Tools/Tax & estate/Roth Conversion Calculator✓ CHECKED AGAINST WORKED EXAMPLES · SEP 29, 2026

How much should I convert to a Roth, and what will it really cost?

See the 2026 tax a Roth conversion adds when it also makes more of your Social Security taxable and can push you over a Medicare premium line, and how much room you have before each one.

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Other income this year, before the conversion$An example, not your numbers: pension, IRA withdrawals, interest and dividends, before any Social Security. Replace it with yours.Social Security benefits this year$An example: the year’s total before Medicare premiums are taken out. Use 0 if you have not started.Amount to convert$An example. The whole amount is treated as pre-tax money and taxable this year.Pre-tax IRA or 401(k) balance$An example. It only sets how many years converting at this pace would take.

Filing status

Taxpayers 65 or older this year

Adds the extra standard deduction and, through 2028, the senior deduction, which shrinks as income rises.

People paying Medicare premiums

The Medicare surcharge is charged per person, once they are on Medicare.

ROTH CONVERSION TAX

$3,900

Converting $20,000 adds $3,900 of federal tax this year, 19.5% on average and 23.3% on the next dollar, because it also makes $11,650 more of your Social Security taxable.

Average rate

19.5%

Next dollar

23.3%

Medicare surcharge

Years to convert all

UNDERSTAND YOUR RESULT

LIBRARY CHAPTERHow Tax Brackets Really WorkWhy a raise never leaves you worse off under the federal income tax, how the 2026 brackets apply in layers, the difference between your marginal and effective rate, and what a deduction is really worth.LIBRARY CHAPTERBeneficiary DesignationsWhy the form on a retirement account or policy overrides your will, how to name primary and contingent beneficiaries, the rules for spouses, minors and trusts, and how the 10-year rule taxes an inherited account.

Terms:Roth conversion ladder Roth versus traditional contributions Beneficiary designation IRMAA (income-related monthly adjustment amount)

What the next dollar of conversion costs, by how much you convert

Tax on the next dollar converted

With no conversion, the next dollar of income costs 22.2%; at your $20,000 it costs 23.3%. Where the rate is higher than the bracket, Social Security is being pulled into taxable income until 85% of it is taxed; the rate falls back to the bracket rate after that. The line steps up at each bracket top and adds 6% of the bracket rate while the senior deduction is shrinking.

How much you can convert before each line

Stop at	Convert up to	Tax added	Average rate
Top of the 10% bracket	Already past	—	—
Top of the 12% bracket	$19,050	$3,684	19.3%
Top of the 22% bracket	$71,245	$15,850	22.2%
Top of the 24% bracket	$164,425	$38,908	23.7%
Top of the 32% bracket	$218,875	$56,332	25.7%
First Medicare line ($109,000)	$53,500	$11,712	21.9%

Converting up to the top of a bracket keeps the conversion out of the next one. These amounts count the extra Social Security taxed and the senior deduction lost as you convert, so they can be smaller than a bracket top minus your income. Past the first Medicare line, the whole tier's surcharge applies to every person on Medicare.

Medicare premium tiers (2026 amounts)

Income (MAGI)	Added per person, a year
Up to $109,000 (yours)	$0
Over $109,000 to $137,000	$1,148
Over $137,000 to $171,000	$2,885
Over $171,000 to $205,000	$4,620
Over $205,000 to $500,000	$6,355
$500,000 or more	$6,936

With the conversion your income is $75,500, in the lowest tier; you would have $53,500 of conversion room before the first line. Premiums are set from your tax return two years earlier, so a conversion now affects premiums in 2028, and the lines rise with inflation until then. The standard Part B premium is $202.90 a month in 2026.

What moves the needle

Each row re-runs the calculation with one change. Click to apply.

