How do you calculate cost basis for Unknown transactions on the 1099-B?

Your financial institution may report transactions where one or more data elements are unknown by the institution and cannot be reported.

In such a case, you have to determine the cost basis from your own records.  If you purchased the investment, the cost basis what you paid for it.  

If you inherited the investment, the cost basis is the market value on the date of death of the person that you inherited it from.

If you received the investment as a gift, your basis is the donor's basis.

In some cases, you may be able to research these items by using a tool returned under a Google search for 'historical stock price lookup tool'.

Post again if you have specific questions that are not addressed here.

See also this TurboTax Help.