Yes, in order to figure out the non-taxable portion of a Traditional IRA distribution, you need to know the amount of non-deductible contributions made.
The ratios are meaningless, the amount that is nontaxable is calculated on Form 8606 each year, based upon the amount of IRA basis that you have.
Your IRA basis is the amount of non-deductible contributions that you made, less any amount you have already distributed tax free.
In the context of an IRA, pre-tax and post-tax are confusing, unless this is an employer plan. If your plan is from an employer, you may have a post-tax amount and they will need to share that with you.
In the much more common scenario where you open an IRA at your bank or broker and contribute to it with your post tax dollars, the only real question is did you deduct your contribution.
If you contributed and did not deduct your contribution and you left the amount in your IRA, you file Form 8606 and report to the IRS that you made a non-deductible contribution.
When you start taking distributions, you figure the amount that is not taxable using Form 8606.
TurboTax addresses this issue very clearly in the IRA section, allowing you to enter non-deductible contributions made in the past. Here is how to find this in TurboTax:
- Select Federal from the left side menu.
- Select Deductions and Credits.
- Expand the list and scroll down to Retirement and Investments.
- Expand the section and click Start to the right of Traditional and Roth IRA Contributions.
- Check the type of IRA accounts you have and then answer the questions about contributions.
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