Some of RMD was after tax contribution. How to calculate taxable portion of RMD?

[dmertz]

From what type of retirement account did you receive the RMD?

What is the code in box 7 of the Form 1099-R?

Is the IRA/SEP/SIMPLE box marked?

[ThomasM125]

Typically, the distribution will be reported on a form 1099-R and box 2 will often show the taxable amount. In that case the difference between box 1 and box 2 is the untaxed amount. Also, often the codes in box 7 will indicate the taxability of the distribution, as mentioned by @dmertz .

[dmertz]

In the case of a distribution from a traditional IRA when you have basis in nondeductible contributions, boxes 1 and 2a will both show the same amount and the taxable amount will be calculated on Form 8606.