Roth conversions and Medicare IRMAA are closely connected—and if you don’t plan the timing, a conversion can raise your Medicare Part B and Part D premiums a couple years later. This guide explains how to coordinate Roth conversions with retirement milestones so you can reduce lifetime taxes without triggering avoidable IRMAA surcharges.

Most retirement planning mistakes don’t come from bad investing. They come from income timing.

Two things make this especially important:

1. Roth conversions are taxed at your marginal rate and increase income in the year you convert.
2. Medicare uses a two-year lookback on MAGI to determine whether you owe IRMAA surcharges for Part B and Part D.

This guide explains how to coordinate tax brackets, Medicare enrollment timing, and Roth conversions—and why converting during lower tax-bracket years is often one of the highest-impact moves available.

If you’re still running a business, here’s how we plan for these issues years in advance…

The big idea: convert in your “low-tax window” to create long-term flexibility with Roth conversions and Medicare IRMAA

Many households have a short window after retirement when income drops:

• Wages stop (or drop sharply)
• Social Security hasn’t started yet (or hasn’t ramped up)
• RMDs haven’t started forcing taxable withdrawals

Those years can be ideal for conversions because you can often convert meaningful amounts while staying in a lower bracket—then enjoy tax-free Roth growth and more control later.

The 3 retirement phases that determine your best Roth conversions and Medicare IRMAA rates

1) Working years

Conversions often cost more because wages stack on top of everything else.

2) “Gap years” (retired, before Social Security and before RMDs)

This is commonly the best window. Income is lower, and you can “fill up” a target bracket intentionally.

3) Social Security + RMD years

RMDs force taxable withdrawals and can push taxable income higher each year. That’s also when Medicare premium surcharges and NIIT become much more relevant.

Retirement Milestones by Age (Medicare, Social Security, TSP, and RMD Timing) to consider for Roth conversions and Medicare IRMAA

Below is a “timeline view” you can use in the article to anchor key decisions around tax brackets, Roth conversions, Medicare enrollment, and future RMD pressure—especially for FERS / TSP households.

Ages 55–57: FERS Minimum Retirement Age (MRA)

• MRA is 55–57 depending on birth year (many current retirees are at 57).
• This is the earliest point many FERS employees start thinking seriously about sequencing:
  • pension start timing
  • health coverage (FEHB)
  • and whether TSP withdrawals will be needed soon or can be delayed

Planning relevance: Your withdrawal strategy and “gap years” (low-income years that are prime for Roth conversions) often start here.

Ages 50 / 55: Early access to TSP in specific situations

There are special situations where TSP funds can be accessed before 59½ without the 10% early distribution penalty, including:

• Separation in or after the year you turn 55 (commonly referred to as the “rule of 55”) for TSP/401(k)-type plans.
• Special category employees (certain public safety roles) may have an even earlier threshold (often tied to age 50).
• Certain early-out / involuntary separation situations can also change the planning path.

Planning relevance: If TSP can be accessed without penalty earlier than an IRA, it may change whether you need to roll TSP to an IRA immediately—and it may change the best Roth conversion timing.

Age 59½: IRA penalty barrier (big flexibility milestone)

• Traditional IRA distributions generally avoid the 10% early distribution penalty once you hit 59½.
• Roth conversion planning becomes more flexible because you can better separate:
  • “money for spending” vs.
  • “money for tax strategy (Roth conversions)”

Planning relevance: Many households aim to avoid using Roth conversion withholding (or pulling conversion taxes from retirement dollars) because it reduces what stays invested in tax-advantaged accounts.

Age 62: Social Security eligibility begins (and the FERS supplement typically ends)

• Social Security can start at 62, but starting at 62 generally means a permanent reduction versus Full Retirement Age (FRA).
• For many FERS retirees, the FERS annuity supplement (if eligible) is designed to bridge income until 62, and often ends at 62.

Planning relevance: The 62–FRA years can still be a Roth conversion opportunity, but you need to model how Social Security timing affects taxable income and Medicare MAGI later.

