Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch tall at a standard temperature and gravity. In aviation, it is the standard unit for altimeter settings and manifold pressure in piston engines. Standard sea-level atmospheric pressure is 29.92 inches of mercury.
Plain English
A way of measuring air pressure by comparing it to how high it would push a column of liquid mercury in a tube. The higher the pressure, the higher the mercury rises, measured in inches.
Context Anchor
You may see inches of mercury during a before-takeoff check on the engine pressure gauge, and in weather or altimeter settings such as 29.92 inches of mercury.
Derivation
The unit comes from early barometers, which were glass tubes filled with mercury. Atmospheric pressure pushed the mercury up the tube, and pressure was reported by measuring how many inches the column rose. The same principle and unit carried into modern aviation instruments, even though no actual mercury is involved anymore.
Why Pilots Care
An incorrect altimeter setting in inches of mercury produces an altitude error that can lead to terrain conflict or airspace deviation.
Grounding Statement
Standard sea-level pressure is 29.92 inches of mercury, meaning the air pressure would hold mercury about 29.92 inches high in a barometer.
Intuition Check
Do not read “inches” here as a length on the airplane. In this term, inches of mercury is a pressure measurement based on how high mercury would rise in a measuring tube.
Example Sentence 1
Tower reported the altimeter setting as 30.05 inches of mercury, so the pilot turned the Kollsman knob until 30.05 appeared in the altimeter window.
Example Sentence 2
During the before-takeoff check the pilot confirmed the local pressure remained 29.82 inches of mercury and rechecked the altimeter reading.