When does a Roth conversion make sense?

What is a Roth conversion?

A Roth conversion moves dollars you saved pre-tax — in a traditional IRA or an old 401(k) — into a Roth IRA. The amount you convert is added to your taxable income for that year, and you pay ordinary income tax on it now.

In exchange, those dollars grow inside the Roth, where qualified withdrawals are tax-free under current law — generally once you’re 59½ and the account has been open five years. And a Roth has no required minimum distributions during your lifetime; the IRS never forces those dollars back out. (The RMD rules are here.)

Strip away the brochure language and a conversion is one decision: pay tax on those dollars today, at a rate you can see, or later, at whatever rate applies when they finally come out. Everything else is arithmetic on those two rates.

Why do the years before Social Security and RMDs get called “the window”?

Picture the year after your last paycheck. Salary: gone. Social Security: not started yet. Required minimum distributions: not until age 73 — or 75 if you were born in 1960 or later. For a stretch of years, your taxable income can drop to its lowest level since you started working.

That stretch matters because federal income tax runs through seven brackets under current law — 10%, 12%, 22%, 24%, 32%, 35%, and 37% — and each band of income pays only that band’s rate. The step that decides most retirement tax math is the third one: the rate jumps ten full points between the 12% and 22% brackets.

In a low-income year, those bottom brackets sit partly empty. A conversion sized to that empty space pays tax at 10% or 12% on dollars that might otherwise be taxed at a higher rate later, once RMDs stack on top of Social Security and refill your brackets every year, for life.

The logic of the window is exactly that simple: the same dollars, converted in different years, can carry meaningfully different tax bills. But notice the word might. Whether your later rate really is higher is the entire question — and the honest answer is, not for everyone.

When does a conversion actually pay for itself?

One comparison decides the whole thing: the rate you’d pay on the conversion today versus the rate you — or your heirs — would pay on the same dollars later. Pay 12% now to avoid 22% later, and the conversion earns its keep. Pay 24% now to avoid 12% later, and you’ve volunteered for a tax increase.

Plenty of retired households come in lower than they expect. Bracket thresholds adjust upward for inflation every year, and retirement income is usually smaller than a working salary. If your rate in retirement will match or undershoot today’s, converting can simply cost you money. Sometimes the right-sized conversion is a small one — and sometimes the honest answer is none at all.

Three more pieces of the math get skipped in most pitches. The conversion tax is best paid from money outside the IRA — pay it out of the converted amount, and less reaches the Roth. The dollars you send to the IRS stop compounding for you the day you send them. And a conversion is permanent: the IRS eliminated the option to undo one for conversions made after 2017.

One factor can flip a “don’t convert” into a “convert”: your heirs. Under the inherited-IRA rules, most non-spouse heirs must empty the account within 10 years. A traditional IRA arrives as taxable income stacked on top of an adult child’s peak earning years; an inherited Roth arrives income-tax-free once the account’s five-year clock has run. A conversion can be roughly a wash for you and still leave your children meaningfully better off — run both sets of numbers.

Why does being married change the math?

The married-filing-jointly brackets are roughly twice as wide as a single filer’s at most levels under current law. Two of you, double the room: a conversion that would shove a single filer into the next bracket can fit comfortably inside a couple’s current one.

But that width is on loan. When one spouse dies, the survivor files as single from the following year — similar income, brackets roughly half as wide. Every dollar still sitting in the traditional IRA at that point gets taxed on the narrower single schedule, either as the survivor’s RMDs or as the heirs’ inherited income.

So for a married couple, the conversion window carries a second deadline no one can see in advance. Converting while you’re both alive and filing jointly uses bracket width that doesn’t survive either one of you. The same filing-status switch hits Medicare premiums even harder — the widow’s IRMAA cliff walks through those numbers.

How can one big conversion raise your Medicare premiums?

Medicare sets your Part B and Part D premiums using your income from two years earlier — this year’s premium is based on the tax return you filed two years back. Above set income thresholds, a surcharge called IRMAA is added. And the thresholds are cliffs, not ramps: one dollar over a line triggers the full next tier’s surcharge.

A Roth conversion counts, in full, toward that income figure in the year you convert. Convert a very large amount in one year, and the bill can arrive twice — once from the IRS the following April, and again two years later as a higher Medicare premium every month. Because of the two-year lookback, conversions from age 63 onward can already shape your very first premiums at 65.

None of that says don’t convert. It says size each year’s conversion with both lines in view — the top of your current tax bracket and the Medicare income threshold for your filing status. Several measured conversions spread across the window often do the same job as one heroic conversion, without the premium spike two years later.

So what’s the first real step?

Put your own numbers on one page: this year’s expected taxable income, the distance to the top of your current bracket, and where the Medicare threshold sits for your filing status. Those distances are your conversion room for the year — a measured answer, not a guess, and one that resets every year the window stays open.

A conversion decision doesn’t live alone, either. It interacts with your withdrawal order, your Social Security timing, and everything else in a coordinated retirement income plan — which is exactly why it belongs inside one.

And because a conversion is a tax decision, run the final call past your own tax adviser or CPA before you convert a dollar — Asset Lift doesn’t prepare tax returns or give tax advice; we build the retirement plan the conversion has to serve. The window is real, it’s measurable, and for many households it’s standing open right now. The measured move is to find out whether yours is.