Yes, you do need to report it separately and to calculate the cost basis from the tax included on your W-2. You will be told the amount already taxed on your W-2 in box 12 or 14. The cost basis for this sale is the price used to calculate your tax on the W-2. For example, if you were taxed at $15 a share on the W-2, then you would determine your cost basis by multiplying $15 a share by the number of shares sold, plus any commission charged on the sale charged by the broker.