What is a Inch of Mercury?
The inch of mercury is the American aviation standard for altimeter settings and historic barometer readings.
Overview
The inch of mercury equals exactly 25.4 mmHg (or about 3,386.4 pascals) and is the American aviation standard for altimeter settings and historic barometer readings. Pilots set their altimeters to the local atmospheric pressure in inHg before takeoff and landing — standard 'sea-level pressure' is 29.92 inHg. American weather reports historically used inHg before transitioning to millibars/hPa, and home barometers in the US still display inHg. The unit relates to the mmHg (25.4 mmHg = 1 inHg), the millibar (1 inHg ≈ 33.864 mbar), the kilopascal (1 inHg ≈ 3.386 kPa), and the psi (1 inHg ≈ 0.491 psi). Aviation's continued use of inHg in the United States reflects regulatory tradition rather than scientific advantage; international ATC outside the US uses hectopascals.
Convert Inch of Mercury to all units
Live resultRelationship to Other Pressure Units
1 inHg equalsVisual reference for how the inch of mercury relates to other pressure units. Each row links to the full converter for that pair.
When Is the Inch of Mercury Used?
- US aviation altimeter QNH settings
- US residential barometers
- US weather forecasts (older)
Sea-level standard: 29.92 inHg. Mountain pass at 3000 m: ~21 inHg. US aviation QNH given in inHg.
Tips for Using the Inch of Mercury
- 1 inHg ≈ 3386 Pa ≈ 33.86 mbar.
- 29.92 inHg = 1013.25 mbar = 1 atm.
- US pilots use inHg; European pilots use hPa (mbar).
Common Mistakes
- Confusing inHg with mmHg — factor of 25.4.
- Mixing US and European altimeter units on a single flight plan.
- Using inHg on scientific papers — prefer Pa.