How does a Roth conversion affect MAGI for IRMAA determination in 2025?

1. Roth conversion = taxable income that becomes MAGI two years later

A Roth conversion is taxed in the year you convert and that taxable amount increases your MAGI for that year; Medicare’s IRMAA uses MAGI from two years before the premium year, so a conversion today will change your IRMAA exposure two years hence ^{[2] [1]}. Multiple planning pieces from advisors and law firms state the same mechanics: conversions count toward MAGI and therefore count toward IRMAA two years later ^{[5] [6]}.

2. IRMAA is a cliff system — one dollar can move you into a much higher premium

IRMAA tiers are “all‑or‑nothing”: crossing a threshold by even one dollar exposes you to the full higher surcharge for Part B and Part D. That cliff behavior makes the interaction with lump‑sum Roth conversions risky: a relatively modest conversion can push you into the next tier and multiply your annual Medicare cost by hundreds or even thousands of dollars ^{[3] [6]}.

3. How big the harm can be — concrete costs and brackets for 2025

Public reporting and calculators for 2025 show a wide range of extra costs. For 2025, one summary lists Part B IRMAA additions that push annual Part B costs by between about $888 and $5,326.80 depending on how far over the threshold you are; Part D surcharges add hundreds more ^[3]. Another summary published in consumer media gives specific monthly surcharge numbers for higher MAGI ranges for 2025 (e.g., specified monthly IRMAA increments for Part B in higher brackets) ^[4].

4. Practical planning responses advisors recommend

Common planning strategies in the coverage include: (a) stagger conversions over several years to avoid a single‑year MAGI spike, (b) perform conversions in lower‑income years, (c) use qualified charitable distributions (QCDs) or other deductions in conversion years to reduce MAGI, and (d) model the impact before executing large conversions ^{[7] [8] [9]}. Financial firms and articles explicitly recommend scenario modeling because the interaction of tax brackets, phaseouts (SALT, QBI), and IRMAA can produce surprise effective tax rates and premium changes ^{[5] [10]}.

5. Tradeoffs: lock in tax-free growth vs. temporary IRMAA pain

Several pieces stress the fundamental tradeoff: Roth conversions remove future RMDs and tax exposure (and Roth distributions later do not count toward MAGI), which can reduce IRMAA risk in later years — but that benefit comes at the cost of higher MAGI (and potentially IRMAA) in the conversion year that feeds the two‑year lookback ^{[6] [11]}. Analysts show examples where large conversions raise marginal tax and IRMAA exposure materially, and they propose splitting conversions to reduce peak MAGI ^{[12] [10]}.

6. Where reporting disagrees or leaves gaps

Coverage consistently agrees on mechanics (conversion = taxable MAGI that affects IRMAA two years later) and on cliff behavior; what varies is the recommended numerical thresholds and how aggressive to be. Some blogs project 2025 MFJ first tiers differently depending on inflation assumptions and rounding ^{[8] [13]}. Available sources do not provide an exhaustive, IRS‑issued 2025 MAGI table in this packet; consumer outlets give bracket examples and surcharge numbers but can vary in presentation and assumptions ^{[4] [3]}. For precise, personal decisions you must run your own numbers or consult a tax pro.

7. Bottom line for someone deciding in 2025

If you convert in 2025 expect that increased MAGI to be visible to Medicare in 2027 and potentially raise your Part B/Part D premiums; the magnitude can be modest or large depending on how far the conversion pushes you across IRMAA thresholds ^{[5] [2]}. The prudent approach shown across sources: model the conversion’s effect on your 2‑year‑lookback MAGI, consider spreading conversions or using MAGI‑reducing tactics in the conversion year, and weigh the long‑term Roth benefits (no RMDs, tax‑free withdrawals not counted for MAGI) against short‑term IRMAA costs ^{[7] [6] [3]}.