Roth Conversion Calculator
Work out the tax cost of converting traditional IRA money to a Roth in 2026, how much of your current bracket is left, and whether the conversion pays off long term.
How to use this calculator
- 1Enter the amount you are thinking of converting and your other taxable income for the year.
- 2Check the room left in your current bracket — converting up to that line and no further is usually the most efficient approach.
- 3Set the rate you expect to pay in retirement. If it is higher than the effective rate shown, the conversion is likely worth doing.
- 4Keep "pay tax from outside" on if you can fund the tax from savings, since that is what makes conversions work.
How the calculation works
Conversion tax = tax(income + conversion) − tax(income). Roth = conversion × (1 + r)^n. Traditional = conversion × (1 + r)^n × (1 − future rate) + side account- conversion tax
- The real marginal cost, computed across every bracket the conversion touches
- side account
- What the tax money would have grown to if it had been invested instead of paid to the IRS
- future rate
- The rate you expect to pay on traditional withdrawals in retirement
The comparison is only fair if the tax paid today is accounted for. Paying it from outside savings means that money is no longer invested, so the traditional side is credited with a side account representing what it would have grown to.
The break-even is simply the effective rate on the conversion. If you expect to pay more than that in retirement, converting wins; if less, it does not. Everything else is refinement.
Because it is a marginal calculation across real brackets, converting exactly enough to fill a bracket without spilling into the next is usually the optimal amount — which is why the remaining room in your current bracket is shown.
Worked example
Converting $50,000 on $90,000 of income
- 1.$90,000 of income less the $16,100 standard deduction leaves about $73,900 of taxable income, inside the 22% bracket.
- 2.The 22% bracket runs to $105,700, so roughly $31,800 of room remains before the 24% bracket begins.
- 3.A $50,000 conversion therefore spills over: part taxed at 22%, the rest at 24%.
- 4.Converting about $31,800 this year and the balance next year would keep the whole amount at 22%.
Result: A blended rate slightly above 22%, avoidable by splitting across two years
What a conversion is actually buying
A Roth conversion moves money from a traditional IRA or 401(k), where it has never been taxed, into a Roth, where it will never be taxed again. The price is that the converted amount is added to this year's taxable income and taxed at ordinary rates.
The decision reduces to one comparison: the rate you pay now against the rate you would have paid later. If you convert at 22% and would have withdrawn at 32%, you have won ten percentage points on that money and all its future growth. If you convert at 32% and would have withdrawn at 12%, you have simply prepaid tax at a worse rate. Everything else — market timing, growth assumptions, account balances — is secondary to that comparison.
This is why conversions cluster in specific windows: the years between retiring and claiming Social Security, a year of low income, a sabbatical, or early retirement before required minimum distributions begin. In those years income is temporarily low, and a conversion fills the empty brackets cheaply.
Bracket filling, and why the amount matters more than the decision
The most common mistake is treating conversion as a yes-or-no question. It is a how-much question. Because brackets are progressive, converting a large amount in one year pushes the later portion into higher and higher rates, while spreading the same total across several years can keep all of it at the lowest available rate.
The technique is called bracket filling: convert exactly enough to reach the top of your current bracket and stop. This calculator reports the remaining room directly, because that figure is usually the right answer for how much to convert.
Several thresholds beyond the rate brackets deserve watching, because a conversion can cross them and trigger costs that dwarf the rate difference.
- IRMAA — Medicare Part B and D surcharges are set by income from two years earlier, and the brackets are cliffs rather than ramps. A conversion that crosses one by a single dollar can add over a thousand dollars a year of premiums per person.
- Social Security taxation — the proportion of benefits subject to tax rises with other income, so a conversion can increase tax on benefits as well as on itself.
- The Net Investment Income Tax — a conversion is not itself investment income, but it raises modified AGI, which can pull other investment income above the 3.8% surtax threshold.
- Capital gains rates — the 0% long-term gains bracket is generous, and a conversion that fills it can push gains that would have been tax-free into the 15% band.
Rules worth knowing before converting
Three mechanical points cause most of the trouble in practice.
The pro-rata rule is the big one. If you hold any pre-tax money in any traditional IRA, a conversion cannot be cherry-picked from after-tax contributions alone — every conversion is treated as proportionally pre-tax and after-tax across all your traditional IRAs combined. This is what defeats naive backdoor Roth attempts by people with existing rollover IRAs, and it produces unexpected tax bills.
Each conversion also starts its own five-year clock. Converted amounts withdrawn within five years, before age 59½, face a 10% penalty on the converted principal even though the tax was already paid. Money you might need within five years is not a good conversion candidate.
Finally, conversions can no longer be undone. Recharacterisation of a conversion was eliminated in 2018, so a conversion made in a year that turns out badly — a market fall, an unexpected bonus, a windfall — is permanent. That argues for converting late in the year when income is known, rather than early on an estimate.
What this assumes, and where it stops
Assumptions
- Federal tax only, using the 2026 schedule and the standard deduction.
- The full converted amount is taxable — no after-tax basis in the traditional IRA.
- A single constant growth rate applies to both accounts, and the retirement tax rate entered applies to the whole withdrawal.
- Where tax is paid from outside funds, the side account is taxed only on its growth at the future rate.
Limitations
- State income tax is not modelled and can materially change the answer, especially if you expect to retire in a different state.
- IRMAA surcharges, Social Security benefit taxation and the Net Investment Income Tax are described but not calculated.
- The pro-rata rule is not applied — if you hold pre-tax money in any traditional IRA, your real taxable amount may be higher than shown.
- A single flat retirement tax rate is a simplification; real withdrawals are taxed through brackets, often at a lower effective rate than the marginal one entered.
Common questions
How much should I convert in one year?
Usually enough to fill your current bracket and no more. Because rates are progressive, converting past the top of a bracket taxes the excess at the next rate up, so spreading a large conversion over several years often costs substantially less than doing it at once. This calculator shows exactly how much room is left in your current bracket, which is typically the right conversion amount for the year.
When does a Roth conversion not make sense?
When you expect to pay a lower rate in retirement than you would pay converting now — which is common for high earners in their peak years who will retire on much less income. It is also weak if you must pay the tax out of the converted money itself, if you might need the money within five years, or if you are close to a Medicare IRMAA threshold where crossing it costs more than the rate saving is worth.
What is the five-year rule on conversions?
Each conversion starts its own five-year clock. If you withdraw converted principal within five years and are under 59½, a 10% penalty applies to that amount even though you already paid income tax on it when converting. Multiple conversions mean multiple clocks running in parallel. The practical implication is simple: do not convert money you may need in the next five years.
What is the pro-rata rule and why does it catch people?
If you hold any pre-tax money in any traditional IRA, you cannot choose to convert only after-tax contributions. Every conversion is treated as proportionally pre-tax and after-tax across all your traditional IRAs combined, so part of it is taxable however the transaction is arranged. This is what defeats backdoor Roth attempts by people with an existing rollover IRA, and it regularly produces tax bills that were not planned for.
Sources
- Roth IRAs — US Internal Revenue Service
- Publication 590-A, Contributions to Individual Retirement Arrangements — US Internal Revenue Service
- Publication 590-B, Distributions from Individual Retirement Arrangements — US Internal Revenue Service
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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