Decoding the Two-Year Lookback Rule for 2027 Premiums

The Social Security Administration (SSA) determines your Medicare Part B and Part D premiums using tax data provided directly by the Internal Revenue Service. However, tax filings lag behind the current operational year. To set your 2027 premiums, Medicare looks back to your most recently processed tax return, which is your 2025 tax filing submitted in early 2026.

This structural gap means decisions you make throughout 2025 regarding Roth conversions, real estate sales, required minimum distributions (RMDs), or business liquidations directly dictate your healthcare overhead in 2027. The Centers for Medicare & Medicaid Services (CMS) will officially publish finalized 2027 IRMAA brackets and base premiums in November 2026; however, waiting until that announcement leaves you zero flexibility to alter your 2025 tax numbers.

To evaluate your potential exposure, you must calculate your Modified Adjusted Gross Income as defined specifically for Medicare. For IRMAA purposes, the IRS formula adds tax-exempt municipal bond interest and untaxed foreign earned income back to your standard Adjusted Gross Income (AGI):

IRMAA MAGI = Adjusted Gross Income (AGI) + Tax-Exempt Interest Income + Foreign Earned Income

Many retirees discover too late that conservative investments like municipal bonds—while free from federal income tax—still count toward your Medicare surcharge threshold. Monitoring this specific MAGI total before December 31, 2025, remains the only reliable method to prevent unwanted Medicare price hikes in 2027.