The Roth TSP Conversion Tax Trap (2026)

Last Updated: April 5, 2026 Reading Time: 10 min

The advice has been everywhere: convert your traditional TSP to Roth, pay taxes now, retire tax-free. It sounds clean. And for some federal employees at the right time, it is.

But a lot of people are getting this wrong. Converting too much, too early, in the wrong tax year creates a bill that wipes out years of projected savings. The TSP launched in-plan Roth conversions on January 28, 2026, and federal employees are moving money without fully understanding what it costs them.

This post covers the part your tax pro might skip: bracket stacking, the IRMAA cliff, the 5-year traps, and the scenarios where a Roth conversion costs you more than it saves.

Key Takeaways

• A Roth conversion is not free. The converted amount is added to your ordinary income and taxed at your marginal rate for the year of conversion.
• Bracket stacking is the core risk. If you are still earning a GS salary, a large conversion piles on top of it and can push you from the 22% bracket into the 32% bracket.
• IRMAA is a hidden cost. Converting $1 over the $109,000 threshold (single) can cost more than $1,000 per year in higher Medicare premiums for two consecutive years.
• Every conversion has its own 5-year clock. Withdraw converted funds before 5 years and before age 59 1/2, and you owe a 10% penalty.
• The best conversion window is often early retirement, when your salary is gone, RMDs have not started, and your tax bracket is at its lowest.
• The mandatory Roth catch-up rule (2026) already forces some high earners into Roth. Adding voluntary conversions on top compounds the tax hit.

Why Roth Conversions Are Not Free

A Roth conversion works like this: pay taxes now, get tax-free growth and withdrawals later. The math works when your current tax rate is lower than your expected future tax rate.

When it does not work: when your current rate is already high, or when secondary effects like bracket jumps, Medicare surcharges, and Social Security taxation drive the real cost well above the stated marginal rate.

The mechanics are straightforward. The IRS treats the converted amount as ordinary income in the year it occurs. It stacks on top of everything else you earned that year. If you converted $80,000 and your salary was already $120,000, the IRS sees $200,000 of ordinary income. Depending on your filing status, that pushes you out of the 22% bracket and into higher territory for a portion of the conversion.

The TSP does not withhold taxes on conversions. That means you owe money you may not have set aside. If you do not make estimated tax payments, you could face an IRS underpayment penalty on top of the tax bill itself.

The Tax Bill You Will Owe: Real Numbers

A $100,000 Roth conversion costs this much at different federal tax brackets in 2026:

Your Bracket	Federal Tax on $100K Conversion	Effective After-Tax Amount
12%	$12,000	$88,000
22%	$22,000	$78,000
24%	$24,000	$76,000
32%	$32,000	$68,000
35%	$35,000	$65,000

Now add state income tax. California taxes ordinary income at up to 9.3%. Virginia taxes it at 5.75%. If you live in a state with income tax, add that rate to your federal rate. A Virginia resident in the 22% federal bracket paying 5.75% state tax faces a combined 27.75% rate on the conversion, turning a $100,000 conversion into a $72,250 net gain.

And the tax bill must come from outside the TSP. If you are under 59 1/2 and you withdraw TSP funds to pay the tax, that withdrawal itself is taxable and subject to the 10% early withdrawal penalty. You end up paying tax on your tax payment.

Bracket Stacking: The Problem Most People Miss

The term "bracket stacking" describes what happens when a Roth conversion rides on top of your existing income instead of replacing it.

Federal employees who are still working get hit hardest. Take a GS-13 in Washington D.C. earning $135,000. After the 2026 standard deduction of $16,100 (single filer), taxable income is $118,900. That person is already in the 24% bracket before any conversion starts.

Now they convert $60,000. Taxable income jumps to $178,900. A portion of that $60,000 gets taxed at 32%, not 22% or 24%. The effective cost of the conversion is higher than the rate they expected to pay.

The takeaway: know your full income picture before you convert. Your salary, side income, pension, and Social Security all count. Conversion income stacks on top of all of it, not beside it.

IRMAA: The Conversion Cost Nobody Talks About

IRMAA stands for Income-Related Monthly Adjustment Amount. It is Medicare's income-based surcharge system that increases your Part B and Part D premiums when your income crosses certain thresholds.

For 2026, IRMAA brackets are based on your 2024 modified adjusted gross income (MAGI). The thresholds:

Filing Status	Income Threshold	Additional Annual Cost
Single / MFS	Over $109,000	$974 per year (Part B + D combined)
Married Filing Jointly	Over $218,000	$974 per year
Single	Over $136,000	Increases further
Married	Over $272,000	Increases further

The maximum IRMAA surcharge in 2026 is approximately $6,936 per year (Part B plus Part D combined). These are cliff brackets, not gradual phases. Going $1 over the first threshold triggers the full surcharge for both Part B and Part D.

