What a Roth conversion is
A Roth conversion moves money from a pre-tax retirement account — a traditional IRA or 401(k) — into a Roth IRA. The amount converted is added to taxable income in the year of the conversion. In exchange, that money and all future growth on it becomes permanently tax-free.
The mechanics are straightforward. The strategy question — whether to convert, how much, and when — is more involved.
Why the years just after retirement matter
For many people, the period between retirement and age 73 represents the lowest-income years of their financial life. Employment income has stopped. Social Security has not yet started, or has just begun. Required minimum distributions have not yet arrived. Taxable income is often lower than it was during the working years and lower than it will be once RMDs begin.
This window — sometimes called the Roth conversion corridor — is when converting pre-tax assets tends to be most efficient. The tax cost of converting is lower, and the long-term benefit of removing assets from the pre-tax pile is higher.
The Roth conversion decision is not whether converting is good or bad in the abstract. It is whether paying tax now at a known rate is preferable to paying tax later at an uncertain one — and how much of that trade-off makes sense in a given year.
How conversion amounts are determined
The goal is to convert enough to make meaningful progress on pre-tax balances without crossing into a higher tax bracket or triggering income-based surcharges. Three thresholds matter most.
| Threshold | What it affects | Why it matters for conversions |
|---|---|---|
| Federal tax bracket boundaries | Marginal tax rate on the converted amount | Converting to the top of the 22% bracket and stopping before 24% is a common anchor point |
| IRMAA income thresholds | Medicare Part B and D premiums (two years later) | Crossing an IRMAA tier adds hundreds to monthly Medicare costs — a conversion that crosses a tier may cost more than it saves |
| Social Security taxation threshold | Percentage of Social Security benefit subject to tax | Above $44,000 in combined income (married filing jointly), up to 85% of Social Security becomes taxable |
In practice, the optimal conversion amount in any given year is found by modeling taxable income with and without the conversion, checking each threshold, and finding the amount that makes progress without unnecessary cost.
The connection to required minimum distributions
Pre-tax IRA and 401(k) balances are subject to required minimum distributions beginning at age 73. RMDs are calculated as a percentage of the account balance and added to taxable income whether or not the money is needed. Larger pre-tax balances produce larger RMDs.
Roth conversions reduce pre-tax balances, which reduces future RMD obligations. This is one of the primary long-term reasons to convert even when the near-term tax cost seems modest — the alternative is potentially larger, unavoidable distributions later at whatever tax rates apply at that time.
Roth accounts are not subject to RMDs during the account owner's lifetime, which also preserves flexibility in later years.
What coordination with other decisions looks like
A Roth conversion does not happen in isolation. The converted amount affects taxable income, which affects Social Security taxation, IRMAA, capital gains rates, and the size of the standard deduction phase-outs. A conversion that appears efficient when viewed alone may look different when these interactions are accounted for.
This is why conversion decisions in a coordinated retirement plan are modeled alongside all other income sources for that year — and recalculated each year as the variables change.