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where
p is the pressure
F is the force
A is the area
Often F is taken to be the of the magnitude of the mean vector force normal to the surface of area A upon which it exerts; the "surface" not necessarily being a that of a body, but for example the cross sectional area of a conduit.
"Pressure is a scalar quantity, but teachers and authors do not appear to believe this in their hearts." (McClelland, 1987)
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·s-2·m-1).
In the above equation:
p is the pressure, measured in pascals
F is the force, measured in newtons
A is the area, measured in square metres
Non-SI measures (still in use in some parts of the world) include the pound-force per square inch ( PSI) and the bar.
The cgs unit of pressure is barye (ba). It is equal to 1 dyn·cm-2.
In the United States air pressure is still measured in inHg — inches of mercury (as in the mercury barometerA barometer is an instrument used to measure atmospheric pressure. A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjust). Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, because it gives the same numbers as the older millibar (mbar).
Pressure is sometimes measured not as an absolute pressure, but as the excess of that pressure above atmospheric pressureAtmospheric pressure is the pressure caused by the weight of air above any area in the Earth's atmosphere. Standard atmospheric pressure atm is discussed in the next section. Air masses are affected by the general atmospheric pressure within the mass, cre, sometimes called gauge pressure. An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure. In technical work, this would be written as "30 PSIG" or, more commonly, "30 psig".
The standard atmosphere (atm) is a curious unitThe word unit means any of several things: One, the first natural number. The natural or usual or smallest measure of something, of which there are multiples and of which there may be fractions. In most physical contexts, for example science and engineeri of pressure, defined to six figures of precision to approximate a reality that varies constantly from place to place and moment to moment. It is approximately equal to typical air pressures at sea level and defined to be
Obsolete manometric units of pressure such as inches or millimeters of mercury are based on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity. They are effectively attempts to define a unit for expressing the readings of a manometer.
Manometric pressure units should no longer be used for scientific or engineering purposes, due to the lack of repeatability inherent in their definitions.