Age 65: Medicare eligibility begins (enrollment timing is the real trap)

• Medicare eligibility generally begins at 65, but eligibility and enrollment timing are not the same thing.
• Most people get a 7-month Initial Enrollment Period (IEP) around 65.
• If you miss enrolling in Part B when required (and you don’t qualify for a Special Enrollment Period), you can be stuck waiting for a limited enrollment window—and the late enrollment penalty can be permanent.

Part B penalty (important correction):
The Part B late enrollment penalty is generally 10% for each full 12-month period you could have had Part B but didn’t (without qualifying coverage). That penalty is added to your monthly premium and typically lasts as long as you have Part B.

Planning relevance: A “good” Roth conversion plan can still turn expensive if it pushes MAGI up right as you’re navigating Medicare enrollment and IRMAA.

Ages 66–67: Social Security Full Retirement Age (FRA)

• FRA is generally 66–67 depending on birth year (for many people today, it’s 67).
• Once at FRA, the Social Security earnings test becomes less restrictive (and after FRA, it no longer applies the same way).

Planning relevance: This is another decision point where income may shift (work stopping/starting, Social Security turning on), which affects your Roth conversion bracket “space.”

Ages 73–75: Required Minimum Distributions (RMDs) begin (and can create tax + IRMAA pressure)

• RMDs apply to traditional retirement accounts (Traditional IRAs, pre-tax 401(k), traditional TSP).
• RMD age is being phased upward:
  • Age 73 currently
  • Age 74 for those who reach that age in 2029
  • Age 75 for those who reach that age in 2033
• Once RMDs start, they often push taxable income higher—reducing Roth conversion flexibility and potentially increasing IRMAA exposure.

Planning relevance: This is exactly why the years before RMDs (especially the post-retirement “gap years”) are often the best time to execute a structured Roth conversion plan.

Roth conversions and Medicare IRMAA: the retirement cost most people don’t model correctly

IRMAA is an income-based surcharge added to:

• Medicare Part B premium, and
• Medicare Part D premium (an additional monthly amount)

Medicare determines IRMAA using your MAGI from two years prior.

That creates a common trap: a Roth conversion today can raise Medicare premiums two years later, even if you’re no longer working.

Planning tip: Don’t avoid conversions because of IRMAA—model conversions with IRMAA tiers in mind and make an intentional tradeoff.

A major “missing” factor: the surviving spouse tax bracket problem for Roth conversions and Medicare IRMAA

A lot of couples plan conversions around their Married Filing Jointly bracket—then one spouse dies and the survivor becomes Single, often with similar retirement income and similar RMD pressure.

That can push the surviving spouse into:

• a higher marginal tax bracket
• higher IRMAA tiers
• NIIT exposure if investment income is meaningful

This is one of the clearest reasons a conversion in the 22% range (and sometimes higher) can be worth considering while both spouses are alive—especially if one spouse’s health is deteriorating and the probability of a filing-status change is real.

Sometimes the right question isn’t:
“What bracket are we in today?”
It’s: “What bracket will the survivor be in later?”

NIIT: conversions don’t create it, but they can trigger it

The Net Investment Income Tax (NIIT) is a 3.8% tax that applies when MAGI exceeds certain thresholds (including $250,000 for MFJ).

Important nuance:

• A Roth conversion is not net investment income.
• But a conversion can raise MAGI enough that interest, dividends, and capital gains become subject to NIIT under the “lesser of” rule.

That’s why conversion planning should check three moving parts at once:

1. your marginal bracket,
2. IRMAA tiers (two-year lookback), and
3. NIIT thresholds.

Medicare timing for married couples: the working spouse can control the other spouse’s Part B timing

If one spouse is still working and has active employer coverage, the other spouse (even if not working) may be able to delay Part B without penalty if they’re covered under that active employer plan, then enroll later through a Special Enrollment Period (SEP).

This matters because:

• It can prevent lifetime Part B penalties.
• It can let you coordinate retirement dates, Medicare effective dates, and conversion timing more intentionally.

Key distinction: this is about current employment coverage, not retiree coverage. Details matter.

