Every dollar you put into a 401(k), 403(b), 457, or traditional IRA came with a silent handshake: no tax now, full tax later, at whatever rate applies when it comes out. For thirty or forty working years, “later” was somebody else’s problem. Then you retire, and the calendar hands you something remarkable: the salary is gone, Social Security may not have started, and required minimum distributions — the IRS’s mandatory withdrawals from pretax accounts — don’t begin until age 73 (or 75 if you were born in 1960 or later).
Between those two dates, your taxable income can fall to nearly nothing while your wealth is at its peak. Retire at 63 and you may have a decade or more sitting in the lowest brackets you have seen since your first job. That is the window. We call it the tax mulligan — a do-over on decades of deferral decisions, granted once, with an expiration date printed on it.
*Illustrative: depends on retirement age, birth year, and when Social Security and pensions begin.
Measured Roth conversions. Each year in the window, you can deliberately move money from pretax accounts to a Roth IRA, paying tax now at the low bracket the window created — instead of later at whatever bracket forced withdrawals push you into. The craft is in the word measured: convert enough to fill the low brackets, and stop before the conversion itself spills into a higher one. Done annually for a decade, this quietly relocates a large share of your retirement money to an account that never produces taxable income again.
Harvesting the 0% capital-gains space. Federal law taxes long-term capital gains at 0% below an income threshold that a low-income retiree can actually stay under. In window years, appreciated taxable holdings can sometimes be sold — and even repurchased — realizing gains at a federal rate of zero and resetting the cost basis. (California taxes gains as ordinary income, so the state side needs its own math — another reason this is a planning exercise, not a rule of thumb.)
Charitable timing. From age 70½, qualified charitable distributions let you give directly from an IRA to charity — the gift counts against future RMDs and never appears in your income at all. If giving is already in your plans, routing it through the IRA in and after the window is often the single most tax-efficient dollar you will ever donate.
Defusing the widow’s penalty. This is the part couples rarely see coming. When one spouse dies, the survivor typically keeps most of the income — both IRAs, often both Social Security-sized cash flows replaced by the larger one — but files as a single taxpayer, with brackets that turn punishing at roughly half the income. Conversions done in the window, while two names are still on the return, are one of the few ways to shrink that future bill in advance.
Respecting the cliffs. Medicare premiums are means-tested: cross an IRMAA income threshold by a single dollar — based on your return from two years earlier — and both spouses’ premiums step up for a full year. The thresholds are published and indexed annually; the point is not to memorize them but to know they exist and steer conversions underneath them. This is precisely why “convert a big lump one year” usually loses to “convert a measured slice every year.”
The alternative to using the window is the RMD snowball. Left alone, a large pretax balance keeps compounding through your 60s and early 70s — which sounds like good news until the required-withdrawal schedule arrives and applies a growing percentage to a grown balance. Forced income arrives whether you need it or not, stacking on top of Social Security and pensions, pushing you into brackets you thought you retired away from, triggering Medicare surcharges, and — for couples — setting up the survivor for the single-filer squeeze at the worst possible moment. The deferral handshake gets honored either way. The only question is whether you choose the rate or the calendar does.
A couple retires at 63 with $1.8 million in pretax accounts and modest taxable savings to live on. From 63 to 72 their taxable income is a fraction of their working years’. Path A: do nothing; the pretax balance compounds for a decade, then RMDs begin on the larger sum and every withdrawal is taxed on top of two Social Security checks. Path B: convert a measured amount each year — enough to fill the lower brackets, stopping short of the next bracket and the Medicare thresholds — shifting a substantial share to Roth by 73. Same lifetime wealth, very different lifetime tax bill and a far softer landing for the surviving spouse. The specific numbers depend entirely on the household; the shape of the choice does not.
The mulligan matters most for three groups: households with large pretax balances relative to their other savings (the classic profile of career educators with 403(b)/457 accounts and business owners who deducted aggressively for decades); anyone retiring several years before RMD age, which is what creates the window in the first place; and couples doing survivor planning, where today’s joint brackets are a resource one spouse will eventually lose. If Social Security hasn’t started yet, the window is wider still — and the order in which you tap accounts becomes its own strategy, which is why we treat withdrawal sequencing as the companion discipline to conversion planning.
The mulligan is not a product and there is nothing to buy. It is arithmetic with a deadline: your projected brackets year by year from retirement to RMD age, the conversion amount that fills each year’s low brackets without tripping the cliffs, and a written sequence for which accounts fund your life in the meantime. It takes a tax return, your account statements, and an afternoon — and it should be coordinated with your CPA, because conversions are irrevocable once made. Start with the withdrawal-sequencing briefing and the 2026 tax guide, then bring your latest return to the conversation.