TSP Roth Conversions and the IRMAA Window Feds Miss

Last Updated: August 2, 2026 Reading Time: 8 min

There's a stretch of retirement most federal employees walk right through without realizing it's the most valuable tax window they'll ever get. You've retired, but Social Security hasn't started, your TSP isn't forcing withdrawals yet, and you're not on Medicare. Your income sits in a valley. Fill that valley with smart TSP Roth conversions and you can keep your Medicare premiums low for the rest of your life. Miss it, and a $1.2 million traditional TSP can quietly hand you an IRMAA surcharge bill every year until you die.

Try it: TSP Roth Conversion Calculator: model your annual conversion amount to stay under IRMAA thresholds.

What IRMAA Actually Costs

IRMAA (the Income-Related Monthly Adjustment Amount) is a surcharge stacked on top of your Medicare Part B and Part D premiums once your income crosses a line. It's not a one-time hit. It applies every year your income stays high, and the standard Part B premium it sits on top of is already $202.90/month in 2026.

The cruel part: it's a cliff, not a ramp. Go one dollar over $109,000 and you owe the full $1,148. For a married couple both on Medicare, every tier counts twice.

Two things the table makes plain. A couple with a solid FERS pension and Social Security at $190,000 MAGI has only $28,000 of safe conversion room, far less than most people assume. And overshooting by even $1,000 costs the same as overshooting by $20,000, because the cliff charges the full tier either way. The widely quoted "$2,300 mistake" figure understates the second cliff: crossing from Tier 1 to Tier 2 runs a couple about $3,473 a year.

Mechanics worth knowing before December: TSP allows up to 26 in-plan conversions a year at a $500 minimum each, so you can convert in slices and stop precisely at your headroom instead of guessing once. Conversions submitted before noon Eastern process same-day; the hard cutoff for a 2026 tax-year conversion is December 31, and TSP withholds no taxes, so the estimated-payment math is on you.

One more asymmetry that decides close calls: if your income drops because you retire, that is a qualifying life-changing event and Form SSA-44 can get a fresh IRMAA determination using your new, lower income. A Roth conversion is explicitly not a life-changing event. Retirement-year income spikes can be appealed away; conversion spikes cannot.

Why the 57-to-64 Gap Is the Sweet Spot

A FERS retiree's income moves through clear phases. Right after you retire at your MRA, your pension starts, the FERS Supplement may bridge you to Social Security, but you have no RMDs, no Medicare premiums, and often no Social Security yet. That's the valley.

Then it fills back up. Social Security can start at 62. Medicare and its premiums begin around 65. And at 73, your traditional TSP starts forcing required minimum distributions whether you need the money or not. A $1.2 million traditional balance throws off a first-year RMD of roughly $45,280 (the IRS divisor at 73 is 26.5). Stack that on a pension and Social Security and your MAGI can land near $199,000, which is IRMAA Tier 3, about $4,615 a year, per person, for life.

The conversions you do in the valley are taxed at 12% or 22%. The income RMDs force on you later can be taxed at 24% and trigger IRMAA on top. Same dollars, very different bill, depending on when they come out.

The Age-63 Trap

This is the detail that catches people. IRMAA uses a two-year lookback: your 2026 tax return sets your 2028 Medicare premiums. If you enroll in Part B at 65, the conversions you run at 63 and 64 are the ones that show up on your very first Medicare bills.

So the strategy has a built-in deadline. The big, aggressive conversions belong in your late 50s and very early 60s, when the lookback doesn't reach your Medicare years. By 63, you taper, sizing each conversion to stay under the IRMAA tier you're willing to live in once premiums start.

The Hold-Harmless Rule Won't Save You From IRMAA

Some retirees assume Medicare's hold-harmless rule caps how much their Part B premium can rise in a year. It won't help here, for two separate reasons that hit federal retirees harder than most.

First, hold-harmless only protects people whose Part B premium is deducted from a Social Security check. A federal retiree who hasn't claimed Social Security yet, or a CSRS retiree who never qualified for it, pays Medicare directly and gets no hold-harmless protection at all.

