How are taxes paid on a IRA to Roth IRA rollover

I have some a good amount of money saved up in a traditional IRA and wanted to wait until I got married (joint filing with a low income spouse) to roll it over into a Roth IRA, which I will be doing this year. My question is how are the taxes paid on a IRA to Roth conversion; is it paid out of the balance being rolled over, or do I need to pay the hefty tax bill with post tax earnings?