I have both deductible and nondeductible retirement accounts. I did a Roth conversion from my pretax account. TurboTax asks my cost basis for my nondeductible account even though the entire conversion was from my pretax account. It then calculated that part of my conversion is not taxable. Is that correct? Why would my nondeductible IRA affect how much of the conversion from my pretax account is taxable?
To confirm you converted funds from your traditional IRA to Roth IRA?
If yes, and you had nondeductible traditional IRA contributions in prior years, reported on Form 8606, then you will enter your basis from line 14 of your last filed Form 8606 during the interview.
If you have more than one traditional/SEP/SIMPLE IRA then they are considered one account for the purpose to calculate the taxable part according to the pro-rata rule. You cannot just only move the taxable part. For each distribution/conversion, the pro-rata rule is applied and therefore each distribution/conversion will have a taxable and non-taxable part as long as you have pre- and after-tax funds in the accounts.
To enter your conversion: