There are numerous units of measurement for pressure. Since pressure is defined as a force per area, the United States commonly uses units of pounds per square inch (PSI), as well as pounds per square foot (PSF). In Europe and Japan, the metric system uses Kilograms per square centimeter (Kg/cm2 ).
Pressure can also be stated in terms of the height of a liquid column. If one pound of water were poured into a glass tube with an area of one square inch, the weight of the water on that area at the bottom of the glass tube is one pound, and the pressure is therefore one PSI. At 39° F, the water column would be 27.68 inches tall. One inch of water column is annotated as 1"WC.
If we replace the water with a heavier liquid, the pressure generated increases. For example, it only takes 2.036 inches of mercury to generate 1 PSI versus 27.68" water column because mercury is so heavy. 1 PSI is equal to 2.036"Hg.
In Europe the metric system is prevalent, so inches are replaced with millimeters. 25.4 mmHg is equal to 1"Hg.
Evangelista Torricelli did a lot of the early work in pressure measurement and invented the barometer. 1 mmHg has been renamed Torr in his honor. The Pascal is named after Blaise Pascal, another early mathematician who discovered that air pressure decreases with altitude and that fluid pressure is the same in all directions.
Other pressure units of measure are the Atmosphere and the Bar, which are both roughly equivalent to atmospheric pressure at sea level on a "standard" day.
Here is a list of the more common units of pressure measurement.
Units of Pressure Measurement
Referred to 1 PSI
1 PSI=27.68"WC (inches of water column)
1 PSI=2.036"Hg (inches of mercury)
1 PSI=51.715 mmHg or Torr
1 PSI=0.068947 Bar
1 PSI=0.06804 Atmospheres (Note that 1 Bar is not exactly 1 atmosphere)
1 PSI=6.8947 KiloPascals or KPa
1 PSI=0.0703 Kg/cm2
1 PSI=2.307 feet of water
