<P> 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 . </P> <P> 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; similarly, the pound - force per square inch (psi) 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 atmosphere (atm) is equal to this pressure, and the torr is defined as ​ ⁄ 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 . </P> <P> Pressure is the amount of force applied perpendicular to the surface of an object per unit area . The symbol for it is p or P . The IUPAC recommendation for pressure is a lower - case p . However, upper - case P is widely used . The usage of P vs p depends upon the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum, and on writing style . </P> <Table> <Tr> <Th_colspan="2"> Conjugate variables of thermodynamics </Th> </Tr> <Tr> <Td> Pressure </Td> <Td> Volume </Td> </Tr> <Tr> <Td> (Stress) </Td> <Td> (Strain) </Td> </Tr> <Tr> <Td> Temperature </Td> <Td> Entropy </Td> </Tr> <Tr> <Td> Chemical potential </Td> <Td> Particle number </Td> </Tr> </Table>

Force per unit area is a measure of