Convert Hectopascals to Standard Atmospheres
Convert Hectopascals (hPa) to Standard Atmospheres (atm) instantly and accurately.
Conversion Formula
atm = hPa × 0.0009869232667
About Hectopascals
A hectopascal (hPa) = 100 Pa = 1 mbar - the WMO-official SI-compatible meteorological pressure unit since 1986, replacing millibar with no numerical change. All modern surface observations (METAR, SYNOP), digital barometers, aviation weather, and weather apps report in hPa. QNH altimeter settings outside North America are in hPa. ICAO standard sea-level pressure is 1013.25 hPa. Continental high-pressure record exceeds 1084 hPa; Typhoon Tip (1979) holds the low record at ~870 hPa. 1 hPa = 100 Pa.
About Standard Atmospheres
The standard atmosphere (atm) = exactly 101,325 Pa - defined by the 10th CGPM (1954) as the reference air pressure at sea level. Used to define STP (0°C and 1 atm) for gas law and thermochemistry. Chemical equilibrium constants (Kp), Henry's law solubility, and thermodynamic tables are tabulated at 1 atm. ISA aviation standard: sea-level 1 atm / 15°C, pressure halving every ~5.5 km. Diving physics: +1 atm per 10 m depth (fresh water) so 30 m = 4 atm absolute. Diamond formation in Earth's mantle: ~50,000 atm. 1 atm = 101,325 Pa.
Quick Reference Table
| Hectopascals (hPa) | Standard Atmospheres (atm) |
|---|---|
| 1 hPa | 0.000986923 atm |
| 2 hPa | 0.00197385 atm |
| 5 hPa | 0.00493462 atm |
| 10 hPa | 0.00986923 atm |
| 25 hPa | 0.0246731 atm |
| 50 hPa | 0.0493462 atm |
| 100 hPa | 0.0986923 atm |