Convert Inch of Mercury to Atmosphere (inHg → atm)

Converting inches of mercury to atmospheres is straightforward: multiply by the conversion factor. Follow these three steps to do it by hand or in your head.

1. 1
   Take your value in inches of mercury

   Start with the number of inches of mercury (inHg) you want to convert.
2. 2
   Multiply by 0.033421

   The conversion factor from inHg to atm is 0.033421. Multiply your value by this number.
3. 3
   Read the result in atmospheres

   The result is your value in atmospheres (atm).

Multiply the value in inches of mercury by 0.033421. For the reverse direction, multiply by 29.921252.

• ✓1 inHg ≈ 3386 Pa ≈ 33.86 mbar.
• ✓29.92 inHg = 1013.25 mbar = 1 atm.
• ✓US pilots use inHg; European pilots use hPa (mbar).

• ✗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.

What is the Inch of Mercury?

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.

• US aviation altimeter QNH settings
• US residential barometers
• US weather forecasts (older)

What is the Atmosphere?

The atmosphere equals exactly 101,325 pascals (the average atmospheric pressure at sea level, latitude 45°) and is a reference unit in chemistry, diving, and engineering. Defined for scientific convenience to represent 'standard atmospheric pressure,' it is widely used in chemistry (gas laws, reaction conditions), aviation (cabin pressure relative to ambient), and scuba diving (depth pressure: every 10 m of seawater adds about 1 atm). Standard reference conditions in chemistry often specify 1 atm and 25°C. The atmosphere relates to the pascal (101,325 Pa = 1 atm), the kilopascal (101.325 kPa = 1 atm), the bar (1.01325 bar = 1 atm), the psi (14.696 psi = 1 atm), the torr (760 torr = 1 atm), and the meter of seawater (10.33 mH₂O = 1 atm). The 'technical atmosphere' (1 at = 98.066 kPa = 1 kgf/cm²) is a slightly different historical unit no longer in use.

• Chemistry standard conditions
• Aviation cockpit pressurisation references
• Pressure tank and vessel ratings