The bracket is not the rate.

The facts, in one place.

Six quotable sentences on the real marginal rate in retirement, 2026 figures.

1. The bracket is not the rate. Once provisional income passes $25,000 single or $32,000 joint, each extra dollar of other income also pulls 50 cents of Social Security into taxable income; past $34,000 and $44,000, 85 cents. The dollar is taxed at the bracket rate 1.85 times.
2. Example: a single retiree, 65 or older, with $30,000 of Social Security and $40,000 of other income sits in the 12% bracket, but the next $1,000 of IRA withdrawal costs $222: a real rate of 22.2%.
3. A couple with $48,000 of benefits and $60,000 of other income: bracket 12%, real rate 21.6% on the next $1,000.
4. The effect ends when 85 percent of the benefit is in income, which is why the real rate falls back to the bracket rate at higher incomes; the peak sits where the 85-percent phase-in meets the 22 percent bracket.
5. Two more slopes hide in the same range: the senior deduction of $6,000 per person falls by 6 percent of income over $75,000 single and $150,000 joint, adding 6 percent of the bracket rate; and the Medicare tiers are cliffs, where one dollar can cost a full year's surcharge two years later.
6. The practical use is sizing: a withdrawal or a Roth conversion is measured against the real rate on the dollars it adds, not the bracket printed on the return, and the shape of the curve says where to stop.

Bracket against real rate.

Two households across the income range where the phase-in bites.

How the rate climbs.

The bracket printed on a return is the rate on the last dollar of taxable income. The real marginal rate is the tax on one more dollar of income, and in retirement the two part company, because that dollar does more than sit in a bracket. Above the provisional-income thresholds it pulls part of the Social Security benefit into taxable income, 50 cents at first and then 85, so the bracket rate is charged on up to 1.85 dollars for every dollar withdrawn. The effect ends when 85 percent of the benefit is in, and the real rate drops back to the bracket.

Two more things move in the same range. The senior deduction phases out at six cents on the dollar over its threshold, which adds six percent of the bracket rate. And the Medicare tiers are cliffs: at each threshold, one dollar of income raises the following year's premium by a whole step, which shows on the curve as a spike. The calculator measures the tax on a chosen step, says which of these drove it, and draws the whole curve so the shape is visible before a withdrawal or a conversion is sized.

Methodology.

1. Inputs. Social Security for the household, other income before the withdrawal, filing status, whether the filers are 65 or older, and the size of the extra withdrawal.
2. The step. The federal return is computed at the income and at the income plus the step (the shared return helper: § 86 taxable benefits, the standard deduction with the age-65 additions and the senior deduction, the year's brackets, the Medicare tier). The income-tax rate on the step is the tax difference over the step; the all-in rate adds the change in the household's yearly Medicare surcharge two years out.
3. The curve. The same computation on each $1,000 of other income from zero to $250,000, income tax only, so the Medicare cliffs do not swamp the picture; the peak is reported with its location.
4. What drove it. The benefit dollars pulled into income by the step, the senior deduction lost, and whether a Medicare tier was crossed, each read from the two returns.
5. Validation. A plain case with no benefits inside the 22 percent bracket returns exactly 22 percent; a case inside the 85-percent phase-in returns 1.85 times the bracket rate; the curve is non-negative and the right length. A transcription error fails the build.
6. Not modeled. Capital gains and qualified dividends (which are stacked and taxed at their own rates, and which the phase-in also affects), state tax, itemized deductions, other phase-outs, the earnings test, and the premium tax credit. Educational, not advice.

Sources.

1. 1. Internal Revenue Service, Rev. Proc. 2025-32 — 2026 inflation adjustments (tax rate tables § 4.01, capital gains § 4.03, AMT exemptions § 4.10). Every 2026 bracket boundary and base amount for individuals and for estates and trusts; the 0% and 15% capital-gain thresholds; the AMT exemption amounts. Retrieved September 4, 2026; verified September 4, 2026.
2. 2. United States Code (Cornell LII), 26 U.S.C. § 86(c) — Base amount and adjusted base amount for taxing Social Security benefits. The $25,000 / $32,000 and $34,000 / $44,000 thresholds, which are not indexed. Retrieved September 4, 2026; verified September 4, 2026.
3. 3. Centers for Medicare & Medicaid Services, 2026 Medicare Parts A & B Premiums and Deductibles (fact sheet, November 14, 2025). The Part B standard premium and deductible, the Part A inpatient deductible, and every row of the Part B and Part D income-related adjustment tables. Retrieved September 4, 2026; verified September 4, 2026.
4. 4. United States Code (Cornell LII), 42 U.S.C. § 1395r(i) — Income-related increase in Part B premium (and, by cross-reference, Part D). That the income used is modified adjusted gross income from the return two years before the premium year, and the mechanism of the tiers. Retrieved September 4, 2026; verified September 4, 2026.
5. 5. Internal Revenue Service, Rev. Proc. 2025-32 — 2026 inflation adjustments (§ 4.14 standard deduction; § 4.42 gift exclusion). The standard deduction amounts, the additional amounts for age or blindness, the $19,000 gift exclusion, and the $194,000 non-citizen-spouse exclusion. Retrieved September 4, 2026; verified September 4, 2026.
6. 6. Internal Revenue Service, One Big Beautiful Bill Act: tax deductions for working Americans and seniors. The $6,000 deduction for individuals 65 and older, its MAGI phase-out thresholds, its 2025-through-2028 window, and its availability to itemizers and non-itemizers. Retrieved September 4, 2026; verified September 4, 2026.

Revision history.

This tool reads the federal and annual records; their histories are below.

September 5, 2026

Added the senior deduction's phase-out rate (6 percent of MAGI over the threshold), which the widow's penalty and QCD tools apply.

September 4, 2026

First release of the annual numbers record and page for 2026, and of the IRMAA tool built on its Medicare section: plan and IRA limits from Notice 2025-67, HSA figures from Rev. Proc. 2025-19, Social Security figures from the Federal Register notice, Medicare premiums and both income-related tables from the CMS fact sheet, the standard deduction and gift exclusion from Rev. Proc. 2025-32, and the senior deduction from the IRS's OBBBA guidance.

September 4, 2026

First release of the federal income-tax record for 2026 and the trust income-tax tool: the estates-and-trusts table, the single and joint tables for comparison, capital-gain thresholds, AMT exemptions, the § 1411 trust threshold rule, and the § 642(b) exemptions, all from Rev. Proc. 2025-32 and the Code.

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