The estate planning angle: why Roth can protect family wealth under the 10-year rule

Many parents and grandparents pass away while their children are in peak earning years. If the inheritance is heavily weighted toward pre-tax accounts (Traditional IRA, 401(k), TSP), beneficiaries often must withdraw the account under the 10-year rule (for most non-spouse beneficiaries).

That can create a tax squeeze:

• The inherited withdrawals stack on top of the beneficiary’s wages/bonus years.
• It can push them into much higher brackets—sometimes 30%+ combined marginal rates depending on income and state.

Why Roth is powerful for beneficiaries (important clarification)

For most non-spouse beneficiaries, inherited Roth IRAs still generally must be emptied within 10 years. The difference is that distributions are generally tax-free (assuming the Roth meets the applicable rules, including the 5-year requirement).

So even when the 10-year timeline still applies, a Roth can dramatically reduce the portion of the inheritance lost to taxes—and can preserve flexibility for beneficiaries to time withdrawals during those 10 years without increasing their taxable income.

How to pick your Roth conversion amount each year (a simple process)

A strong plan is repeatable:

1. Project income if you do nothing
   Wages, pensions, investment income, capital gains, deferred comp, part-time work, etc.
2. Pick a target bracket
   Many strategies prioritize filling lower brackets first, then evaluate higher brackets only with a specific reason.
3. Check IRMAA two years out
   Know which tier you’ll land in before you decide the conversion amount.
4. Check NIIT exposure
   Especially if investment income is meaningful and MAGI is near the threshold.
5. Stress-test for survivor taxes
   Run a scenario where one spouse files Single with similar retirement income and RMD pressure.
6. Stress-test for heirs
   If you expect kids to inherit during peak earning years, model the effect of the 10-year rule on their bracket.

State planning note: Iowa excludes Roth conversion income for eligible taxpayers 55+

If you’re an Iowa resident, Iowa guidance provides a retirement income exclusion for those who qualify (including being 55+), which can make the Iowa income tax on Roth conversions effectively $0 for many eligible taxpayers.

Important: this is a state rule. It does not change federal taxable income, federal MAGI for NIIT, or Medicare MAGI for IRMAA.

Resources

Medicare costs and IRMAA overview (Medicare PDF): https://www.medicare.gov/publications/11579-medicare-costs.pdf

CMS 2026 Part B premiums and deductibles: https://www.cms.gov/newsroom/fact-sheets/2026-medicare-parts-b-premiums-deductibles

Medicare: Working past 65 (includes employer/spouse coverage concepts): https://www.medicare.gov/basics/get-started-with-medicare/medicare-basics/working-past-65

Medicare: Avoid Part B/Part D penalties: https://www.medicare.gov/basics/costs/medicare-costs/avoid-penalties

Medicare Interactive: Part B Special Enrollment Period (SEP) PDF: https://www.medicareinteractive.org/wp-content/uploads/Medicare-Part-B-SEP.pdf

Medicare Interactive: Part B costs for higher incomes (IRMAA explainer): https://www.medicareinteractive.org/understanding-medicare/health-coverage-options/original-medicare-costs/part-b-costs-for-those-with-higher-incomes

OPM FEHB eligibility and retirement coordination: https://www.opm.gov/healthcare-insurance/healthcare/eligibility/

IRS: Retirement topics — Beneficiary (inherited IRA/Roth basics): https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-beneficiary

IRS Publication 590-B (Distributions from IRAs) PDF: https://www.irs.gov/pub/irs-pdf/p590b.pdf

Fidelity: Inherited IRA / inherited Roth IRA 10-year rule explainer: https://www.fidelity.com/retirement-ira/inherited-ira-rmd

Iowa retirement income guidance (55+ exclusion): https://revenue.iowa.gov/taxes/tax-guidance/individual-income-tax/retirement-income-tax-guidance

Kiplinger: IRMAA overview: https://www.kiplinger.com/retirement/medicare/what-is-the-irmaa

Kiplinger: RMD overview / calculation context: https://www.kiplinger.com/retirement/retirement-plans/required-minimum-distributions-rmds/603196/calculate-your-rmds