Bitcoin is considered property by the IRS and not currency. When you transfer bitcoin the difference in the fair market value at the time of transfer versus the purchase price or fair market value at time of acquisition is reported on schedule D as a gain or loss. IRS Notice 2014-21 answers many common bitcoin tax questions. Coinbase which is a large US exchange provides customers with a capital gain/loss calculator and has proposed in their reporting dispute with the IRS to begin 1099-B reporting to customers.