Your Roth conversion window is smaller than you think
The years between retiring and the start of RMDs and Social Security are often your lowest-tax years of the whole retirement — and the cheapest time to convert traditional TSP and IRA dollars to Roth. Every year you wait, that window narrows: one fewer low-bracket year, a bigger balance, a larger RMD. Convert now at 12%, or later at 40.7%.
1. The cheapest tax years you’ll ever have
For many federal retirees there’s a quiet stretch of years — often from retirement in the early 60s until required minimum distributions begin at 73 or 75 — when taxable income temporarily falls. The paychecks have stopped, Social Security may not have started, RMDs aren’t forcing income out yet, and the result is a window of unusually low brackets. That window is the single best opportunity in the entire retirement to do something valuable: a Roth conversion — moving traditional balances to Roth at a low tax rate.
The logic is simple. A traditional TSP or IRA is a tax bill waiting to happen — every dollar comes out taxable, eventually on the IRS’s schedule through RMDs. A Roth conversion lets you choose to pay that tax now, voluntarily, at today’s rate, in exchange for tax-free growth, tax-free withdrawals, and no future RMDs. Done in the low-bracket gap years, you’re paying the bill at a discount.
The catch is in the headline: the window is smaller than it looks, and it’s shrinking. This dispatch covers why it closes, how to fill your brackets without overfilling them, and how to see your own conversion headroom this year.
Roth conversions in the gap years are unusually powerful because they defuse three separate retirement traps at once. They shrink the traditional balance that drives the RMD tax torpedo, so future required distributions — and the Social Security taxation they trigger — are smaller. They leave a surviving spouse with tax-free Roth dollars instead of fully taxable traditional ones, softening the widow’s penalty when filing status flips to single. And they reduce the income that pushes you into Medicare IRMAA surcharges later. Few financial moves address that many problems with a single decision, which is why advisors treat the gap-year conversion window as one of the highest-value planning opportunities in all of retirement — and why letting it pass unused is so costly.
2. Why the window shrinks every year
“I’ll get to it” is the most expensive sentence in conversion planning, because three things work against you with every year you delay.
One fewer low-bracket year. The gap is finite — it ends when RMDs and Social Security begin. Each year you don’t convert is a year of low-bracket headroom you can never get back. Conversions spread across many years stay in low brackets; conversions crammed into a few years spill into high ones.
The traditional balance keeps growing. Left alone, your traditional TSP and IRA compound — which sounds good but means a larger balance to convert later, and a larger forced RMD when the time comes. The mountain you’re trying to move gets taller while you wait.
The later tax rate is usually higher. Once RMDs and Social Security are flowing, any conversion stacks on top of that income, at your highest marginal rate — possibly inside the tax-torpedo zone where the effective rate hits 40.7%. The whole point of converting in the gap years is to do it before that income arrives.
Every gap year you skip is gone for good — and the balance you didn’t convert keeps growing into a bigger RMD, taxed later at a higher rate. The window doesn’t just close; it shrinks from both ends.
3. Filling the brackets without overfilling them
The art of conversions is filling your low brackets up to the brim — but not over. Convert too little and you waste cheap headroom; convert too much and you spill into a higher bracket or trip an IRMAA tier.
2026 bracket tops (MFJ): 12% → $100,800 · 22% → $211,400
2026 bracket tops (Single): 12% → $50,400 · 22% → $105,700
Watch the IRMAA cliff: ~$218k MFJ / ~$109k single
The usual approach is to pick a target bracket you’re comfortable paying — often the top of the 12% or 22% bracket — and convert just enough each year to reach it, but no more. Over a decade of gap years, that steady, deliberate filling can move a large traditional balance to Roth at an average rate far below what the eventual RMDs would cost.
Two ceilings sit above the bracket you choose. The first is the next tax bracket. The second — once you’re within two years of Medicare — is the IRMAA tier, which is a cliff: cross it by a dollar and you owe the full surcharge two years later. Before age 63, IRMAA doesn’t constrain you at all, which is exactly why the early gap years are the best time to convert aggressively.
4. See your headroom — and respect the rules
The estimator shows how much you can convert this year to reach a target bracket, and the blended rate you’d pay on it.
Roth Conversion Headroom Estimator
Shows the room between your current taxable income and the top of a target bracket, and the blended federal rate on filling it. Simplified 2026 illustration — ignores state tax, the Social Security worktable, and IRMAA detail. Not advice.
“Current taxable income” means your income after deductions but before the conversion. The blended rate is federal only; your state may also tax the conversion. Mind the IRMAA cliff if you’re within two years of Medicare.
