A Roth conversion is not one decision. It is one decision that quietly changes four others.
Converting moves money from a pre-tax account to a Roth account and adds the converted amount to your taxable income for that year. That single change ripples into your Medicare premiums, how much of your Social Security is taxed, what rate your capital gains are taxed at, and what you owe your state. Run the conversion first and check those afterward, and you can turn a sound long-term move into a costly one.
This checklist runs in the order the decisions actually depend on each other.
Step 1 — Establish your baseline income before the conversion
Before you model any conversion amount, write down what your income already looks like without it: pensions, Social Security, required minimum distributions, interest, dividends, realized capital gains, and any part-time earnings.
The conversion amount stacks on top of this baseline. Two households converting the same dollar amount can land in completely different places because their starting points differ. The baseline is what determines how much headroom you actually have.
Step 2 — Identify which tax bracket you are filling, and where it ends
A conversion is most often used to fill up the remainder of a bracket you are already in, rather than to push into the next one. Find the top of your current bracket and subtract your baseline income. That difference is your working room.
This is also where multi-year planning starts to matter. A series of smaller conversions across several years frequently produces a lower lifetime tax cost than one large conversion, because each year uses only the lower brackets.
Step 3 — Check the Medicare IRMAA thresholds before you commit
This is the step most often skipped, and the most expensive one to miss.
Medicare charges higher Part B and Part D premiums to households above certain income levels. The surcharge is called IRMAA, the Income-Related Monthly Adjustment Amount. In 2026 it begins at $109,000 of modified adjusted gross income for a single filer and $218,000 for a married couple filing jointly.
Two features make this hazardous during a conversion year.
It is a cliff, not a slope. Crossing a threshold by one dollar moves you into the entire next tier. There is no partial step.
It looks back two years. Your premiums are set using the return you filed two years earlier. A conversion done this year shows up in your Medicare premiums two years from now, long after the conversion feels finished.
At the highest tier, the combined Part B and Part D surcharge reaches $6,936 per year. A conversion that clears a threshold by a small margin can cost several thousand dollars in premiums that no one budgeted for.
Step 4 — Model the effect on how your Social Security is taxed
The share of your Social Security benefit subject to federal income tax depends on your other income. Adding a conversion to that calculation can increase the taxable portion of a benefit you are already receiving.
The practical result is that the conversion is taxed and it simultaneously increases tax on income you were already collecting. Model both together, never separately.
Step 5 — Check what it does to your capital gains rate
Long-term capital gains are taxed at rates that depend on your taxable income. Because a conversion raises taxable income, it can push gains that would have been taxed at a lower rate into a higher one.
If you were also planning to realize gains, whether selling appreciated stock, rebalancing a concentrated position, or selling property, sequence those decisions against the conversion rather than treating them as unrelated.
Step 6 — Add your state's treatment
State income tax rules on conversions vary widely. Some states tax the converted amount as ordinary income, some offer retirement income exclusions, and some have no income tax at all.
If you are considering relocating, or you split the year between two states, the timing of a conversion relative to your residency can change the result substantially. Confirm the rules for the state that will actually claim you in the conversion year.
Step 7 — Confirm where the tax payment comes from
Paying the conversion tax from the converted funds themselves reduces the amount that ends up growing in the Roth, and if you are under 59 and a half the withheld portion may be treated as a distribution.
Paying from outside funds keeps the full converted amount invested. This is a small mechanical detail that materially changes the outcome, and it is decided before the conversion, not after.
Step 8 — Write down the deadline
Roth conversions must be completed within the calendar year. There is no equivalent of the April filing-season grace period that applies to IRA contributions. A December conversion counts for that year. January 2nd counts for the next one.
Because the year's final income picture is not clear until late in the year, many households do the analysis in the fall and execute in December, early enough to avoid custodian processing delays.
The order matters more than the arithmetic
Most conversion mistakes are not calculation errors. They are sequencing errors: the conversion happened, and only afterward did someone check the IRMAA threshold, the Social Security calculation, or the capital gains stacking.
Run steps 1 through 8 in order, and the arithmetic tends to take care of itself.