Calculate the total acquisition cost of all shares of a particular stock purchased. It is important to consider that you may have purchased the stock at different prices. For example, if you purchased 2,000 shares at $14, 3,000 shares at $16 and 1,000 shares at $20, you would calculate the total cost of all shares as follows [(2000*14)+(3000*16)+(1000*20)] = (28,000+48,000+20,000) = $96,000.

Calculate the total number of shares purchased. Continuing the same example, you would add (2000+3000+1000)= 6,000 shares.

Divide the total acquisition cost divided by the total quantity of stock purchased. Continuing the same example, you'd divide $96,000 by 6,000. This calculation results in an average share price of $16 per share.