In principle, you should know the cost basis of the stocks you owned, as you became the owner by acquisition, inheritance or gift.
If you do not know or remember the cost basis, you should do some research to find it or make an educated guess for it, otherwise the whole proceeds will be taxable as capital gains.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
