In fluid mechanics, the pressure results from a normal compressive force acting on an area. The pressure ‘P’ is defined as force (F) per unit area (A). In SI units the unit of measurement of pressure is Newtons per square meter (N/m^{2} ) or Pascal (Pa).

Since Pascal is a small unit of measurement of pressure, the pressure is usually referred to in kilo Pascal (kPa) or even in Mega Pascal (M Pa). The standard atmospheric pressure (P_{atm}) at sea level is 101.3 kPa or 1.013 bar.

The gauge pressure (P_{gauge}) is the pressure recorded by the gauge or manometer device. In engineering calculations, absolute pressure is used and the conversion from gauge pressure to absolute pressure is carried out using the following equation.

Absolute pressure = gauge pressure + atmospheric pressure

P_{absolute} = P_{gauge} + P_{atm}

‘**Gauge pressure**‘ is the difference between ‘**absolute pressure**‘ and ‘**atmospheric pressure**‘.

Zero gauge pressure is atmospheric pressure. Also, zero absolute pressure in an ideal vacuum.

Fig. 1 gives the relation between gauge and absolute pressures.

The gauge pressure is negative whenever the absolute pressure is less than atmospheric pressure; it can be called as vacuum.

For negative pressures

_{absolute}= P

_{atm}– P

_{gauge}