Once your taxable income - which includes both short term and long term capital gains - breaks through the top of the last tax bracket that taxes long term capital gains at 0%, only the amount of long term capital gain that's put you into the next tax bracket gets taxed at the 15% rate. So if your taxable income including your LTCG put you $1 into one of the tax brackets where LTCG get taxed at 15%, only $1 of the LTCG would get taxed, no matter what the actual dollar amount of the LTCG.
Tom Young
