ounce, it is measuring liquid amount by volume.
Example Sentence 1
The bottle of oil additive holds 16 fluid ounces, enough to treat one engine oil change.
Example Sentence 2
The oil can label lists capacity in both quarts and fluid ounces for small top-offs.