A few rules to respect as you convert:
Mind the IRMAA two-year lookback. Conversions at 63 or later can raise your Medicare premiums at 65+. Before 63, convert freely on that front; at 63+, size conversions against the IRMAA tiers. (See the tax torpedo for how IRMAA stacks with RMDs.)
Pay the conversion tax from outside the account. Ideally you pay the tax on a conversion from taxable savings, not by withholding from the conversion itself — that way the full amount lands in the Roth and keeps compounding tax-free.
Know the five-year rule. Each conversion starts a five-year clock, and a Roth IRA needs five years open for earnings to be fully tax-free. For most retirees past 59½ converting in the gap years, this is a non-issue — but it’s a reason to open a Roth IRA early.
Think in spouses, not just in years. Converting during your joint-filing years is also a hedge against the widow’s penalty — Roth dollars don’t get taxed harshly when the survivor files single. (See the widow’s penalty.)
The case for gap-year conversions is strong, but it isn’t unconditional, and overdoing it is a real mistake. A conversion only wins if you convert at a lower rate than you’d otherwise pay later — so converting so aggressively that you push yourself into the 24% or 32% bracket today, to avoid a 22% rate tomorrow, defeats the purpose. The tax is due now and in cash, which has its own cost: dollars spent on conversion tax aren’t compounding for you. And the calculus depends on assumptions about future tax rates, your balance, and your spending that no one can know for certain. The right answer is rarely “convert everything” or “convert nothing” — it’s a deliberate, year-by-year filling of the brackets you’re willing to pay, ideally modeled with a tax professional who can see your whole picture. The goal is a discount, not a record-setting tax bill.
This dispatch reflects 2026 federal brackets and IRMAA thresholds, which adjust annually, and RMD ages under SECURE 2.0. Tax law can change — current individual rates are scheduled under existing law, and future legislation could move brackets up or down, which affects the conversion calculus. The estimator is a simplified federal illustration; it does not capture state taxes, the Social Security worktable, or your full return. Model your specific situation with a tax professional before converting.
Frequently asked questions
What is a Roth conversion and when does it make sense?
A Roth conversion moves money from a traditional, pre-tax account — a traditional IRA, or the traditional TSP rolled to an IRA — into a Roth account. You pay ordinary income tax on the converted amount in the year you convert, and in exchange that money grows tax-free and comes out tax-free in qualified withdrawals, with no future required minimum distributions on a Roth IRA. Conversions make the most sense when you can convert at a lower tax rate today than the rate you’d otherwise pay later. The classic setup is the gap years between retiring and the start of RMDs and Social Security, when many retirees temporarily sit in a low bracket before those income sources kick in. Converting then — filling up the 12% or 22% bracket deliberately — locks in a low rate now and shrinks the traditional balance that would otherwise drive large, harshly taxed RMDs at 73 or 75. It makes less sense if converting would push you into a much higher bracket than you’ll face later, or if you’ll need the converted money within five years and are under 59 and a half.
Will a Roth conversion affect my Medicare premiums (IRMAA)?
It can, and the timing is the key. A Roth conversion increases your modified adjusted gross income in the year you convert, and Medicare’s IRMAA surcharges are based on your MAGI from two years earlier. So a conversion done at age 63 or 64 can raise your Medicare Part B and Part D premiums at age 65 or 66. Because IRMAA is a cliff — crossing a tier by even a dollar triggers the full surcharge — a large conversion that nudges you over a tier can be expensive. The practical implications: conversions done well before age 63 don’t affect IRMAA at all, because Medicare hasn’t started and the two-year lookback hasn’t begun, which makes the early gap years especially valuable for aggressive conversions. Once you’re within two years of Medicare or already enrolled, size your conversions to stay under the relevant IRMAA tier, or accept the surcharge knowingly as part of the trade-off. The conversion can still be worth it even with an IRMAA bump, but you want to choose it on purpose rather than be surprised.
What is the Roth five-year rule?
There are actually two five-year rules, and they trip people up. The first applies to earnings: for your Roth IRA earnings to come out completely tax-free, your first Roth IRA must have been open for at least five years and you must be over 59 and a half. The second applies specifically to converted amounts: each conversion starts its own five-year clock, and if you’re under 59 and a half, withdrawing converted principal before that clock runs out can trigger a 10% penalty. For most federal retirees doing conversions in their early-to-mid 60s, the penalty version is not a concern, because they’re already past 59 and a half. The remaining consideration is making sure a Roth IRA has been open at least five years before tapping earnings tax-free — which is a good reason to open and fund a Roth IRA early, even with a small amount, to start the clock. If you’re converting in the gap years and won’t touch the money for a while, the five-year rules usually pose no obstacle, but it’s worth confirming your situation with a tax professional.