What is the formula for calculating elasticity?

The basic idea of elasticity is how a percentage change in one variable causes a percentage change in another variable.

$$\text{Elasticity of variable 1 with respect to variable 2}=\dfrac{\text{\% change in variable 1}}{\text{\% change in variable 2}}$$

Midpoint Method is used to calculate point elasticity between two points whether there is a variable increase or decrease. This formula uses the same base for both cases.