Definition
A unit of pressure measurement, abbreviated inHg, equal to the pressure exerted by a column of mercury one inch high at a standard temperature and gravity. Standard atmospheric pressure at sea level is 29.92 inches of mercury. Used in aviation for altimeter settings, manifold pressure readings, and atmospheric pressure reporting.
Plain English
A way of measuring air pressure based on how high it can push a column of mercury in a tube. The higher the column, the higher the pressure.
Context Anchor
Seen in altimeter settings, weather information, and engine instruments such as manifold pressure gauges.
Derivation
The unit comes from early barometers, where pressure was measured by how high it pushed liquid mercury up a sealed glass tube. Mercury was used because it is dense, so the column stayed a manageable height. The inch is simply the unit used to measure that height.
Why Pilots Care
Correct altimeter settings in this unit prevent altitude errors during flight and approach.
Analogy
Think of it like measuring weight by how far it pushes down on a spring. With inches of mercury, pressure is measured by how high it can hold up a column of mercury.
Grounding Statement
Picture a tube of mercury standing in a dish of mercury. The weight of the air pushing down on the dish pushes mercury up the tube. On a standard day at sea level, it rises 29.92 inches.
Intuition Check
Do not read “inch” here as a distance the aircraft travels or a physical length on the airplane. In this term, an inch of mercury is a pressure measurement.
Example Sentence 1
Before takeoff, the pilot set the altimeter to 30.05 inches of mercury, matching the current setting reported by the tower.
Example Sentence 2
During cruise the manifold pressure stabilized at 22 inches of mercury.