Second, even for retirees drawing Social Security, the statute excludes IRMAA-payers from hold-harmless entirely. The moment a Roth conversion pushes your MAGI over an IRMAA threshold, the surcharge applies in full two years later, uncapped by whatever your COLA was. There is no protection to plan around; the only defense is keeping conversion-year MAGI under the cliff.

The aggressive ladder costs more tax up front, during low-bracket years, and saves roughly $35,000 in Medicare surcharges over a decade, plus it shrinks the traditional balance that drives every future RMD. Run your own numbers in the TSP Calculator and check your retirement income timeline with the FERS Retirement Calculator before you pick a conversion size.

How to Actually Do It

A few mechanics decide whether this works or backfires.

• Pay the tax from outside the TSP. The TSP does not withhold on an in-plan Roth conversion (the feature launched January 28, 2026). If you pay the tax out of the converted money, you've defeated most of the benefit. Use a taxable account.
• Fill the bracket, don't blow past it. Convert enough each year to reach the top of your target tax bracket or the bottom of the next IRMAA tier, then stop. The goal is a steady ladder, not one giant conversion that spikes a single year.
• Watch what stacks on the conversion. A conversion raises your MAGI, which can also make more of your Social Security taxable. Model the whole picture, not just the conversion in isolation.
• Remember the match stays traditional. Agency matching contributions always land in your traditional balance and aren't converted automatically. Part of your account will stay traditional and subject to RMDs no matter what.
• OBBBA removed the deadline panic. The 2025 law permanently extended the current tax brackets, so there's no sunset forcing your hand. Today's 22% and 24% rates are a stable baseline to plan around.

Frequently Asked Questions

What is IRMAA and how does it affect federal retirees?

IRMAA is the income-related surcharge added to Medicare Part B and Part D premiums once your modified adjusted gross income passes a threshold. In 2026 it starts at $109,000 for single filers and $218,000 for married filing jointly. The first tier adds $1,148 per person per year on top of the standard premium, and it climbs to $6,936 at the top tier. Medicare uses your income from two years earlier to set it.

Why is age 57 to 64 the best time to convert TSP to Roth?

After you retire but before Social Security, RMDs, and Medicare begin, your taxable income drops to its lowest point in retirement. That valley is room you can fill with Roth conversions taxed at a low rate. Once RMDs start at 73, your traditional TSP forces income out whether you want it or not, often pushing you into IRMAA permanently. The gap years are a one-time window that never reopens.

Why does age 63 matter for IRMAA?

IRMAA uses a two-year lookback. The income on your 2026 return sets your 2028 Medicare premiums. If you enroll in Part B at 65, the conversions you do at 63 and 64 are the ones that show up on your first Medicare bills. A large conversion at 63 can spike your premiums right as you start paying them, so that's the year to be most careful about sizing.

Where does the tax on a TSP Roth conversion come from?

From outside the TSP. The TSP does not withhold taxes on an in-plan Roth conversion, so you owe the income tax on the converted amount at filing. Paying it from a taxable account, rather than from the converted balance, is what makes the strategy work. The in-plan conversion feature launched January 28, 2026.

Do Roth TSP balances have required minimum distributions?

No. Under SECURE 2.0, Roth TSP balances are exempt from lifetime RMDs, which is the whole point of converting before 73. Note that agency matching contributions always go into your traditional balance and are not converted automatically, so a portion of your account stays traditional and subject to RMDs.

Related Resources

• TSP Roth In-Plan Conversion Guide 2026: The mechanics, eligibility, and five-year rule.
• The Roth TSP Conversion Tax Trap: When converting too much backfires.
• The TSP RMD Tax Problem at Age 73: What happens if you don't convert in time.
• TSP Calculator: Model your balance and conversion scenarios.

Sources: 2026 Medicare Part B premiums and IRMAA, CMS, TSP Roth in-plan conversions, 2026 tax brackets, Tax Foundation. Worked example figures are FedTools 2026 analysis using verified IRMAA tiers and the IRS Uniform Lifetime Table.