What is PSI?

PSI (or P.S.I, psi or p.s.i) is an acroynm for pounds per square inch, a measurement of pressure that is often used in automotive circles. The metric equivalent for psi is the Pascal, although most psi measurements equate better to kiloPascals (kPa). Other units for pressure include the bar and the atmosphere (atm). For the mathematically minded, 1 bar = 14.504 psi = 100 kPa = 0.9869 atm.

'Hg' is the chemical symbol for mercury. It is sometimes used as an incorrect version of inches of mercury (inHG) or millimeters of mercury (mmHG), the unit of measurement for vacuum. This unit was derived from the technique of measuring vacuum by pulling mercury up a glass tube. Vacuum, which is simply negative pressure, is also commonly measured in bars, inches of water (inH2O), and Pascals (Pa). In this case, 1 bar = 29.53 inHg = 401.463 inH2O.