Pressure is defined as force per unit area. Pressure above atmospheric is mostly measured in pounds per square inch (PSI), or in kilo gram per square centimeter (kg/cm2) gauge or absolute. For lower pressure (below about 10 psi), inches or cms. of water or mercury column are frequently used. For near atmospheric pressure, we refer more commonly to millimeters of mercury column (or torr).

The SI unit of pressure is Newton per square meter, or PASCAL (Pa). One atmospheric pressure (atm) is equal to 101325 Pa or 101.325 Kpa. More recently, bar is also being used for higher ranges. A bar is 0.987 atm, or approximately 14.5 PSI. A conversion table for more commonly used pressure units is given here.
Pressure Conversion Table:
From                atmospheric     Psi                   Bar                  mmHg (torr):
Atmospheric    1.000               14.696             1.0133             760.00
(1.033 kg/cm2)
Psi                   0.068               1.000               0.0689             51.50
Bar                  0.987               14.504             1.000               750.06
Kg/         0.9678             14.223             0.9807             735.56
MmHg(torr)    0.0013             0.019               0.013               1.00
Mm(WC)         0.0001             0.0014             0.0001             0.0736HgHJH
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