Atmospheric Pressure Converter: Pa, hPa, psi, atm, mmHg
Pressure is force divided by area, and atmospheric pressure is the weight of the air column pressing down on every surface. The SI unit is the pascal (Pa), equal to one newton per square metre, but it is tiny: standard sea-level pressure is 101,325 pascals. Meteorologists therefore use hectopascals (hPa) and millibars (mbar), which are identical: 1,013.25 of either equals standard atmospheric pressure. Aviation and barometry use inches of mercury (inHg) in the United States and millimetres of mercury (mmHg) worldwide, both referencing the height of a mercury column balanced by air pressure. The bar is convenient for engineering: 1 bar is close to 1 atm, and 1,000 mbar equals 1 bar. Pounds per square inch (psi) is used in the US for tire pressures and pneumatic systems. The atmosphere (atm) is the exact reference: 101,325 Pa by definition. Kgf/cm² is used in some Asian and Russian engineering standards. This converter handles all eleven units simultaneously: enter any value and every other field updates instantly using BIPM-standard conversion factors.
Conversion factors (relative to the pascal)
All factors below are BIPM-standard. The pascal is the SI base unit; all other units are defined exactly or with high-precision factors.
| Unit | Symbol | Pascals (exact or rounded) |
|---|---|---|
| Pascal | Pa | 1 |
| Hectopascal | hPa | 100 |
| Kilopascal | kPa | 1,000 |
| Megapascal | MPa | 1,000,000 |
| Bar | bar | 100,000 |
| Millibar | mbar | 100 |
| Atmosphere | atm | 101,325 (exact) |
| Millimetre of mercury | mmHg | 133.322 |
| Inch of mercury | inHg | 3,386.39 |
| Pound per square inch | psi | 6,894.757 |
| Kilogram-force per cm² | kgf/cm² | 98,066.5 |
Reference pressures
| Condition | hPa | Pa | mmHg | inHg | psi |
|---|---|---|---|---|---|
| Standard atmosphere (ISA) | 1,013.25 | 101,325 | 760.00 | 29.921 | 14.696 |
| Typical high-pressure system | 1,030 | 103,000 | 772.6 | 30.42 | 14.94 |
| Typical low-pressure system | 980 | 98,000 | 735.1 | 28.94 | 14.21 |
| Record high (Tosontsengel, 2001) | 1,084 | 108,400 | 813.1 | 32.01 | 15.72 |
| Record low (Typhoon Tip, 1979) | 870 | 87,000 | 652.6 | 25.69 | 12.62 |
| Top of Everest (~8,849 m) | 337 | 33,700 | 252.8 | 9.95 | 4.89 |
| Blood pressure (systolic typical) | 16 | 1,600 | 120 | 4.72 | 0.23 |
Atmospheric pressure converter: frequently asked questions
What is atmospheric pressure?
Atmospheric pressure is the force per unit area exerted by the weight of the air column above a given point. At sea level, this pressure averages 101,325 pascals (1,013.25 hPa or approximately 1 bar). Atmospheric pressure decreases with altitude as there is less air above. It also varies with weather: high-pressure systems typically sit above 1,013 hPa, and low-pressure systems fall below. Barometers measure atmospheric pressure, and the readings are fundamental to weather forecasting.
What is the difference between absolute and gauge pressure?
Absolute pressure is measured relative to a perfect vacuum (zero pressure). Gauge pressure is measured relative to the local atmospheric pressure, so gauge pressure plus atmospheric pressure equals absolute pressure. Tire pressure gauges read gauge pressure: a reading of 32 psi means 32 psi above local atmospheric pressure. Medical blood pressure readings are also gauge values referenced to atmospheric pressure. All conversions on this page use absolute pressure.
What is a hectopascal (hPa) and is it the same as a millibar?
Yes, 1 hectopascal (hPa) is exactly equal to 1 millibar (mbar). Both equal 100 pascals. Meteorologists historically used millibar, but the WMO now recommends hectopascal as the SI-coherent unit. Weather maps and forecasts use hPa and mbar interchangeably. Standard atmospheric pressure is 1,013.25 hPa, and weather systems typically range from about 950 hPa (strong cyclone) to 1,040 hPa (strong anticyclone).
What is standard atmospheric pressure?
Standard atmospheric pressure (ISA sea-level standard) is defined as exactly 101,325 pascals. This is equivalent to 1,013.25 hPa (or mbar), 760 mmHg (Torr), 29.921 inHg, 14.696 psi, 1.01325 bar, and 1.033 kgf/cm². This value is used as a reference for altitude, aviation, chemistry, and engineering calculations. The BIPM and ISO have adopted this as the standard reference pressure.
How does atmospheric pressure relate to weather forecasting?
Barometric pressure trends are among the most reliable short-term weather indicators. A rising barometer generally signals improving weather (high pressure, clear skies), while a falling barometer signals approaching storms or low pressure. A rapid drop of more than 3 hPa per hour is considered a very rapid pressure fall and indicates fast-approaching severe weather. Meteorologists issue advisories when central pressure in tropical cyclones drops below defined thresholds.
Official sources
- Bureau International des Poids et Mesures (BIPM): SI units and conversion factors.
- NOAA National Weather Service: Atmospheric pressure definitions and weather data.
- World Meteorological Organization (WMO): International meteorological standards.
- NIST Special Publication 330: The International System of Units.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.