Midpoint formula

The elasticity $\epsilon$ can be calculated using the midpoint formula:

$\epsilon = 100*\frac{\frac{Q_1 - Q_0}{\bar{Q}}}{ \frac{P_1 - P_0}{\bar{P}}}$

where $X_0$ is the initial quantity, $X_1$ is the new quantity, $\bar{X}$ is the average quantity, $P_1$ is the new price, $P_0$ is the initial price and $\bar{P}$ is the average price.

The average quantity is calculated:

$\bar{Q} = \frac{Q_1 + Q_0}{2}$

The average price is also calculated:

$\bar{P} = \frac{P_1 + P_0}{2}$