How it's computed

FORMULA

Income with the conversion = other income + conversion + taxable Social Security

Provisional income = other income + conversion + ½ benefits; taxable benefits = the smaller of 85% of benefits and (50% of the excess over the base amount, then 85% of the excess over the adjusted base amount + the smaller of ½ benefits or ½ the gap), with base amounts $25,000 / $34,000 single and $32,000 / $44,000 joint

Federal tax = 2026 brackets on income − the standard deduction (with the aged addition) − the senior deduction (each $6,000, reduced by 6% of income over $75,000 single / $150,000 joint)

Tax added = tax with the conversion − tax without it; next-dollar rate = tax on the next $100 converted ÷ 100

Medicare surcharge = people × 12 × (Part B + Part D monthly amount for the tier that your income (MAGI) reaches)

The conversion is treated as entirely pre-tax money, fully taxable in the year it happens, with the tax paid from outside the account. Other income is $30,000 and Social Security $30,000; nothing else is on the return (no gains, no state tax, no tax-exempt interest, no ACA subsidy).
Up to 85% of Social Security can be taxable (26 U.S.C. § 86; the base amounts are not indexed). A conversion adds to income before benefits are counted, so it can raise the tax on the next dollar to as much as 1.85 times the bracket rate.
The senior deduction ($6,000 per person 65 or older) and the extra standard deduction apply for tax years 2025 through 2028. The senior deduction shrinks by 6 cents for each dollar of income over $75,000 ($150,000 joint), which adds to the cost of a conversion in that range.
Medicare premiums (IRMAA) use the 2026 lines and amounts from CMS. Your premium year is two years after the conversion and the lines are indexed, so treat the surcharge as an estimate; each line is a cliff, and one dollar over adds the whole tier.
Nothing here tells you how much to convert: it shows the cost of the amount you enter and the room before each line. The amounts are examples until you replace them.

WORKED EXAMPLE · SAMPLE NUMBERS

Without a conversion, $30,000 of benefits and $30,000 of other income leave $13,850 of the benefits taxable, income $43,850 and tax $2,116. Converting $20,000 makes $25,500 of the benefits taxable, income $75,500 and tax $6,016: $3,900 more, 19.5% of the conversion. The next $100 costs $23.32.

SOURCES

[1]26 U.S. Code § 86: Social security and tier 1 railroad retirement benefitsLegal Information Institute, Cornell Law School [2]Topic no. 423, Social Security and equivalent railroad retirement benefitsInternal Revenue Service [3]Publication 915, Social Security and Equivalent Railroad Retirement BenefitsInternal Revenue Service [4]2026 Medicare Parts A & B Premiums and Deductibles (income-related monthly adjustment amounts)Centers for Medicare & Medicaid Services, November 2025 [5]Roth IRAs (conversions)Internal Revenue Service [6]Rev. Proc. 2025-32: 2026 inflation-adjusted tax items (brackets, standard deduction, aged addition)Internal Revenue Service, 2025 [7]26 U.S. Code § 151: Allowance of deductions for personal exemptions (senior deduction, (d)(5)(C))Legal Information Institute, Cornell Law School

HSBuilt by Hussain Sehorewala · checked against worked examples · Sep 29, 2026

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Questions about this result

That depends on your other income, your tax rates now and later, and when you expect to need the money, and no calculator can pick it for you. This page shows the cost of an amount you choose and how much room there is before each bracket top and before the first Medicare premium line. On the example, a single filer 65 or older with $30,000 of other income and $30,000 of Social Security, converting $20,000 adds $3,900 of federal tax, 19.5% on average.

Converting adds to the income used to decide how much of your Social Security is taxable, up to 85%. Until 85% of the benefits are taxable, each extra dollar of income brings in more taxable benefits too, so it costs up to 1.85 times its bracket rate. On the example, before any conversion the next dollar already costs 22.2%, a 12% bracket times 1.85; converting $20,000 takes taxable benefits from $13,850 to $25,500, the 85% limit, and the next dollar then costs 23.3% with the senior deduction shrinking.

It can, two years later. Medicare Part B and Part D premiums go up in tiers when your modified adjusted gross income from two years earlier passes a line, $109,000 for a single filer and $218,000 for a married couple in 2026. Each line is a cliff: one dollar over adds the whole tier, $1,148 a year per person at the first tier. On the example, converting $100,000 lifts income to $155,500 and adds $2,885 a year, and the room before the first line is $53,500.

Because a conversion also makes more of your Social Security taxable and shrinks the senior deduction, so taxable income grows faster than the conversion. On the example, the 12% bracket ends at $50,400 of taxable income, but only $19,050 of conversion fits under it.

No. State tax, capital gains, ACA subsidies and the net investment income tax are not modelled, and the conversion tax is assumed to be paid from money outside the account. Paying it from the account would make part of the withdrawal taxable too.

No. The 2026 lines are shown because they are the latest published; they are adjusted for inflation each year, so the line that will apply to a conversion made now is likely higher.

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