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Pressure means how much something is pushing on something else. It is expressed as force per unit area: P=F/A

In technology, pressure is often specified in multiples of atmospheric pressure.

It can also be defined as the thrust [compressive force acting perpendicularly to the surface of a body] acting per unit area.

Since pressure depends on both force and area, the same amount of force applied by a hand will make less pressure than when applied by something smaller, like a finger.

Pressure is also related to density. By the ideal gas law, for example, doubling the pressure can put twice as much gas in a bottle.

To measure the amount of pressure people use these units:

  • Pounds per square inch - (psi, PSI, lb/in2, lb/sq in). This U.S. customary units is not elsewhere. Normal atmospheric pressure is 14.7 psi, which means that a column of air one square inch in area rising from the Earth's atmosphere to space weighs 14.7 pounds.
  • Atmosphere - atm. Normal atmospheric pressure is called 1 atmosphere. 1 atm = 14.6956 psi = 760 torr.
  • Bar - bar. The bar is almost the same as the atmosphere unit. One bar = 750.062 torr = 0.9869 atm = 100,000 Pa.
  • Millibar - mb or mbar. There are 1,000 millibar in one bar. This unit is used by meteorologists who find it easier to refer to atmospheric pressures without using decimals. One millibar = 0.001 bar = 0.750 torr = 100 Pa.
  • Pascal - Pa is the SI unit of pressure. It was named after the French physicist Blaise Pascal. 1 pascal = a force of 1 newton per square meter (1 newton = the force needed to accelerate 1 kilogram one meter per second per second = 1 kg.m/s2; this is actually quite logical for physicists and engineers, honest). 1 pascal = 10 dyne/cm2 = 0.01 mbar. 1 atm = 101,325 Pascals = 760 mm Hg = 760 torr = 14.7 psi.

Pressure is directly proportionate to force and inversely proportionate to area.

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