Roth conversions can be a powerful tool in retirement planning: they remove money from tax-deferred accounts (401(k), traditional IRA) and put it into tax-free buckets (Roth IRA). That lowers future required minimum distributions (RMDs) and can simplify long-term tax planning. But conversions raise taxable income in the year you do them—and that can trigger Medicare IRMAA surcharges, increase taxes on Social Security benefits, and create short-term tax pain.

This guide walks you through a practical, step-by-step process to execute Roth conversions while minimizing Medicare IRMAA exposure and other near-term tax shock. It covers 401(k) and IRA conversion choices, in-plan Roth options, the interaction with pensions and Social Security, and a timeline you can apply in 2026 and beyond.

Why timing matters: the three short-term consequences

  • Medicare IRMAA: The Social Security Administration (SSA) uses your modified adjusted gross income (MAGI) from two tax years earlier to set Medicare Part B and Part D premiums. Large conversions in Year X can raise premiums in Year X+2.
  • Social Security taxation: Roth conversions increase combined income (AGI + tax-exempt interest + half of Social Security) in your tax return year and can raise the fraction of Social Security that is taxable.
  • Income-tax bill: Conversions are taxable events. You need cash outside retirement accounts to pay the tax or accept reduction in retirement assets.

Core principles before you convert

  1. Inventory accounts and incomes: List all 401(k)s (pre-tax and Roth), traditional IRAs, Roth IRAs, taxable brokerage, pension projections, and expected Social Security start dates and amounts.
  2. Confirm plan mechanics: Does your employer plan allow in‑service rollovers or in‑plan Roth conversions? Do you have after‑tax or pre‑tax balances? Rolling a Roth 401(k) into a Roth IRA eliminates Roth 401(k) RMDs later; leaving money in a Roth 401(k) subjects it to RMDs.
  3. Know RMD timing: RMD rules and the "RMD starting age" can change; check current IRS guidance. Conversions do not count toward an RMD in the year an RMD is required — you must first satisfy the RMD from traditional accounts before converting additional funds that same year.
  4. Model taxes two years out: Because IRMAA is set by the SSA using tax-year data from two years earlier, plan conversions with the X -> X+2 lag in mind.
  5. Arrange cash to pay taxes: Avoid paying conversion taxes with the converted funds unless you are prepared for the reduced retirement principal and potential penalty consequences for younger account owners.

Step-by-step conversion calendar

Step 1 — Two-plus years before you need to control Medicare premiums

Decide the Medicare premium year you want to avoid higher IRMAA surcharges for (call this Year M). Work backward: the SSA will use your tax return from Year M-2. That means conversions you do in Year M-2 matter for premiums in Year M.

Action items:

  • Project your expected MAGI in Year M-2 without conversions (include pension and anticipated taxable Social Security).
  • Identify IRMAA cliff points and surtax bands from the SSA (they are adjusted periodically; check current thresholds). If your projected MAGI falls safely below the first surcharge band, you may have room to convert without triggering IRMAA.
  • If you’re close to a band, plan smaller conversions spread across years to “fill” lower tax brackets without crossing IRMAA boundaries.

Step 2 — One year before (Year M-1): refine and execute partial conversions

Use current-year results and any late changes (pension start, delayed Social Security claim) to fine-tune conversion amounts. Remember: conversions executed in Year M-1 impact Medicare premiums in Year M+1, so decide whether that matters for your timeline.

Action items:

  • Run a tax projection for Year M-1 and Year M (include conversions). Use a tax-preparation tool or tax advisor to estimate the marginal impact on AGI, taxable Social Security, and bracket creep.
  • If you are subject to the Alternative Minimum Tax (AMT) or near a higher bracket threshold, spread conversions across more years.
  • If you have a pre-tax 401(k) and your plan allows in‑service Roth rollovers, compare in-plan Roth conversions vs. rollovers to a Roth IRA. Converting inside the plan keeps funds in the employer environment and may have different distribution rules; rolling to a Roth IRA usually removes future RMDs.

Step 3 — The RMD year: prioritize RMD first, then consider conversions

Once RMDs begin, you cannot use the RMD itself for conversion relief that year — RMDs must be taken from pre-tax accounts before doing conversions. However, you can still do conversions in the same year after satisfying the RMD.

Action items:

  • Take any required RMDs early in the year to preserve flexibility for conversions later that year.
  • If your RMD pushes you into an undesirable tax bracket or IRMAA band in the relevant tax year, shifting conversions to earlier years (before RMDs begin) or to later years (if you can tolerate higher tax now) may be preferable.

Step 4 — Pay taxes strategically

Best practice: use taxable cash or taxable brokerage proceeds to pay conversion taxes. Using retirement funds reduces the tax-free base and may jeopardize long-term goals.

