IC 50 is calculated with a software known as graph pad prism. Though initially people use graph to extrapolate it from a graph of concentration against % inhibition.
You need to calculate the scavenging activity of different concentrations of your extract (5-7 levels). Then you plot the scavenging activity results against the concentration of the extract. In the equation of the linear regression curve obtained, you replace y-value (the scavenging activity) by 50% (50) and you will obtain the IC50 value (x-value).