Pressure is defined as the force per unit area acting on a surface in a direction perpendicular to that surface and is denoted as P. Scientifically P is equal to F / A, where P is the pressure, F is the normal force and A is the area. Pressure is transformed to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a basic parameter in thermodynamics and it is conjugate to volume. Unlike stress, pressure is defined as a scalar quantity. The gradient of pressure is called the force density.

For gases, pressure is at times measured, not as an absolute pressure, but relative to atmospheric pressure. Such measurements are sometimes called gauge pressure. A paradigm of this is the air pressure in an automobile tire, which may be said to be 220 kpa, but is really 220 kpa over atmospheric pressure.

Because atmospheric pressure at sea level is about 100 kpa, the absolute pressure in the tire is consequently about 320 kpa. In scientific work, this is written a gauge pressure of 220 kpa. Where space is restricted, such as on pressure gauges, graph labels and table headings, the use of a modifier in parentheses, such as kpa gauge, is allowed. In non-SI scientific work, a gauge pressure is sometimes written as 32 psig, although the other techniques clarified above that avoid connecting characters to the unit of pressure are chosen.

Though pressures are usually positive, there are numerous conditions in which a negative pressure can be encountered. The pressure due to the weight of a fluid is known as Hydrostatic pressure. P is equal to gh where is the density of the fluid which is the practical density of fresh water is equal to 1000 kg/m3, g is the acceleration due to gravity about 9.81 m/s2 on Earth’s surface and h is the height of the fluid column in meters or feet. The pressure a fluid applies when it is forced to stop moving is called as Stagnation pressure.