Action items:

  • Make estimated tax payments or adjust withholding to avoid underpayment penalties.
  • Plan conversions in years with low other income (e.g., years before taking Social Security or before a pension kick-in) to maximize use of lower brackets.

Practical tactics and trade-offs

Spread conversions to avoid IRMAA cliffs

Because IRMAA has discrete thresholds, a single large conversion can push you into a surcharge that costs thousands annually. Instead, split a target total conversion into multiple years to stay under key brackets in the SSA lookback year.

Coordinate with Social Security claiming

Social Security filing decisions materially affect MAGI and taxability. If you plan to delay Social Security (to increase future benefit) but need lower MAGI in a certain tax year to avoid IRMAA, you can schedule conversions before you start benefits. Conversely, starting Social Security earlier might make a conversion less attractive in that tax year because it increases taxable Social Security.

Use pension start timing as a lever

Pension pay start dates are often negotiable within a small window. Pushing a pension start from late December to January, for example, shifts taxable income between years and can be used to keep a tax year’s MAGI under an IRMAA threshold.

In-plan Roth vs. Roth IRA rollover

Converting a 401(k) to a Roth 401(k) inside the plan may be simpler, but Roth 401(k)s are subject to RMDs. Rolling a Roth 401(k) to a Roth IRA before RMDs begin removes that RMD requirement. If you’re still employed and plan rules allow, you may be able to delay RMDs from that plan by leaving balances in the employer plan—another planning lever.

Short illustrative examples (numbers are illustrative)

Example A — Staggered conversions to avoid IRMAA

Mrs. Lee will retire in 2027 and expects Medicare in 2029 (she wants to avoid higher Part B premiums in 2029). SSA will use her 2027 tax return to assess IRMAA for 2029. She projects a baseline MAGI of $85,000. The first IRMAA surcharge band starts at about $95,000 in her scenario. Rather than converting $60,000 in 2027 (which would push MAGI to $145,000), she converts $20,000 in each of 2025, 2026, and 2027. Each year’s MAGI remains under the surcharge band in the crucial lookback years, and she reduces future RMDs without triggering higher Medicare premiums.

Example B — Pension timing and a conversion window

Mr. Patel has a small defined-benefit pension and a large traditional IRA. He can start the pension on either December 2026 or January 2027. Because he plans a Roth conversion in 2026, he delays pension payments until January 2027 to keep his 2026 MAGI lower and avoid an IRMAA cliff in 2028. He pays the conversion tax using taxable account proceeds.

Checklist: decisions to make and questions to ask your advisor

  • Which accounts are pre-tax vs Roth vs taxable?
  • Does my 401(k) allow in‑service Roth conversions or in‑plan Roth rollovers?
  • When do my RMDs start (check current IRS guidance)?
  • Which Medicare premium year am I trying to protect and what is its SSA lookback year?
  • How will conversions interact with expected Social Security taxation and pension payments?
  • Do I have cash outside retirement accounts to pay conversion taxes?
  • Have I modeled multiple year scenarios, including worst‑case for IRMAA and marginal tax rate changes?

Common mistakes to avoid

  • Doing a large one‑off conversion without checking the two-year SSA lookback and triggering IRMAA.
  • Using converted funds to pay conversion taxes (erodes Roth’s benefit and may create penalties if under age 59½).
  • Assuming Roth 401(k) balances are free from RMDs — they are not, unless rolled to a Roth IRA before RMDs apply.
  • Forgetting that conversions are irreversible — recharacterizations of Roth conversions were eliminated by tax law changes in 2018.

When to consult a professional

If your projected MAGI lies near IRMAA thresholds, if you have a complex mix of pension, 401(k), IRAs, and taxable sources, or if you face state income tax consequences for conversions, work with a tax advisor or fee-only financial planner who understands the Medicare lookback rule, RMD timing, and Social Security taxation nuances. Small timing shifts (pension start dates, Social Security month of first benefit, conversion year) can save thousands over time, but they require modeling.

Next steps you can take this month

  1. Pull last two years’ tax returns and calculate MAGI as SSA defines it (AGI + tax‑exempt interest).
  2. Check current SSA IRMAA tables and note the thresholds relevant for you.
  3. Create a two- to five-year conversion plan that staggers taxable income and accounts for RMD start dates.
  4. Confirm plan rules with your 401(k) administrator and, if applicable, request the paperwork for in‑plan conversions or rollovers.
  5. Schedule a meeting with a tax advisor to model Medicare premiums and Social Security tax implications.

Roth conversions are a long-term tax planning lever with both immediate costs and future benefits. Thoughtful timing—particularly with the SSA’s two-year MAGI lookback for IRMAA—lets you capture Roth advantages while avoiding unintended Medicare premium and Social Security tax increases. Plan with a multi-year horizon, keep taxes payable from non-retirement sources where possible, and consult a professional for scenarios that sit close to critical thresholds.