Inches Of Mercury to Hectopascals (inHg to hPa)

1 inch of mercury equals 33.8638864 hectopascals (rounded).

How to convert inches of mercury to hectopascals

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

hectopascals = inches of mercury × 33.8638864

Example: 10 inches of mercury = 338.638864 hectopascals.

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

Hectopascal (hPa)

Metric (SI)Current unit

100 pascals, equal to one millibar. The standard unit in weather forecasts and barometers.

Weather forecasters rarely think in pascals, and the hectopascal exists largely to avoid changing their numbers. When SI arrived, meteorologists generally switched from the millibar to the hectopascal, which equals exactly 100 Pa and exactly one millibar, so existing readings carried over unchanged. Observations are typically reported in hPa, following the World Meteorological Organization recommendation, though many scientific fields dislike the hecto prefix because they prefer steps of 1,000.

A typical sea-level reading is about 1,013 hPa. The lowest non-tornadic pressure ever measured was 870 hPa, in Typhoon Tip on 12 October 1979, and the highest sea-level pressure, 1,084.8 hPa, came at Tosontsengel, Mongolia, on 19 December 2001, at a station above 750 meters. At low altitudes, pressure drops about 12 hPa for every 100 meters you climb.

Aviation weather reports, such as the QNH in a METAR, use hectopascals or millibars in most countries, while the US, Canada and Japan quote altimeter settings in inches of mercury.

1 hectopascal = 100 pascals (exactly).

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

Related conversions

See all pressure units.