I have sold some stock that was given as a gift and need to determine the capital gains. How do I calculate the capital gain?

[Bsch4477]

Cost basis is that of the donor when you received it. 

[Hal_Al]

Your capital gain is the difference in what you sold it for and your cost basis. The cost basis in a gift is the giver's original cost basis; usually what he paid for it**.

Unknown basis. You have to report the sale on your tax return. Lacking any cost basis, the IRS will consider the entire sale amount as taxable. So, you need to make your best effort to determine the original cost basis, even if (worst case) it's a guess. Historical prices of publicly traded stocks are  readily available on the internet and should, at least, satisfy the IRS that your basis wasn't zero; just "google" “Historical Stock Prices". I use http://bigcharts.marketwatch.com/historical/. The stockholder relations dept. at the company may be able to help.

**There is an exception. If using the givers's original cost basis results in a capital loss, for you; you must use the fair market value of the stock, on the date of the gift, instead.