Here's a pattern I see almost every year: someone in their late sixties comes in, brings me a stack of retirement account statements, and asks about Required Minimum Distributions. And I have to tell them that a decade of low-tax opportunity has quietly slipped by.
That's what this piece is about. Not the Roth conversion itself — which most people vaguely understand — but the window when it becomes uniquely powerful, why it exists, and what closes it.
What a Roth conversion actually is.
A Roth conversion is what it sounds like: you move money from a pre-tax retirement account (traditional IRA, 401(k), 403(b)) into a Roth IRA. You pay ordinary income tax on the amount you convert in the year you convert it. In exchange, that money grows tax-free from that point forward, and qualified withdrawals in retirement are tax-free.
Two features are worth flagging:
- No income limits on conversions. Unlike direct Roth contributions (which phase out at higher incomes), anyone with a pre-tax retirement account can convert, regardless of income.
- No earned income requirement. You can convert whether you're working or retired.
So the question isn't can you convert — almost anyone can. The question is when, and how much.
The window: why it exists.
Roth conversions are only worth doing when the tax you pay to convert is lower than the tax you'd pay later. For most people, that requires a specific set of circumstances — and those circumstances tend to line up in a specific stretch of years.
The typical shape:
Window is closed
Your salary fills up the tax brackets. Any conversion stacks on top and gets taxed at your marginal rate — often 24% or higher for professionals. Rarely worth it.
begins
Window opens
Wages stop. Social Security hasn't started yet (if you're delaying). RMDs don't begin until 73. Your taxable income drops dramatically. Suddenly the 12% bracket has room — and often the 22% and 24% brackets do too.
starts
Window narrows
Up to 85% of Social Security becomes taxable, filling the lower brackets. Room for conversions shrinks — sometimes dramatically.
begin at 73
Window largely closes
Required Minimum Distributions from traditional accounts are taxed as ordinary income and can't be avoided. Combined with Social Security, they often fill the brackets that conversions were meant to fill. Additional conversions on top now get taxed at high rates.
The gap between "retirement begins" and "RMDs begin" — which can be as short as three years or as long as fifteen — is the window. For someone who retires at 62 and delays Social Security to 70, the window is eleven years long and might be the most tax-planning-relevant decade of their financial life.
Traditional tax advice is retrospective: file this year's return, respond to this year's questions. Roth conversion planning is prospective: it requires modeling twenty years forward. Most preparers don't have the time (or the software) to do that, so the window closes quietly.
The bracket-filling framework.
Once the window is open, the goal isn't "convert as much as possible." The goal is to convert up to the top of a specific tax bracket without spilling into the next one.
Here's the intuition. The 2026 federal brackets for a married couple filing jointly look roughly like this (rounded, and simplified):
- 10% — up to about $24,000 of taxable income
- 12% — up to about $97,000
- 22% — up to about $207,000
- 24% — up to about $395,000
- Higher brackets above that
A retired couple whose only income is, say, $30,000 in interest and dividends has enormous space in the 12% and 22% brackets. Converting enough to "fill up" the 22% bracket — paying tax now at 22% — often beats leaving that money in a traditional account where RMDs will eventually force withdrawals at 24% or higher (potentially much higher after one spouse dies and the survivor moves to single filing status).
The exact bracket to target depends on the family's expected long-term tax bracket. That's what a multi-year projection is for.
Watch the second-order effects.
The tax on the conversion itself is only part of the cost. Several other things move when your income spikes for a year:
IRMAA (Medicare premium surcharges)
Once you're on Medicare, your Part B and Part D premiums are tied to your income from two years earlier. A large conversion at age 65 can bump your Medicare premiums at 67. The IRMAA surcharges come in step-function increases, meaning one dollar of extra income can push you into a new tier and add thousands of dollars in Medicare costs the year the surcharge hits.
ACA subsidy cliffs (for early retirees under 65)
If you're on marketplace insurance before Medicare eligibility, your ACA subsidies are income-tested. A conversion can eliminate your subsidy for the year — which for a couple can mean $10,000 or more of lost subsidy on top of the conversion tax itself.
Capital gains stacking
Long-term capital gains sit on top of your ordinary income for tax purposes. Filling the ordinary-income brackets with a conversion can push previously-tax-free capital gains (in the 0% LTCG bracket) up into the 15% or 20% bracket. This matters for retirees planning capital-gains harvesting in the same window.
State taxes
Some states tax Roth conversions the same way they tax any retirement distribution — sometimes at rates as high as 5-9%. Pennsylvania, for what it's worth, generally does not tax Roth conversions for individuals age 59½ or older, which makes Pennsylvania one of the more conversion-friendly states in the country. This is a real, structural advantage for retirees who live here.
Paying the conversion tax out of the converted funds themselves is almost always a mistake. It shrinks the amount that gets Roth treatment and (if you're under 59½) can trigger a 10% early-withdrawal penalty. The math almost always works better when the tax is paid from a taxable brokerage account instead.
Who has this window, and who doesn't.
The window is meaningful for:
- Anyone retiring between 55 and 72, especially with significant traditional retirement account balances
- Early retirees delaying Social Security to 70
- Couples who expect one spouse to outlive the other by many years — single filing brackets are much narrower
- People with modest pensions or no pensions
- Anyone who expects their income tax rate to be higher in the future than it is today
The window is less useful for:
- People still in their highest-earning working years (their current bracket is often the highest they'll ever be in)
- Retirees with such large pensions and Social Security that they'll be in the top brackets regardless
- People who expect to leave most of their traditional account to charity — those dollars pass tax-free anyway
- People in poor health who don't expect to live long enough for the Roth to grow
The most common mistakes.
Waiting too long.
The single most common mistake. People think about Roth conversions when they start getting their first RMD notice at 73. By then the window is essentially closed. The best time to start planning conversions is the year you retire.
Converting too much in one year.
A single large conversion often pushes into higher brackets, triggers IRMAA, and costs more per dollar than a series of smaller multi-year conversions would. The window exists so you can spread conversions across it — not so you can do one big one.
Ignoring IRMAA and ACA.
Both are step functions that can add thousands to the cost of a conversion. Both are commonly missed when people run "tax cost" calculations on their own.
Not projecting multiple years.
A single-year conversion analysis almost always understates the long-term value (or, sometimes, overstates it). The right question is: what does our tax picture look like over the next fifteen years with conversions versus without?
Converting without a plan for the tax.
The conversion is taxable in the year it happens. Failing to withhold enough — or failing to make an estimated payment — can trigger underpayment penalties. Coordinate the conversion timing with quarterly estimated tax payments.
When to think about it.
The best time to model a conversion is late fall — usually October or November of the year in question. By that point you have a reasonably clear picture of the year's income, capital gains, and deductions. You can convert an amount calibrated to a specific bracket target, then complete the conversion before December 31.
The best time to plan conversions is the year you retire, or the year before. Not the year you turn 72.