Yes- This is confusing. It looks like the two agencies have different definitions for MAGI for different purposes.
Even TurboTax lists "non-taxable Social Security" to be included in MAGI
https://turbotax.intuit.com/tax-tips/irs-tax-return/what-is-the-difference-between-agi-and-magi-on-y...
For Social Security IRMAA purposes, I believe this can be misleading- (just my opinion).
I believe for SS IRMAA purposes, my 15% non-taxed amount is NOT to be added back in (See SSA doc reference below).
But I am not going to risk it - I will add that 15% back in as a safety margin when calculating how much Roth conversions I can do.
Of course standard disclaimer applies- rely on a tax professional.
FYI- Copied from the SS Handbook at their site:
https://www.ssa.gov/OP_Home/handbook/handbook.25/handbook-2501.html
What is Modified Adjusted Gross Income (MAGI)?
Modified Adjusted Gross Income is the sum of:
The beneficiary's adjusted gross income (AGI) (last line of page 1 of the IRS Form 1040 (U.S. Individual Income Tax Return)), plus
Tax-exempt interest income (line 8b of IRS Form 1040)
2501.1How Is MAGI Used?MAGI is used to determine if an Income-Related Monthly Adjustment Amount (IRMAA) applies. It is provided by IRS and is generally information that is two years prior (but not more than 3 years prior) to the year for which the premium is being determined. We will use the appropriate sliding scale table (�2503) to determine the IRMAA.
