For equity shares purchased before 1st February 2018, the cost of acquisition is calculated through the following steps –
Fair market value of an investment is calculated by multiplying the number of purchased shares with their highest price, as on 31st January 2018.
The lesser value between the fair market value and the actual sale value of the investment is chosen.
It is then compared with purchase value of the share, and the higher value between the two is chosen. This value is the cost of acquisition of the asset.