It depends on how your Form 1099-R is reported and how much nondeductible contributions you have made.
If you have made any nondeductible contributions to your traditional IRA, you do not pay taxes on the entire conversion amount. Those nondeductible amounts are so-called the basis. To see how this amount is calculated, I suggest you review your Form 8606 Nondeductible IRAs.
For more information about the Form 8606, click here Nondeductible Contribution
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