Methodology and sources

Inputs: filing status (single or married filing jointly), each person’s age on December 31, 2026, and 2026 income: Social Security benefits, pensions and annuities, planned IRA and 401(k) withdrawals, wages or self-employment income, taxable interest and ordinary dividends, qualified dividends and long-term gains, and tax-exempt interest. Optionally, itemized deductions.

How it works. For every conversion amount from $0 to $400,000 in $1,000 steps, the whole 2026 federal return is figured again from scratch:

1. Taxable Social Security, by IRS Publication 915, Worksheet 1. Provisional income is half the benefits, plus all other income (the conversion included), plus tax-exempt interest. Above the first base amount, up to half of the excess becomes taxable; above the second, 85% of the excess is added, never more than 85% of the benefits.
2. Adjusted gross income (AGI) is all other income plus taxable Social Security.
3. Deduction: the larger of the standard deduction (plus the additional amount for each person 65 or older) or the itemized amount entered.
4. Senior deduction: $6,000 for each person 65 or older, each reduced by 6% of MAGI over the threshold, as on Schedule 1-A. Its MAGI is AGI here (the calculator doesn’t ask about excluded foreign income). It applies whether or not you itemize.
5. Taxable income is AGI less both deductions.
6. Tax: the ordinary part through the 2026 rate schedule; qualified dividends and long-term gains stacked on top at 0%, 15% and 20%, by the Qualified Dividends and Capital Gain Tax Worksheet, keeping the smaller result as its last line does.

The cost of each step is the difference in total tax between that step and the one before. That captures Social Security becoming taxable (up to $1.85 of income for each converted dollar), the senior deduction shrinking, and gains being pushed out of the 0% rate.

Room to the top of a bracket is the largest whole-dollar conversion that keeps ordinary taxable income at or below that bracket’s top. It is found by figuring the return again at each amount, not by subtracting, because each converted dollar can add more than a dollar of taxable income.

Medicare IRMAA is shown when anyone is 63 or older in 2026, since 2026 income sets 2028 premiums. Its MAGI is AGI plus tax-exempt interest. 2028 thresholds aren’t published yet, so the 2026 thresholds are used; they usually rise each year. The yearly cost of a tier is (Part B surcharge + Part D surcharge) x 12 x the number of people 63 or older.

Simplifications: tax is figured with the tax rate schedules, not the IRS Tax Table, which applies below $100,000 of taxable income and can differ by a few dollars. Adjustments to income (such as half of self-employment tax or HSA contributions), self-employment tax itself, capital losses, and the additional deduction for blindness are not included. Ages are as of December 31.

Not modeled: state tax, the net investment income tax, AMT, the QBI deduction, the effect on premium tax credits before 65, the five-year rules for Roth withdrawals, and tax rates in future years.

Inputs: filing status, the income year (2026 or 2027), MAGI for that year, and how many people will be on Medicare in the premium year. MAGI can be typed in or built from income: the builder figures taxable Social Security by Publication 915, Worksheet 1, then adds all other income and tax-exempt interest.

How it works. Medicare uses the tax return from two years before the premium year, so 2026 income sets 2028 premiums. MAGI here is adjusted gross income plus tax-exempt interest. The tier is read from the CMS table exactly as written: each tier starts above the limit below it and runs up to and including its own limit, except the top tier, which starts at its limit.

Results: the monthly Part B and Part D surcharges per person for that tier, the yearly cost for the household (monthly surcharge x 12 x the number of people on Medicare), the room below the next tier, and how far above the tier below the income sits.

Simplifications: 2028 and 2029 thresholds aren’t published yet, so the 2026 thresholds and surcharges are used; they usually rise each year. Married filing separately is not included. The surcharges are on top of the standard Part B premium and of your own Part D plan’s premium, which are not included.

Life-changing events: Form SSA-44 asks Social Security to use a more recent, lower year after marriage, divorce or annulment, the death of a spouse, work stopping or being reduced, the loss of income-producing property or pension income, or an employer settlement payment.

Inputs: birth year, each account’s type and its balance on December 31, 2025, whether the only beneficiary is a spouse more than 10 years younger, and an assumed growth rate for the ten-year table.

How it works. The age when RMDs begin comes from the final regulations: 73 for those born from 1951 through 1959 and 75 for those born in 1960 or later. The first RMD is for the year that age is reached and may wait until April 1 of the next year, but then two RMDs fall in that next year. Each year’s RMD is the account’s balance on December 31 of the year before, divided by the Uniform Lifetime Table factor for the age reached that year. Each account is figured separately; IRA amounts may then be taken from any one IRA, while each employer plan’s amount must come from that plan.

Spouse more than 10 years younger: the Joint and Last Survivor Table applies instead, so the calculator stops and says so.

Ten-year table: starts from the total of the balances entered and assumes each year’s RMD is taken and the rest grows at the rate entered. The rate is an assumption, not a forecast.

Qualified charitable distributions: shown for anyone 70½ or older in 2026. With a birth year only, someone who turns 70 in 2026 may or may not reach 70½ during the year, so the calculator says it depends on the birth month.

Not modeled: inherited accounts, Roth accounts, the exception that lets someone still working delay a current employer’s plan, and RMDs missed in earlier years.

Inputs: filing status, each person’s age on December 31, 2026, and MAGI, typed in or built from income as in the IRMAA calculator.

How it works. Each person 65 or older by the end of the year gets $6,000. Each person’s $6,000 is reduced by 6% of MAGI above $75,000 ($150,000 on a joint return), as Schedule 1-A figures it, so a couple who both qualify lose it entirely at $250,000. MAGI for this deduction is adjusted gross income plus excluded foreign and territory income; the calculator treats it as AGI. The deduction is allowed whether or not you itemize, for tax years 2025 through 2028.

The cost of $1,000 more income inside the phase-out is the deduction lost ($60 for each qualifying person) times the federal bracket rate. The bracket is estimated from MAGI less the standard deduction (with the 65-or-older amounts) and the senior deduction, assuming all of it is ordinary income.

Not modeled: married filing separately (not allowed this deduction), the Social Security number requirement, the add-back of excluded foreign and territory income, other income that is taxed at the capital gains rates, and state tax.

Inputs: the year the owner died, the owner’s birth year, the beneficiary’s relationship to the owner and birth year, the balance on December 31, 2025, and an assumed growth rate for the table.

Who it covers. An individual beneficiary who is not an eligible designated beneficiary, of an owner who died in 2020 or later. A spouse, a child under 21, a disabled or chronically ill beneficiary, and one not more than 10 years younger than the owner may have options beyond the 10-year rule, and estates and trusts follow their own rules: the calculator stops for each of these and says a review is needed.

How it works. The account must be empty by December 31 of the tenth year after the year of death. If the owner died on or after their required beginning date (April 1 of the year after reaching their RMD age), a minimum is also due in each of years 1 to 9. That minimum is the balance on December 31 of the year before, divided by the beneficiary’s life expectancy from the Single Life Table for their age in the year after death, less one for each year since. The IRS waived the penalty for those yearly minimums for 2021 through 2024, so they apply from 2025. Nothing is due from the beneficiary in the year of death itself.

When the date matters: if the owner died in the same year as their required beginning date, the answer turns on whether it was before April 1, so the calculator asks. For owners born in 1949 or earlier, the beginning date depended on their birth month, and where that leaves it unclear the calculator says so rather than guess.

Not modeled: the owner’s own RMD for the year of death, inherited Roth IRAs, successor beneficiaries, and accounts left to more than one beneficiary.