A square inch (plural: square inches) is a unit of area, equal to the area of a square with sides of one inch. The square inch is a common unit of measurement in the United States and the United Kingdom. Pounds per square inch (psi), a unit of pressure, are derived from this unit of area.
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Square inch in the context of Pressure
Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure.
Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre (N/m); similarly, the pound-force per square inch (psi, symbol lbf/in) is the traditional unit of pressure in the imperial and US customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the unit atmosphere (atm) is equal to this pressure, and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.
Square inch in the context of Pounds per square inch
The pound per square inch (abbreviation: psi) or, more accurately, pound-force per square inch (symbol: lbf/in), is a unit of measurement of pressure or of stress based on avoirdupois units and used primarily in the United States. It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately 6.895 kilopascals.
The pound per square inch absolute (psia) is used to make it clear that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi (101 kilopascals), this will be added to any pressure reading made in air at sea level. The converse is pound per square inch gauge (psig), indicating that the pressure is relative to atmospheric pressure. For example, a bicycle tire pumped up to 65 psig in a local atmospheric pressure at sea level (14.7 psi) will have a pressure of 79.7 psia (14.7 psi + 65 psi). When gauge pressure is referenced to something other than ambient atmospheric pressure, then the unit is pound per square inch differential (psid).