Pressure is force per unit area:

P

=

F A

Place a brick on the floor with its largest-area face down. If you then stand it up on its end, it exerts the same force on the floor but a much larger pressure, because the contact area is less.

The force exerted on an object by a fluid is toward the object and perpendicular to its surface. At a microscopic level, the force is associated with the atoms and molecules in the fluid colliding elastically with the object.

The SI unit for pressure is the pascal.

1 Pa = 1 N/m²

There are plenty of other pressure units. Converting from one unit to another can be done using the following relationships:

1 atm = 1.01 x 10⁵ Pa = 14.7 lb/in² = 760 torr

pounds / square inch = lb/in² is also abbreviated as psi.

1 torr = 1 millimeter of mercury = 1 mm Hg.