Question

 How to derive a formula for Pressure?

Answer 1

If you mean: What is the formula for pressure?
Then the answer to that would be: Pressure = Force/Area
The Force is applied by the body and the Area is the total area of the object.

The Pressure Formula is given by Pressure = Density x Gravity x Height

The Pressure Formula is used to calculate pressure, force, area, density, height and gravity if some of these quantities are given. Pressure is expressed in Pascal (Pa).

Answer 2

We already know that the Pressure is defined as Force acting on per unit area.
P = Force/A
We should also know that the Thrust is a type of force.
Pressure for Fluids.
We have to find the volume of the column of fluid covering by the cylindrical shape.
V = Area of cross section x height
V = A x h     .....(1)
Thrust on the cylindrical column = Weight of the fluid
= m x g
= (V x d) x g             (since, m = V x d)
Since Thrust = (A x h) x d x g          [from equation (1)]
But, Pressure = Thrust/Area
So, P = Ahdg/A = hdg
Therefore Pressure due to a fluid column = P = hdg.