A Roth conversion moves money from a tax-deferred account into a Roth account, and you pay ordinary income tax on the amount converted in the year you do it. The question is never whether conversions are good in the abstract. It is whether paying tax now, at a rate you know, beats paying it later, at a rate you do not.
The core comparison
Strip away the complexity and a conversion is a bet on one variable: your marginal rate today versus your marginal rate when the money would otherwise have come out. Convert when today’s rate is lower. Don’t when it is higher. Everything else is refinement.
What makes this actionable is that many households have a predictable window where their rate is temporarily low — and they can see it coming years in advance.
The window most people miss
Consider a couple who retires at 63. Between 63 and the year Social Security begins, their taxable income may be modest. Required minimum distributions have not started. For several years they may sit in the 12% or 22% bracket, with room before the next threshold.
Once RMDs begin and Social Security is flowing, that same couple may find themselves pushed into a higher bracket permanently — by withdrawals they are forced to take, from an account that has continued to grow. Filling the lower brackets during the gap years converts money at 22% that would otherwise come out at 24% or more, every year, for life.
Effects beyond the bracket
Conversion income is ordinary income, and it ripples:
- IRMAA surcharges. Medicare premiums step up at specific income thresholds, based on your return from two years prior. Crossing a threshold by a single dollar triggers the full surcharge.
- Taxation of Social Security. Additional income can increase the share of benefits subject to tax, producing effective marginal rates well above the nominal bracket.
- Capital gains stacking. Long-term gains sit on top of ordinary income. Conversion income can push gains from the 0% rate into 15%.
- ACA premium credits. For pre-Medicare retirees on a marketplace plan, conversion income can sharply reduce subsidies.
This is why conversions are sized to a target — a bracket ceiling, an IRMAA threshold, a credit cliff — rather than done in one large move.
Conditions that favor converting
- You are in a temporary low-income year: early retirement, a sabbatical, a business loss.
- Markets are down, so the same number of shares converts at a lower tax cost.
- You can pay the tax from taxable funds rather than from the conversion itself. Paying from outside preserves the full amount inside the Roth and is what makes the math work.
- You expect to leave assets to heirs in higher brackets, who would otherwise inherit a tax-deferred account subject to the ten-year distribution rule.
- You want to reduce future RMDs, which Roth accounts do not have.
Conditions that argue against
Converting makes little sense if you expect a materially lower rate later, if you would have to pay the tax out of the converted amount while young enough for that to compound painfully, if you are charitably inclined and plan to use qualified charitable distributions from the IRA, or if a large conversion would breach a cliff — IRMAA, ACA — that costs more than the bracket arbitrage gains.
A conversion is a multi-year strategy executed in annual increments, sized against a specific threshold, and checked before year end when the year’s actual income is known.
Run the projection before December, not after. Conversions cannot be undone — recharacterization of conversions was eliminated in 2018 — so the sizing decision has to be right the first time.
