Definition
A unit of length in the U.S. customary system equal to 1/12 of a foot, or 2.54 centimeters. In aviation, it appears most often in pressure readings (inches of mercury, in. Hg) for altimeter settings and manifold pressure, and in measurements of rainfall, snowfall, ice accretion, and runway markings.
Plain English
A small unit of length — twelve of them make a foot. In aviation, it's used for short measurements and, very commonly, for measuring pressure (inches of mercury) on the altimeter.
Context Anchor
Seen in FAA abbreviation lists, aircraft information, signs, placards, and written notices where space is limited.
Derivation
From the Latin uncia, meaning 'one-twelfth part.' The same root gives us 'ounce.' Knowing this helps explain why an inch is one-twelfth of a foot — it's literally a twelfth-part.
Why Pilots Care
Small measurements can matter in aviation. Reading IN correctly helps prevent confusing inches with another unit of length.
Intuition Check
Do not read IN as the everyday word “in” when it appears as a measurement abbreviation. After a number, IN means inch or inches.
Example Sentence 1
The current altimeter setting is 30.05 in. Hg.
Example Sentence 2
Confirm propeller ground clearance is at least 9 IN before taxi.