Units and SI factors
All units are referenced to the SI base Pa (Pascal).
| Unit | Name | Factor to SI | 1 unit = ? SI |
|---|
Pressure in engineering practice
Pressure is force distributed over an area, and its SI unit is the pascal (Pa) — one newton per square metre. Because a pascal is tiny, real systems use kilopascals, megapascals or the convenient bar (exactly 100 000 Pa). In imperial practice the dominant unit is the psi (pound-force per square inch), while the millimetre of mercury (mmHg) and the atmosphere (atm) survive in medicine, chemistry and vacuum work.
Where each unit shows up
- bar / psi — automotive tyres (about 2.2–2.5 bar or 32–36 psi) and pneumatic tooling.
- MPa — hydraulic systems, injection moulding and high-pressure process lines, often 10–35 MPa.
- kPa / mbar — HVAC duct static pressure and weather maps (standard atmosphere ≈ 101.325 kPa ≈ 1013.25 mbar).
- mmHg / atm — blood pressure (≈ 120/80 mmHg), scuba depth and laboratory vacuum.
Conversion gotchas
Always clarify whether a reading is gauge (relative to ambient) or absolute; adding atmospheric pressure changes the number by about 1 bar. Note that psi means pound-force per square inch, not pound of mass. And although mmHg and torr are often used interchangeably, they differ by about 1.4 parts in 107 — negligible for everyday work. Two reference values worth memorising: 1 MPa = 10 bar and 1 bar ≈ 14.5 psi, and 1 atm = 760 mmHg.