The amount in box 2a of a Form 1099-R reporting a regular distribution from an IRA is meaningless, indicated by box 2b Taxable amount not determined being marked. For a traditional IRA distribution, if you have indicated that there is no basis in nondeductible traditional IRA contributions, TurboTax knows that the amount in box 1 is to be used as the taxable amount. If you have indicated that you do have basis in nondeductible traditional IRA contributions, TurboTax knows to determine the taxable amount by preparing Form 1099-R Part I using the amount in box 1, the amount of your basis and the year-end value in your traditional IRAs.
Review the Form 8606 prepared by TurboTax to see the calculation of the taxable amount.
