You have a tax-deferred account. Maybe it’s a 401(k) from a previous employer, a rollover IRA, or a traditional IRA you’ve been feeding for years. The balance is real, and so is the question: should you convert some of it to a Roth IRA now, pay the tax bill with outside money, and let the rest grow tax-free? The answer depends on a comparison that feels slippery—your tax rate today versus your tax rate in retirement. But retirement tax rates are not a single number. They are a series of guesses about future income, future tax law, and future you. This article walks through a framework for comparing a Roth conversion against that uncertainty, using specific dollar amounts, time horizons, and a patient, numbers-first approach. We’ll look at marginal tax brackets, the Social Security tax torpedo, Medicare premium cliffs, and the quiet power of tax diversification. By the end, you’ll have a repeatable method for testing whether a conversion makes sense in your own numbers—even when the future refuses to give you a straight answer.
Start With the Only Rate You Actually Know
Before you model anything, anchor yourself in the present. A Roth conversion adds the converted amount to your ordinary income for the year. That means the tax cost is your marginal rate—the rate on the next dollar of income—not your average or effective rate. If you are a married couple filing jointly with $120,000 of taxable income in 2025, you are in the 22% federal bracket. Converting $30,000 would push $30,000 into that 22% space (and possibly a sliver into 24% if you cross the threshold). State taxes sit on top. A California resident might pay 9.3% on that same income, making the total marginal cost 31.3%. That is the number you are betting against: 31.3 cents per dollar converted.
Write that number down. It is the only piece of this decision you can know with certainty. Everything else is a range of possibilities, and your job is to see how often the future rate beats that number—and by how much.
Build a Retirement Income Picture, Not a Single Bracket Guess
Many people assume their tax rate will be lower in retirement because they won’t have a paycheck. That can be true, but it is not automatic. A married couple with $40,000 of Social Security, $30,000 of pension income, and $50,000 of required minimum distributions (RMDs) from traditional IRAs could have a surprisingly high marginal rate—especially when Social Security taxation phases in. The key is to model your provisional income and see where each additional dollar of IRA withdrawal lands.
Provisional income is adjusted gross income plus tax-exempt interest plus half of Social Security benefits. For a married couple, if provisional income exceeds $44,000, up to 85% of Social Security benefits become taxable. That creates a zone where an extra dollar of IRA withdrawal causes more than a dollar of taxable income, pushing the marginal rate well above the stated bracket. A couple in the 12% bracket can face a marginal rate of 22.2% or even 40.7% on a narrow band of income. If your future RMDs will push you into that zone, a Roth conversion today at 22% or 24% can look like a bargain.
Map Your Future Income Sources
Grab a spreadsheet or a piece of paper. List the income you expect in your first full year of retirement—or the year RMDs begin, whichever is later. Include:
- Social Security benefits (use your most recent statement)
- Pension income, if any
- Rental or business income
- Part-time work you actually plan to do
- RMDs from traditional IRAs and 401(k)s (roughly 4% of the balance at age 73, rising with age)
- Interest, dividends, and capital gains from taxable accounts
Now calculate the tax on that income using current brackets. Then add $10,000 of IRA withdrawals and see what happens to the tax bill. The increase divided by $10,000 is your projected marginal rate. If that rate is higher than your rate today, a conversion now can save you money. If it is lower, converting now would be a net cost. But don’t stop there—uncertainty means you need to test a few scenarios.
Stress-Test the Variables That Matter Most
Future tax rates are not a single number. They are a distribution shaped by four big unknowns: your portfolio growth, your spending needs, future tax law, and your lifespan. You cannot predict any of them perfectly, but you can build a small set of scenarios that bracket the likely outcomes.
1. Portfolio Growth and RMDs
If your traditional IRA grows faster than expected, RMDs will be larger, pushing you into higher brackets. A 65-year-old with $800,000 in a traditional IRA might see it grow to $1.4 million by age 75 at a 6% return—or $2.1 million at 8%. The RMD at 75 on $2.1 million is about $85,000, compared to $57,000 on $1.4 million. That difference alone can shift the marginal rate from 22% to 24% or higher, especially when stacked on Social Security. Run your numbers with a low-return, medium-return, and high-return assumption. See where the marginal rate lands in each case.
