Let us consider a small area d A where the force d F is acting. Pressure is defined as the force per unit area.

So p = d f / d A

If the force F is acting uniformly over the area A, then the pressure p is given by

P = F / A

The S.I. unit of pressure is given by

P => N/m^{2} => p_{a} (pascal)

Sometimes bar is used for pressure unit.

1 bar = 10^{5} N/m^{2} = 10^{5} Pa= 100 KP a

and 1 atm = 101.3 KP a

1 torr = 1 mm of Hg.