Why the window exists at all
Three things usually stop at once when someone retires: salary, bonus, and the payroll deductions that came with them. What replaces them arrives later and in stages — Social Security when you claim it, required minimum distributions at 73, and pension income whenever it starts. In between, taxable income can fall to a fraction of what it was.
That gap is not a planning trick. It is an accident of how the rules are sequenced. But because the US tax system is progressive, low-income years are cheap years to deliberately recognize income — and a Roth conversion is simply choosing to recognize income now, at a rate you can see, instead of later at a rate you cannot.
The decision is not whether to pay tax on pre-tax savings. It is when, and at what rate. Doing nothing is also a choice — it just defers the rate to whatever Congress and your future required distributions decide.
What closes the window
- Required minimum distributions. Under SECURE 2.0, these begin at 73 for anyone reaching that age in 2023 or later, rising to 75 for those born in 1960 or after. Once they start, they add income you cannot decline.
- Claiming Social Security. Up to 85% of the benefit can become taxable depending on your other income, and the interaction is not linear.
- The death of a spouse. The survivor files single, often on similar income, and single brackets are roughly half as wide. This is the most commonly overlooked closure.
- A pension or annuity starting. Fixed income you cannot turn off narrows the room for anything voluntary.
How the arithmetic actually works
A conversion pays off when the rate you pay today is lower than the rate the money would have faced later. That is the whole test. Everything else — growth rates, time horizons, where the tax is paid from — changes the size of the answer, not its direction.
| Convert now | Leave it alone | |
|---|---|---|
| Tax paid | At today’s known rate | At a future unknown rate |
| Future growth | Tax-free | Tax-deferred, then taxed on withdrawal |
| Required distributions | None during your lifetime | From age 73 or 75 |
| Effect on heirs | Ten-year rule, tax-free withdrawals | Ten-year rule, taxable withdrawals |
Pay the tax from outside the account
Every dollar withheld from the conversion itself is a dollar that stops compounding tax-free — and if you are under 59½, withholding from the IRA can trigger a 10% early-withdrawal penalty on the portion used to pay the tax. Converting a smaller amount you can pay for from taxable savings usually beats converting a larger amount you cannot.
Size it to a bracket, not to a feeling
Converting an entire balance in one year is almost always the wrong move, because it drags the top of the conversion into brackets far above where it started. The usual approach is to fill a chosen bracket deliberately, year after year, and stop at its ceiling.
The second-order effects that catch people
Converted amounts count toward modified adjusted gross income, and MAGI drives more than income tax. Medicare Part B and D premiums are set by IRMAA on a two-year lookback, so a conversion at 63 can raise premiums at 65. For anyone receiving ACA premium subsidies before Medicare, the cliff is sharper still.
None of that makes conversions a bad idea. It makes unmeasured conversions a bad idea. These effects are knowable in advance, which is exactly why the sequence is worth modeling before the year ends rather than discovering it on a return.
It cannot be undone
The Tax Cuts and Jobs Act removed the ability to recharacterize a conversion from 2018 onward. Before that, a conversion could be reversed if markets moved against you. Now it is final for the tax year — which is the strongest argument for running the numbers first.
This article is general information, not individualized investment, tax, or legal advice, and it does not account for your circumstances. Tax rules referenced are those in effect at the date shown and change over time. Consult a qualified professional before acting.