Determine the initial cost basis of your investment by calculating the original amount of money you spent to purchase the asset or security. For instance, suppose you purchased 250 shares of a company's stock at $4 per share. The cost basis for that investment would be $1,000.
Track any adjustments you make to your initial investment. For instance, suppose that two months later, you buy 100 shares of the same stock for $5 per share.
Calculate your adjusted basis by factoring in the change to your investment. For example, if you had bought 100 more shares of stock at $5 per share, your adjusted basis would now be $1,500. That is, you add the $1,000 you originally spent to the $500 you spent when you bought more shares.