Common units for pressure include the Pascal, atmosphere, bar, Torr, millimeters of mercury and pounds per square inch. These are abbreviated as Pa, atm, bar, Torr, mmHg, and psi, respectively.

Pressure is force applied over a given area. This means that as the area the force is pushing against increases, the resulting pressure decreases. Because pressure is the division of force by area, units of pressure are derived from units of force and area. Area is derived from units of length. The International System of Units, SI, uses the Pascal as its standard unit for pressure. However, in chemistry, it is not uncommon to find problems involving atmospheres, millimeters of mercury or, more rarely, the Torr.

The Pascal is defined as one Newton, the SI unit of force, per square meter. The Pascal is named after the famous French mathematician Blaise Pascal. Pascal's experiments with pressure led him to develop Pascal's law, which states that an increase in pressure at any point in a fluid-filled container results in an equal increase in pressure at every other point in the confined space. Engineers design hydraulics with Pascal's law in mind to transfer forces for brakes or various other pieces of machinery.