Pressure conversion starts with the pascal
The pascal is the SI coherent unit of pressure, equal to one newton per square meter. CalcOS converts each supported unit through pascals and rounds only the displayed result.
Unit conversion changes the numerical unit only. It does not change whether a measurement is absolute, gauge, or differential pressure.
Common pressure definitions
| Unit | Pascal relationship | Common context |
|---|---|---|
| hectopascal | 100 Pa exactly | weather and atmospheric pressure |
| kilopascal | 1,000 Pa exactly | engineering and tire pressure |
| bar | 100,000 Pa exactly | industrial and fluid systems |
| standard atmosphere | 101,325 Pa exactly | reference conditions |
| torr | 101,325/760 Pa exactly | vacuum measurement |
| psi | ≈ 6,894.757293 Pa | US customary engineering |
Absolute pressure versus gauge pressure
Absolute pressure is referenced to a vacuum, while gauge pressure is referenced to ambient atmospheric pressure. Converting 30 psi gauge to kilopascals produces a gauge value; it does not become absolute pressure. That change requires the applicable atmospheric reference as an additional input.
Why conventional liquid-column units are excluded
Values such as inches of water and millimeters of mercury can depend on conventional density and temperature definitions. CalcOS excludes ambiguous liquid-column variants from this package until their exact conventions are named explicitly.
Authoritative references: BIPM SI Brochure, NIST SP 811 Appendix B.9, and NIST Pressure and Gas Flow Conversions.