Will take first RMD this Dec. 2018 at age 70 1/2 from Rollover IRA (rolled over from employer 401k that had BOTH before and AFTER tax contributions).

[bigmosickle]

The 401k rollover was done before 2009 when I left this employer.  Assume that the Rollover IRA value was $100,000 on Dec. 31, 2017 and that the total after tax contributions were $10,000, or 10% of the total IRA.  I have no other accounts requiring an RMD.  How do I handle the 10% after tax contributions (my cost basis?)?  Obviously I'm clear that the December 2018 RMD is 100,000 / 27.4 = $3649.64 . . . but is only 90% of it taxable which equals $3284.68?  Thank you.

[macuser_22]

When you enter the 1099-R for the distribution the followup questions will ask about prior non-deductible "basis" in the account and then the total 2018 year end value of all Traditional, SEP and SIMPLE IRA accounts that existed.

The taxable amount of the 2018 distribution will be calculated by pro-rating the non-deducible basis between the 2018 distribution and the remaining total year end value on lines 6-15 on a 8606 form with the remaining carry forward basis on line 14 to be used for future distributions. 

[dmertz]

After indicating that you have nondeductible contributions to your traditional IRAs, click the EasyGuide button and mark the checkbox to indicate that you transferred money from an employer's retirement plan, then enter the amount of basis that rolled over from the 401(k) ($10,000 in your example).