Pound per square inch

The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in2) is a unit of pressure in the U.S. customary units. It is the pressure exerted by a force of one pound-force applied to an area of one square inch.

1 psi approximately equals 6,894.757 Pa, where pascal (Pa) is the SI unit of pressure.

Absolute pressure versus gauge pressure
Bourdon tube pressure gauges, vehicle tire gauges and many other types of pressure gauges are zero referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure. However, absolute pressures are zero referenced to a complete vacuum. Thus, the absolute pressure of any system is the gauge pressure of the system plus the local atmospheric or ambient pressure.

An example of the difference is between gauge and absolute pressure is the air pressure in a vehicle tire. A tire pressure gauge might read 32 psi (220 kPa) as the gauge pressure, but that means the pressure is 32 psi (220 kPa) above atmospheric pressure. Since atmospheric pressure at sea level is about 14.7 psi (101) kPa, the absolute pressure in the tire is therefore about 46.7 psi (321 kPa).

In the U.S. cutomary units, gauge pressure and absolute pressure are very commonly abbreviated as psig and psia respectively. In the above example, the tire pressure would commonly be written as 32 psig or 46.7 psia.

In technical writing, using the SI metric system of units, the use of kPa(g) or kPa(a) is not recommended. Instead, for the example above, it is recommended to write a gauge pressure of 220 kPa or an absolute pressure of 321 kPa. Where space is limited, such as on pressure gauge dials, table headings or graph labels, the use of a modifier in parentheses, such as kPa (gauge) or kPa (absolute), is strongly encouraged.