This year's contribution cannot be converted in isolation from any existing traditional (including rollover) IRA(s). It's best explained by example. Let's say you have a $95,000 balance in all your existing traditional IRAs and that balance consist of $45,000 in deductible contributions, $10,000 in previous non-deductible contributions and $40,000 in earnings (interest, dividends & capital gains). This year you make a $5000 non-deductible contribution and convert $5000 to a Roth. Only 15% of the $5000 conversion ($750) will be tax free. Your basis, in all your IRAs, is $15,000 (the previous $10,000 of non-deductible contributions plus this year's $5000 contribution). TurboTax will divide that $15,000 basis by the $100,000 balance ($95K+5K) to arrive at the 15% tax free ratio. This is the way the IRS requires it to be done. The calculations will be shown on form 8606.
See: http://www.bankrate.com/finance/retirement/drawback-one-type-roth-conversion.aspx
http://thefinancebuff.com/how-to-report-backdoor-roth-in-turbotax.html
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-enter-a-backdoor-roth-ira-convers...
https://ttlc.intuit.com/community/retirement/discussion/back-door-roth-ira/01/2552058#M168628