2. Spending Shocks and the Surviving Spouse
Retirement spending is not a flat line. A new roof, a health event, or a decision to help a child with a home down payment can spike withdrawals in a single year, pushing you into a higher bracket. More importantly, when one spouse dies, the survivor files as a single taxpayer. The same RMDs now face narrower brackets and a lower standard deduction. A couple in the 12% bracket can easily see the survivor land in the 22% or 24% bracket. A Roth conversion done while both are alive and filing jointly can reduce future RMDs and soften that jump.
3. Tax Law Changes
The Tax Cuts and Jobs Act of 2017 lowered individual rates through 2025. Unless Congress acts, brackets revert to pre-2018 levels in 2026. The 22% bracket becomes 25%, the 24% bracket becomes 28%, and the 12% bracket becomes 15%. If you are considering a conversion, the window between now and the end of 2025 offers historically low rates. That does not mean you should convert everything—it means the math tilts slightly more in favor of converting some dollars now, especially if you are in the 22% or 24% bracket today and expect to be in a similar or higher bracket later.
4. Medicare Premium Cliffs
Modified Adjusted Gross Income (MAGI) determines your Medicare Part B and Part D premiums two years later. For 2025, if your MAGI exceeds $206,000 (married filing jointly), you pay an extra $419.30 per month per person for Part B—that’s over $5,000 per year per person in additional premiums. A large Roth conversion can push you over that cliff, effectively adding a hidden surtax to the conversion. The cliff is steep: one dollar over the threshold triggers the full surcharge. If you are near a Medicare tier, it may be smarter to convert up to the edge of that tier rather than crossing it.
A Repeatable Framework for the Conversion Decision
Here is a step-by-step method you can use each year, ideally in November or December when you have a clear picture of your current-year income.
- Calculate your current-year marginal rate. Include federal, state, and any phaseouts (child tax credit, IRMAA, net investment income tax). This is your cost to convert.
- Estimate your future marginal rate in three scenarios: baseline (expected returns and spending), high-return (portfolio outperforms), and low-return (portfolio underperforms). For each, include RMDs, Social Security, and any other income.
- Check the surviving-spouse scenario. Run the numbers as if one spouse passes away at 78, leaving the other with the same RMDs but filing single.
- Compare the current rate to the future rates. If the current rate is lower than the future rate in most scenarios, a conversion likely adds value. If the current rate is higher in most scenarios, a conversion likely destroys value. If the rates are close, consider converting a smaller amount to hedge.
- Watch for cliffs. IRMAA tiers, Social Security taxation thresholds, and the net investment income tax threshold ($250,000 married filing jointly) can make the effective marginal rate much higher than the statutory bracket. Stay just below these cliffs unless the long-term math is overwhelmingly in your favor.
Example: The Smiths at Age 62
John and Mary Smith are both 62, recently retired, with $1.2 million in traditional IRAs. They have not started Social Security and have no pension. Their taxable income from part-time work and dividends is $60,000, putting them in the 12% bracket. They are considering converting $50,000 this year, which would fill the 12% bracket and spill $10,000 into the 22% bracket. Their blended federal rate on the conversion is about 14%. State tax adds 5%, for a total of 19%.
They model their RMDs starting at age 73. With 6% growth, their IRA reaches $2.2 million, producing an RMD of about $88,000. Add $50,000 of Social Security (taxable portion around $42,500) and their taxable income is roughly $130,500, landing in the 22% bracket. In the surviving-spouse scenario, the same RMD plus Social Security pushes the single filer into the 24% bracket. In both cases, the future rate is higher than the 19% they would pay today. A conversion now—even up to the top of the 22% bracket—looks like a net win. They decide to convert $100,000, paying $22,000 in federal tax from a taxable brokerage account, and repeat the analysis each year.
Why Tax Diversification Matters More Than Precision
You will never know the exact optimal conversion amount. Tax law changes, market returns vary, and life throws curveballs. What you can control is the shape of your accounts. Having money in three tax buckets—tax-deferred, Roth, and taxable—gives you flexibility to manage your taxable income in retirement. If RMDs are pushing you into a higher bracket, you can pull from Roth to cover the gap. If you need a large lump sum for a one-time expense, you can take it from Roth without triggering a tax spike or Medicare surcharge. If you want to leave tax-free money to heirs, Roth is the most efficient vehicle.
