Inches Of Mercury to Millibars (inHg to mbar)

1 inch of mercury equals 33.8638864 millibars (rounded).

How to convert inches of mercury to millibars

To convert inches of mercury to millibars, multiply the number of inches of mercury by 33.8638864. The factor is rounded to 10 significant digits. To go the other way, multiply millibars by 0.0295299833 (rounded).

millibars = inches of mercury × 33.8638864

Example: 10 inches of mercury = 338.638864 millibars.

About these units

Inch Of Mercury (inHg)

US & ImperialCurrent unit

About 3,386.39 pascals. US weather reports and aircraft altimeters quote barometric pressure in inches of mercury.

Barometers in the United States still speak in inches. The inch of mercury is the pressure exerted by a 1-inch (25.4 mm) column of mercury under standard gravity, which is a conventional value of about 3,386.389 pascals. Because mercury's density depends on temperature, older literature that used a 60 degrees Fahrenheit basis gives about 3,376.85 pascals instead.

Its modern home is weather and flying. US weather reports quote barometric pressure in inHg, and North America and Japan give altimeter settings in it. Aircraft at higher altitudes set their altimeters to a standard 29.92 inHg, which is where flight levels come from, and that number equals one standard atmosphere.

Mechanics know it from vacuum and boost gauges. Technicians describe refrigerant-system vacuum in inHg, automotive cooling system fill tools typically hold 22 to 27 inHg of vacuum, and railways once used it for vacuum brakes.

1 inch of mercury = 3,386.38864 pascals (approximately).

Sources: Wikipedia: Inch of mercury; Wikipedia: Standard atmosphere (unit); Wikipedia: Atmospheric pressure

Millibar (mbar)

Metric (SI)Current unit

One thousandth of a bar, equal to one hectopascal. Standard atmospheric pressure is 1,013.25 mbar.

Millibar and hectopascal are the same pressure with different pedigrees. The millibar was introduced alongside the bar by Vilhelm Bjerknes, and meteorologists worldwide adopted it because the values are convenient: a thousandth of a bar puts ordinary air pressure near 1,000. Standard atmospheric pressure is 1,013.25 millibars.

When SI came in, some meteorologists switched to hectopascals, which carry exactly the same numbers. The millibar did not vanish, though. The symbol mb is still common, and Americans know the unit from US hurricane and cyclone reports.

A few readings give the scale. The record-low non-tornadic pressure of 870 hPa in Typhoon Tip in 1979 equals 870 millibars, and falling pressure of about 12 hPa per 100 meters of altitude means a mountain road can shave tens of millibars off a barometer reading in a short climb.

1 millibar = 100 pascals (exactly).

Sources: Wikipedia: Bar (unit); Wikipedia: Hectopascal; Wikipedia: Atmospheric pressure

Related conversions

See all pressure units.