Standard atmosphere
| Value | 101,325 Pa |
| Status | Exact by definition — no uncertainty |
| Source | 10th CGPM (1954); SI Brochure, 9th edition |
| Categories | ThermodynamicEngineering & Trade |
| pascal | 101,325 Pa |
| kilopascal | 101.325 kPa |
| megapascal | 0.101325 MPa |
| bar | 1.01325 bar |
| atmosphere | 1 atm |
| millimeter of mercury | 759.99989 mmHg |
| pound per square inch | 14.695949 psi |
| foot of water column | 33.898539 ft H₂O |
| gigapascal | 0.000101325 GPa |
| kip per square inch | 0.014695949 ksi |
| inch of mercury | 29.921256 inHg |
| inch of water column | 406.78246 in w.g. |
Learning zone
The standard atmosphere is a defined unit, not a measurement: exactly 101 325 pascals. It began as the pressure of a 760 mm column of mercury at 0 °C under standard gravity, which is why the torr and the millimetre of mercury are still within a whisker of each other. The 10th CGPM fixed the modern value in 1954, deliberately choosing a round number of pascals rather than a round number of millimetres.
Being defined, it is exact and carries no uncertainty — unlike the actual atmospheric pressure at your location, which is a weather variable and averages nearer 101.0 kPa at sea level over a year. Use it as the reference for absolute pressure, for the standard boiling point of water, and for converting gauge readings: absolute = gauge + 101.325 kPa.