IRA Balance impact on taxes due

[ForeignGal]

MFJ, Retired, Age 57, I did an tIRA conversion to Roth. I know I have to pay taxes on that. However, when I put my total IRA balance in the IRA questionnaire in Turbotax online, my tax liability goes up substantially. I have read about pro rata tax but I am still confused why this happens. If I remove the total IRA balance entry, tax drops down. Can someone please explain simply.

[dmertz]

Until you enter your year-end balance, TurboTax produces an incorrectly low taxable amount for the Roth conversion by assuming that your year-end balance is zero and applying too much of your basis in nondeductible traditional IRA contributions.  Once you enter the year-end value, TurboTax corrects the original calculation to produce the correct, higher taxable amount of the conversion by applying less of your basis.

[ForeignGal]

but then why my partner's ( age 60) entry of IRA balance does not change the tax? Why only mine?

[dmertz]

Apparently your partner has no basis in nondeductible traditional IRA contributions, so Form 8606 is not involved in determining a nontaxable portion of the distribution.  The entire distribution is taxable no matter the year-end balance.

[ForeignGal]

THANK YOU.