The trap: a conversion in 2026 that pushes your MAGI over the first IRMAA threshold will trigger higher Medicare premiums in 2028, two years later. IRMAA is re-determined annually based on income from two years prior, so a single large conversion in a high-income year locks in surcharges for two consecutive years.

For a married couple converting $150,000 with combined income of $200,000: the conversion pushes MAGI from $200,000 to $350,000, crossing multiple IRMAA tiers. The additional Medicare cost could be $5,000 or more over two years. That amount has to go into your conversion cost math before you decide.

When Roth Conversions Make Sense

Roth conversions are a real tool. The problem is not the tool, it is timing and size.

Conversions work well when you are in a low-income year. The prime window for most federal employees is the gap between retirement and age 73, when required minimum distributions begin. Retire at 57 with a small FERS supplement and no Social Security yet, and your taxable income might drop to $40,000 to $60,000. Converting $30,000 to $40,000 per year in that window costs 12% to 22% in federal taxes. That rate is often lower than what you would pay once all income streams are running.

Time horizon matters too. A conversion at 45 gives the converted balance 20-plus years of tax-free growth. A conversion at 68 gives you less time to recoup the upfront cost, and the math gets harder.

The ability to pay taxes from outside the TSP is a prerequisite, not a preference. Using external cash means the full converted balance keeps compounding. Tapping the TSP itself to cover the tax bill shrinks the principal and can trigger penalties if you are under 59 1/2.

Large traditional TSP balances eventually become an RMD problem. Every dollar converted to Roth is one fewer dollar subject to required distributions starting at 73 or 75. Reducing the RMD base can prevent forced distributions in your late 70s that push you into higher brackets.

Tax diversification is the longer game. Having both traditional and Roth money in retirement lets you pick which account to draw from based on that year's income. Some years you might want taxable withdrawals. Others you might not. Roth gives you the choice.

When Conversions Do Not Make Sense

If you are a GS-14 or GS-15 in a high-cost locality, you are already in the 24% or 32% federal bracket. Converting $100,000 now at 32% to avoid paying 22% in retirement moves money in the wrong direction. That is the whole game: pay less, not more.

No cash outside the TSP to cover the tax bill? Then stop before you start. Using TSP funds to pay conversion taxes creates a second taxable event, and a penalty if you are under 59 1/2.

Many federal employees look at their combined retirement income and realize they will land in the 12% or 22% bracket. FERS pension plus Social Security plus modest TSP withdrawals often adds up to less than a working GS salary. If that is your situation, paying a higher rate today to prepay a lower rate tomorrow is not a good trade.

The IRMAA cliff is a specific case worth checking separately. Pull your 2024 MAGI. If you are within $30,000 to $40,000 of the $109,000 (single) or $218,000 (married) threshold, model the premium impact before converting. Two years of higher Medicare costs can add thousands to the real cost of the conversion.

Already subject to the mandatory Roth catch-up rule? Your catch-up contributions are going to Roth automatically. For a $8,000 catch-up in the 24% bracket, that is roughly $1,920 more in federal taxes this year compared to traditional. Layering large voluntary conversions on top of that makes 2026 a very expensive tax year.

Conversion Timing Strategies That Work

The bracket-filling approach is the one that actually holds up under scrutiny. Rather than converting as much as possible in one year, convert only enough to reach the ceiling of your current bracket without crossing into the next. If taxable income after deductions is $80,000 and the 22% bracket tops out around $103,350 for single filers in 2026, you have roughly $23,000 of 22% room available. Convert up to that ceiling, stop, and come back next year.

The early-retirement gap is where most of the opportunity sits. Between your last day of work and age 73, your income often drops sharply. Salary is gone. Social Security may not have started. Pension payments may be modest. That window can last 10 to 15 years for someone who retires at 58 or 60. Converting $30,000 to $50,000 per year inside that window, while you are in the 12% or 22% bracket, beats converting at 32% while you are still earning a full salary.

Spread conversions across multiple years. The TSP allows up to 26 conversions per year per account. There is no reason to dump everything in one tax year and create a spike.

Account for the two-year IRMAA lag. Conversions in 2026 set your 2028 Medicare premiums. A high-income year, whether from a conversion, a home sale, or anything else, can lock you into elevated premiums two years out. Build that cost into the plan.

The 5-Year Rule Trap

Roth conversions carry a 5-year rule that is separate from, and in addition to, the general Roth 5-year rule for earnings. This is where people get surprised.

There are two clocks to track. The first covers earnings: to withdraw Roth earnings tax-free, the account must have been open at least 5 years. The clock starts January 1 of the year of your first Roth contribution or conversion, whichever is earlier. If you started Roth TSP contributions in 2020, that clock is already satisfied.

