Roth conversions and taxes
Should I convert to a Roth, and how much?
A one-year 2026 federal tax estimate built for retirees: it shows how much of Social Security is taxed, how much room is left in your bracket and under the next Medicare IRMAA tier, and what each additional dollar of conversion really costs.
Last reviewed: September 4, 2026 · model build 20260906-05 · 2026 federal parameters
What this calculator answers
Enter filing status, ages, ordinary income, Social Security, qualified dividends and gains, tax-exempt interest, and an optional conversion amount. It returns the federal tax before and after the conversion, the effective marginal rate on the conversion and its components, the amount that fits under the next bracket and the next IRMAA tier, a break-even comparison against leaving the money tax-deferred, and the full line-by-line calculation.
Worked example
Married filing jointly, both 65 or older, $60,000 of IRA withdrawals and pension, $40,000 of Social Security, $10,000 of qualified dividends. Taxable Social Security is $34,000 (85%), deductions are $35,500 standard plus $12,000 senior deduction, taxable income is $56,500, and federal tax is about $5,084. Converting $50,000 raises federal tax to about $12,354 — an extra $7,270, or 14.5% of the conversion, even though the couple stays in the 12% bracket: MAGI rises to $154,000, so the senior deduction starts to phase out. About $53,400 more ordinary income fits under the 22% bracket, and MAGI can rise about $114,000 before the first IRMAA tier.
Key 2026 numbers
| Item | Single | Married filing jointly |
|---|---|---|
| Standard deduction | $16,100 | $32,200 |
| Additional deduction, age 65+ | $2,050 | $1,650 per spouse |
| Senior deduction (2025–2028) | $6,000, phased out at 6% of MAGI above $75,000 (gone at $175,000) | $6,000 per eligible spouse, each phased out at 6% of MAGI above $150,000 (both gone at $250,000) |
| 12% bracket ends / 22% ends / 24% ends | $50,400 / $105,700 / $201,775 | $100,800 / $211,400 / $403,550 |
| 0% long-term gains up to | $49,450 | $98,900 |
| Social Security taxation thresholds | $25,000 / $34,000 | $32,000 / $44,000 |
| Net investment income tax (3.8%) above | $200,000 | $250,000 |
| First IRMAA tier (2026 MAGI → 2028 premiums) | $109,000 | $218,000 |
| Standard Part B premium | $202.90 per month; IRMAA adds $81.20 to $487.00 (Part B) and $14.50 to $91.00 (Part D) per person | |
How the calculation works
Taxable Social Security follows IRC §86 using provisional income (other income plus tax-exempt interest plus half of benefits). Deductions are the larger of standard-plus-age addition or itemized, plus the senior deduction computed per person as on IRS Schedule 1-A. Qualified income is stacked on top of ordinary income and taxed at 0/15/20%. NIIT applies to the investment income you identify. The marginal-cost curve reruns the whole calculation for each additional $2,500 of conversion. The break-even compares a Roth balance growing tax-free (with conversion tax paid from a taxable account) against the same amount left tax-deferred and taxed at your expected future rate.
Limitations
Not modeled: the alternative minimum tax, credits, itemized-deduction limits, capital-loss carryforwards, non-deductible IRA basis, Roth five-year rules, ACA premium credits, married-filing-separately for spouses living apart, and actual state rules (the state field is a rough flat allowance). Figures are for tax year 2026. The full household plan uses simpler effective tax rates and does not reconcile to this tab.
Confirm any decision with an official Social Security statement, your plan custodian, and a qualified adviser. See the methodology and changelog for every rule the model applies.
Frequently asked questions
How much of my Social Security is taxable?
Up to 50% when provisional income is between $25,000 and $34,000 (single) or $32,000 and $44,000 (joint), and up to 85% above those amounts. The thresholds are not indexed to inflation.
What is the $6,000 senior deduction?
A temporary deduction for 2025–2028 for people 65 and older, available whether or not you itemize. It is reduced by 6% of modified AGI above $75,000 (single) or $150,000 (joint), calculated per person, so a couple with both spouses eligible loses all $12,000 by $250,000 of MAGI.
What income triggers IRMAA in 2028?
Your 2026 modified AGI (AGI plus tax-exempt interest) above $109,000 single or $218,000 joint. Medicare uses a two-year lookback, so a 2026 Roth conversion affects 2028 premiums.
Should I pay conversion tax from the IRA or from savings?
Paying from a taxable account keeps the full converted amount growing tax-free and is assumed by the break-even figure; paying from the conversion itself is less favorable and, before 59½, may incur a penalty on the amount withheld.