What is the cost basis of shares I bought in the United States?

The IRS says the basis of stocks or bonds you buy is generally the purchase price plus costs of purchase, such as commissions and recording or transfer fees. If you can identify the shares you sold, their basis is the cost of those particular shares. If you got the shares other than by purchase, the basis is usually the fair market value, or the previous owner's adjusted basis. You adjust the basis for certain events after the purchase. This page does not work out a gain or a tax bill.

Continue on IRS · Publication 551