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/m2 => pa (pascal)
Sometimes bar is used for pressure unit.
1 bar = 105 N/m2 = 105 Pa= 100 KP a
and 1 atm = 101.3 KP a
1 torr = 1 mm of Hg.