Calculating IRA to Roth conversion non-deductible contributions from previous tax returns

[Matt18]

I have a rollover IRA that consist of IRA deductible and non-deductible contributions and rollover from three different 401k plans.  I need to calculate the IRA contributions that were non-deductible for the Roth conversion.  Besides looking for 8606 are there other was to find the non-deductible contributions from my previous tax returns?

On a couple tax returns, I see a line 16 on 1040 labeled "rollover" with the 16a being the rollover amount. In one year, there's an amount in 16b. In another year, 16b is blank.   Are rollover amounts, deductible or non-deductible contributions?

[dmertz]

After-tax basis rolled over from 401(k) plans needs to be reported as an adjustment to your basis in nondeductible traditional IRA contribution on the first Form 8606 Part I that you are required to file after the rollover.  The pre-tax money when rolled over to the traditional IRA remains pre-tax.

Your Forms 1099-R reporting the rollovers from the 401(k)s to your traditional IRAs might have reported the after-tax portion in box 5.

 In TurboTax, you report an adjustment to basis by indicating that you made nondeductible traditional IRA contributions, clicking the EasyGuide button, then marking the box to indicate that you rolled over money from an employer plan and entering the amount of after-tax money rolled over to the traditional IRA.

[Matt18]

@dmertz Thanks for the answer.  I don't have an 8606 for any of my 401k rollovers, but for one rollover I see an amount in the 1099-R box5.  In the other ones, box 5 is empty.  Is there another way to calculate the after-tax contributions to my 401k?

Am I allowed to calculate it based on my records? Or do I need to base it on the 1099-R?  

[dmertz]

You would not have any Forms 8606 showing an adjustment of basis if you have had no reason to file Form 8606 Part I (for a new nondeductible contribution made to a traditional IRA or for a distribution or Roth conversion from your IRAs).

Determining your after tax basis would have to come from you records, either from box 5 of Forms 1099-R as previously mentioned or from statements from your 401(k)s.