What is “””HG” in Aviation? (Inch Of Mercury)

Inch of Mercury (inHg) is a unit used in aviation to measure atmospheric pressure. It refers to the height of a column of mercury in a barometer when atmospheric pressure is being measured. This unit is widely used in aircraft altimeters and is an important metric for pilots to monitor during flight. Understanding the concept of Inch of Mercury is crucial for pilots, as it directly affects the altitude and performance of an aircraft.

The Significance of Inch of Mercury in Aviation

Inch of Mercury plays a vital role in aviation because it helps pilots determine the altitude at which they are flying. The altimeter, an instrument used in aircraft to measure altitude, is calibrated using atmospheric pressure. The value of Inch of Mercury allows pilots to set the correct reference pressure on their altimeter, ensuring accurate altitude readings during flight.

When the aircraft is on the ground, the altimeter displays the pressure at sea level, known as the “barometric pressure.” However, as the aircraft climbs to higher altitudes, the atmospheric pressure decreases. Pilots need to adjust the altimeter to compensate for this change in pressure to accurately measure the aircraft’s height above the ground.

Inch of Mercury is particularly relevant for pilots because it is used as a reference unit when setting altimeter pressure. The standard atmospheric pressure at sea level is typically around 29.92 inHg. Pilots adjust their altimeters to this value, which ensures that the instrument provides accurate altitude readings based on the pressure variations.

During flight, as the pressure decreases with altitude, the altimeter will indicate an increase in altitude without any actual change in height above the ground. This phenomenon, known as “Pressure Altitude,” is calculated based on the difference between the altimeter setting and the standard atmospheric pressure at sea level. It is essential for pilots to monitor the changes in Inch of Mercury to accurately determine their aircraft’s altitude and its relation to the ground.

Measuring Inch of Mercury

Inch of Mercury is measured using a device called a mercury barometer. This instrument consists of a long glass tube filled with mercury, inverted in a container also filled with mercury. As the atmospheric pressure changes, it exerts force on the surface of the open container, causing the mercury level inside the tube to rise or fall.

The height of the mercury column indicates the pressure exerted by the atmosphere. One inch of height represents approximately 0.4912 pounds per square inch (psi) of atmospheric pressure. Hence, the term “Inch of Mercury” refers to the height of the mercury column in inches when measuring pressure.

Modern aircraft altimeters do not use mercury, but instead use aneroid capsules (a sealed metal container) that expand or contract based on the atmospheric pressure. These capsules are connected to a mechanical mechanism that moves the needle on the altimeter dial, which displays the corresponding altitude.

The conversion between Inch of Mercury and other units of pressure, such as millibars (mb) or hectopascals (hPa), is vital in aviation. One inHg is equivalent to 33.864 millibars or hectopascals. It is essential for pilots to be familiar with these conversions to ensure accurate readings when using weather reports or communicating with air traffic control.


Inch of Mercury is an important metric in aviation used to measure atmospheric pressure. It is crucial for pilots to understand this unit, as it directly impacts altitude readings and aircraft performance. Monitoring changes in Inch of Mercury allows pilots to make accurate altitude adjustments and navigate safely during flight. With its significance in aviation, pilots rely on this unit to ensure precision and safety in their journeys.


1. Aviation Glossary. (n.d.). Inch of Mercury. Retrieved from https://aviationglossary.com/inch-of-mercury/

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