Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface. Mathematically, the formula is Pressure = Force / Area.

The pascal (Pa) is the SI derived unit of pressure or stress. It is equivalent to one newton per square meter. Non-SI pressure units are pound per square inch (psi) and bar.

Air pressure is the force on you by the weight of tiny particles of air. Barometers are used to measure the air pressure in inches of mercury or in millibars. Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The air in your body balances outside pressure so you don't feel it.