Article — Barometric Pressure Conversion
Barometric pressure conversion: hPa, inHg, mbar, and mmHg in one place
Barometric pressure at sea level under standard conditions is 1013.25 hPa, 29.9213 inHg, 760 mmHg, or 1 atm. These are the same pressure expressed in seven units that survive across weather services, aviation, medicine, and engineering. Converting between them is exact, not a measurement.
Hectopascals (hPa) and millibars (mbar) are the same number; the WMO renamed mbar to hPa in 1985 to align with SI. The US still uses inches of mercury (inHg) for aviation altimeters. Russia and some Eastern European meteorological services keep mmHg. Knowing which unit a forecast or aviation chart uses prevents misinterpretation of weather and altitude data.
What barometric pressure measures
Barometric pressure is the weight of the column of atmosphere above a point on Earth's surface. It is also called atmospheric pressure or air pressure. The pressure changes with altitude (less air above means lower pressure) and with weather conditions (rising or falling air, varying temperature, and density). A standard atmosphere at sea level under 15 degrees Celsius is defined as exactly 1013.25 hPa for reference purposes.
The Pascal (Pa) is the SI unit. Hectopascals (hPa) and kilopascals (kPa) are convenient SI multiples. The other units (inHg, mmHg, atm, psi) are accepted alongside SI but predate it. Switching between them is one of the most common conversions in meteorology and aviation.
The lowest sea-level barometric pressure ever recorded was 870 hPa (25.69 inHg) in the eye of Typhoon Tip on 12 October 1979. The highest reliable reading was 1085.7 hPa (32.06 inHg) at Tosontsengel, Mongolia, on 19 December 2001. The natural range at Earth's surface spans roughly 200 hPa.
History of barometric pressure units
Evangelista Torricelli's 1643 mercury barometer gave the first repeatable measurement of barometric pressure: a 760 mm mercury column at sea level under standard temperature. That observation fixed the millimetre-of-mercury (mmHg) and inch-of-mercury (inHg) as natural units. Blaise Pascal's 1648 mountain experiment confirmed that pressure drops with altitude.
The bar emerged in 1909 as a meteorological unit: 1 bar = 100,000 Pa. The millibar (1/1000 of a bar) became the world weather standard. In 1985 the WMO renamed millibar to hectopascal to fit the SI structure. The numerical value did not change. Aviation in the United States kept inHg because mechanical altimeters had been calibrated in inches since the 1930s.
1013.25 hPa = 1013.25 mbar1013.25 hPa = 29.9213 inHg1013.25 hPa = 760 mmHg1013.25 hPa = 1 atm = 14.696 psi1013.25 hPa = 101.325 kPaConverting hPa to inHg and other units
To convert hPa to inHg, divide by 33.8639. To go from hPa to mmHg, divide by 1.33322. To get atm, divide by 1013.25. To get psi, multiply by 0.0145038. To get kPa, divide by 10. Each factor is exact to twelve significant figures or more.
The most common bidirectional conversion is hPa ↔ inHg, because US weather data is reported in inHg while the rest of the world uses hPa. A common practical pair is 1000 hPa = 29.53 inHg and 1020 hPa = 30.12 inHg, so the inHg scale on a US barometer typically runs from about 28.5 to 31.0.
How pressure changes with altitude
Near sea level, barometric pressure drops about 1 hPa per 8 m climbed. The drop is approximately exponential because air density itself decreases with height. The International Standard Atmosphere (ISA) gives a smooth formula valid up to about 11 km: p(h) = 1013.25 × (1 - 0.0065h/288.15)^5.255 hPa, with h in metres.
Some altitude landmarks: Mexico City (2,240 m) sits at about 780 hPa. La Paz, Bolivia (3,640 m) is around 650 hPa. Mount Everest summit (8,848 m) is about 337 hPa — roughly one-third of sea-level pressure. Aircraft cruise altitudes (10-12 km) sit between 200 and 300 hPa, which is why cabins must be pressurised.
Reading weather from barometric pressure
Rising barometric pressure signals improving weather (descending air, clearing skies). Falling pressure signals approaching storms (rising air, condensation, precipitation). Stable pressure means stable weather. The rate of change matters more than the absolute value: a 5 hPa drop in three hours is a strong storm warning.
- Standard sea level = 1013.25 hPa = 29.92 inHg
- Strong high-pressure ridge = 1030 hPa = 30.42 inHg
- Typical mid-latitude low = 1000 hPa = 29.53 inHg
- Tropical storm = 990 hPa = 29.24 inHg
- Major hurricane (Cat 3) = 945-960 hPa = 27.9-28.4 inHg
- Cat 5 hurricane ≤ 920 hPa = 27.17 inHg
- Typhoon Tip (1979 record) = 870 hPa = 25.69 inHg
Barometric pressure in aviation
Aircraft altimeters work by measuring barometric pressure. The pilot sets a reference pressure (called QNH in the rest of the world, or "altimeter setting" in the US) on the Kollsman window. The altimeter then shows altitude above mean sea level. In the US the setting is in inHg (29.92 standard); elsewhere it is in hPa (1013 standard).
Above the transition altitude (typically 18,000 ft in the US), all altimeters switch to the standard 29.92 inHg / 1013 hPa setting so all aircraft are flying off the same reference. This is called the flight-level system, used for traffic separation.
For pilots: every 1 inHg change in altimeter setting equals about 1,000 ft of altitude difference. If you set 29.92 but the local altimeter is 30.10, your indicated altitude is 180 ft too low. Always set the current local pressure before takeoff and landing.
Common barometric-pressure mistakes
Three mistakes recur. First, treating "1013 mbar" and "1013 hPa" as different values. They are the same — the WMO simply renamed the unit. Second, mixing station pressure with Mean Sea Level Pressure (MSLP). A weather report from Denver (1,609 m elevation) at 1013 hPa MSLP corresponds to roughly 840 hPa station pressure. Without the correction, station readings from different elevations cannot be compared. Third, expecting mmHg blood-pressure values to be barometric values. Blood pressure of 120/80 mmHg is normal physiology; atmospheric pressure is 760 mmHg. The unit is the same but the magnitudes differ by an order of magnitude.
Weather stations report Mean Sea Level Pressure (MSLP), not station pressure, so all stations can be compared on a single weather chart. A barometer at 1,600 m altitude reading 840 hPa actually corresponds to 1013 hPa MSLP. If your home barometer never reads 1013 hPa at sea level, it might be miscalibrated or set to station pressure. Recalibrate using a nearby airport's altimeter setting.