This is the quiet power of tax diversification. It is not about perfectly predicting your future bracket. It is about building a portfolio that can adapt to whatever bracket you end up in. A Roth conversion is one tool to get there. Even a partial conversion—filling the 12% or 22% bracket in a low-income year—can shift the balance meaningfully over a decade. For a deeper look at how small, consistent moves add up, see What a Ten-Dollar Weekly Bump Actually Does to Your Retirement Number. The same compounding logic applies to tax savings: a conversion that saves 10% on $50,000 of future withdrawals puts an extra $5,000 in your pocket, which can then compound for decades.
When a Conversion Is Likely a Bad Idea
Not every conversion makes sense. Here are clear cases where the math usually works against you:
- You are in your peak earning years. If your marginal rate today is 32% or higher, and you expect to be in the 22% or 24% bracket in retirement, converting now locks in a higher tax bill. Wait until you have a lower-income year—perhaps after retiring but before RMDs and Social Security begin.
- You would need to pay the conversion tax from the IRA itself. Withdrawing $30,000 to convert and using $6,600 of it to pay the tax means only $23,400 goes into the Roth. That is almost always worse than leaving the money in the traditional IRA, because you lose the compounding on the $6,600. Pay the tax from a taxable account or don’t convert.
- You are within two years of needing Medicare and the conversion would push you over an IRMAA cliff. The surcharge can be larger than the long-term tax savings. Run the numbers carefully.
- You plan to leave the IRA to charity. Qualified charitable distributions from a traditional IRA can satisfy RMDs tax-free after age 70½. Converting those dollars to Roth would mean paying tax unnecessarily.
FAQ: Roth Conversions and Future Tax Uncertainty
How do I know if my future tax rate will be higher than my current rate?
You cannot know for certain, but you can build a reasonable estimate. Start with your expected retirement income sources—Social Security, pensions, RMDs, part-time work—and calculate the tax using current brackets. Then test a few scenarios: higher portfolio returns, lower returns, and the surviving-spouse case. If your projected marginal rate is higher than your current rate in most scenarios, a conversion is worth considering. If it is lower in most scenarios, a conversion probably does not make sense. The goal is not precision; it is to see whether the odds tilt in your favor.
What is the Social Security tax torpedo, and how does it affect Roth conversions?
The Social Security tax torpedo refers to the way additional income can make more of your Social Security benefits taxable, creating a marginal tax rate that is higher than your tax bracket. For example, a married couple in the 12% bracket can face a marginal rate of 22.2% or even 40.7% on a narrow range of income because each extra dollar of IRA withdrawal causes $0.85 or $0.50 of Social Security benefits to become taxable. If your future RMDs will push you into this zone, a Roth conversion today—at a lower marginal rate—can reduce future RMDs and help you avoid the torpedo.
Should I convert my entire IRA to Roth in one year?
Almost never. Converting a large IRA all at once would push most of the conversion into the highest tax brackets, trigger Medicare surcharges, and possibly cause underpayment penalties. A better approach is to convert incrementally over several years, filling the lower brackets each time. For example, a married couple with $60,000 of other income might convert up to the top of the 22% bracket ($201,050 in 2025) each year, spreading the tax bill over a decade. This keeps the marginal rate on each conversion dollar as low as possible.
Does a Roth conversion make sense if I might move to a lower-tax state in retirement?
It can, but you need to weigh the state tax savings against the federal math. If you live in California (9.3% state tax) and plan to retire to Florida (0% state tax), converting now means paying state tax you could avoid later. In that case, the federal rate comparison needs to be even more favorable to justify the conversion. Run the numbers with and without state tax to see the breakeven point. Sometimes it is better to wait until you have relocated.
The Bottom Line
Comparing a Roth conversion against future tax bracket uncertainty is not about finding a perfect answer. It is about building a decision process you can repeat, using the numbers you have today and reasonable guesses about tomorrow. Start with your current marginal rate. Model your future income in a few scenarios. Watch for cliffs. Pay the tax from outside money. And remember that tax diversification—having money in different tax buckets—is often worth more than a precisely optimized conversion amount. The goal is not to pay the least tax in any single year. It is to keep more of what you’ve saved over your entire retirement, and to give yourself options when life does not follow the spreadsheet.