Convert Inch of Mercury to Millibar (inHg → mbar)

Converting inches of mercury to millibars 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 33.86389

   The conversion factor from inHg to mbar is 33.86389. Multiply your value by this number.
3. 3
   Read the result in millibars

   The result is your value in millibars (mbar).

Multiply the value in inches of mercury by 33.86389. For the reverse direction, multiply by 0.02953.

• ✓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 Millibar?

The millibar equals exactly 100 pascals and is the legacy meteorology unit, identical to the modern hectopascal (hPa). Most weather services worldwide transitioned from millibars to hectopascals in the late 20th century, but the unit persists in aviation weather reports, marine forecasts, and older barometric instruments. Standard sea-level atmospheric pressure is 1,013.25 mbar (or hPa). Hurricane and typhoon central pressures are reported in millibars: a major Category 5 hurricane like Hurricane Andrew (1992) had a central pressure of about 922 mbar. The millibar relates to the hectopascal (1 mbar = 1 hPa exactly), the bar (1,000 mbar = 1 bar), the pascal (100 Pa = 1 mbar), and the atmosphere (1,013.25 mbar = 1 atm). Despite the official preference for hPa, the millibar is retained in some traditional contexts and older texts.

• Weather reports and atmospheric pressure
• Altimetry and aviation
• Historical weather records