Pressure is force distributed over area. The SI pressure unit is the pascal, but real instruments and industries use several practical units: psi for many US gauges, bar for equipment and European specifications, kPa for metric labels, atm for reference pressure, and Torr for vacuum work.
Gauge, absolute, and differential pressure
Pressure values need both a unit and a reference. Gauge pressure is measured relative to local atmospheric pressure, absolute pressure is measured relative to vacuum, and differential pressure compares two points. Unit conversion changes psi to bar or kPa, but it does not recover a missing psig, psia, barg, or bara label.
Gauge pressure
Common on tire gauges, compressors, pumps, and many shop instruments. Often written psig or barg.
Absolute pressure
Common in gas laws, vacuum systems, and thermodynamic calculations. Often written psia or bara.
Differential pressure
Used across filters, ducts, pumps, cleanrooms, and flow systems where pressure difference matters more than absolute level.
Pressure unit scale
The chart compares the pressure units in this directory against the pascal, the SI pressure unit used as the conversion bridge.
Where each pressure unit is used
| Unit | Typical context | Practical note |
|---|---|---|
| Pascal (Pa) | SI formulas, standards, stress, sensors | Precise but often small for everyday readings |
| Kilopascal (kPa) | Tires, HVAC, labels, engineering | Metric-friendly and readable for common pressures |
| Bar | Compressors, pumps, European gauges | Close to atmospheric pressure but not equal to atm |
| Psi | US tires, hydraulics, air tools, gauges | Watch psig vs psia in technical work |
| Atmosphere (atm) | Chemistry, gas laws, reference pressure | Fixed standard value, not live local air pressure |
| Torr | Vacuum, lab instruments, vapor pressure | Defined from standard atmosphere |
Common pressure-unit mistakes
- Dropping gauge or absolute notation when it changes the meaning of the value.
- Treating bar and atmosphere as exactly the same because they are close in size.
- Comparing a rounded tire label value with a precise calculator result and assuming one must be wrong.
- Using psi in a metric engineering formula without first converting to pascals or kilopascals.
- Using Torr or atm for ordinary equipment pressure when kPa, bar, or psi is the clearer unit.
Pressure unit guides
Each guide explains a pressure unit as a unit, not just as a converter input.
Pressure converters
Use converter pages when you need a direct numerical answer.