The second clock covers converted principal, and it resets with every conversion. Convert $50,000 in January 2026, and that $50,000 cannot be withdrawn penalty-free until January 1, 2031, if you are under 59 1/2. Pull it out in 2028 and you owe a 10% early withdrawal penalty on the principal, even though you already paid income tax on it when you converted.

People assume the money is accessible because they paid taxes on it. It is not. You paid taxes to move it to Roth. You still have to wait 5 years (or reach 59 1/2) to access the principal without penalty.

After 59 1/2, and once the Roth account has been open 5 years, all of this goes away. Withdrawals are fully tax-free, principal and earnings both.

The timing implication for federal employees retiring in their late 50s: if you convert at 57 and plan to retire at 58 and tap those funds before 62, the 5-year window may still be open. Model the dates before you convert.

The Mandatory Roth Catch-Up Rule (2026)

For federal employees age 50 or older who earned more than $150,000 in 2025 (based on Social Security wages, W-2 Box 3), catch-up contributions to TSP must go to Roth starting January 1, 2026. This is automatic, managed by payroll, and not optional.

The catch-up limits for 2026:

• Ages 50 to 59 and 64 and older: $8,000
• Ages 60 to 63 (super catch-up): $11,250

In the 24% bracket, mandatory Roth catch-up on $8,000 costs roughly $1,920 more in federal taxes than the same amount going to traditional TSP. Not a reason to avoid it, but worth knowing before you also plan voluntary in-plan conversions.

If you are already subject to mandatory Roth catch-up, 2026 is probably not the year to add large voluntary conversions on top. You are already putting after-tax dollars in at scale. More conversion income in the same year compounds the hit without giving you more bracket room to work with.

Calculate Your TSP Conversion Scenarios

Before converting, run the numbers for your specific situation. Use our free TSP Calculator to model how Roth versus traditional balances affect your long-term retirement income and withdrawal strategy.

Project Your TSP Growth →

Frequently Asked Questions

How much tax will I owe on a TSP Roth conversion?

The full converted amount is added to your ordinary income for the year. At the 22% bracket, a $100,000 conversion creates a $22,000 federal tax bill. TSP does not withhold taxes, so you must pay from external funds or through estimated tax payments. State income taxes apply on top of federal.

Can a Roth TSP conversion trigger higher Medicare premiums?

Yes. Medicare IRMAA surcharges kick in when your modified adjusted gross income exceeds $109,000 (single) or $218,000 (married filing jointly) based on your income two years prior. A large conversion can push you over an IRMAA cliff and cost more than $1,000 per year in additional premiums for two years.

What is the 5-year rule for TSP Roth conversions?

Each in-plan conversion starts its own 5-year clock. If you withdraw converted funds before 5 years have passed and before age 59 1/2, you owe a 10% early withdrawal penalty on that amount. This rule tracks each conversion separately, so a 2026 conversion cannot be tapped penalty-free until 2031.

When does a Roth TSP conversion NOT make sense?

Converting generally does not make sense if you are in your highest earning years, have no cash outside the TSP to pay the tax bill, expect a lower tax bracket in retirement, or would trigger IRMAA surcharges. The best conversion windows are low-income years before RMDs begin.

What is the mandatory Roth catch-up rule for 2026?

Federal employees age 50 or older who earned more than $150,000 in 2025 must direct all catch-up contributions to Roth TSP starting January 1, 2026. The standard catch-up limit is $8,000, and employees ages 60 to 63 can contribute up to $11,250 under the SECURE 2.0 super catch-up rule.

Can I undo a TSP Roth in-plan conversion?

No. TSP Roth in-plan conversions are irrevocable. Once processed, the money cannot be moved back to your traditional balance. This is different from pre-2018 Roth IRA rules, when conversions could be reversed through recharacterization.

Related Resources

• TSP Roth In-Plan Conversion Guide 2026: How the new in-plan conversion feature works, rules, and step-by-step process
• TSP Roth vs Traditional: Which Is Right for You in 2026?: Decision framework for choosing contribution type
• Tax Planning for Federal Retirees 2026: Comprehensive guide to pension, TSP, and Social Security taxation
• SECURE 2.0 Mandatory Roth Catch-Up Guide: Who is affected by the mandatory catch-up rule and what to do about it
• TSP Calculator: Model your TSP growth and retirement projections

Sources

• TSP.gov: Roth In-Plan Conversions
• TSP Bulletin 25-3: 2026 Contribution Limits
• IRS Revenue Procedure 2025-32: 2026 Tax Inflation Adjustments
• IRS Final Regulations on Mandatory Roth Catch-Up
• Federal News Network: Why Government Employees Should Think Carefully About Roth Conversions
• FedSmith: Mastering Roth Conversions for Federal Employees
• Kiplinger: 2026 IRMAA Brackets and Surcharges
• Tax Foundation: 2026 Tax Brackets and Federal Income Tax Rates