If you are asking how IRMAA is calculated for 2026, the mechanics are more straightforward than the paperwork suggests: Medicare compares your Modified Adjusted Gross Income (MAGI) from two years earlier against a set of income brackets that the Centers for Medicare & Medicaid Services (CMS) publishes each year, and if your MAGI clears a bracket threshold, you pay a fixed surcharge on top of the standard Part B premium for the entire year. For 2026, CMS set the standard Part B premium at $202.90 a month, with surcharges that push the total as high as $689.90 a month for the highest-income beneficiaries.¹

The calculation itself has only three moving parts: your MAGI, your tax filing status, and the bracket that MAGI falls into two years before the premium year. What catches most retirees off guard is not the arithmetic; it is the timing. The MAGI that determines your 2026 premium was set when you filed your 2024 tax return, which means a Roth conversion, a large IRA withdrawal, or a capital gain realized two years ago may already be shaping what you pay for Medicare this year. The sections below walk through how that estimate is actually built, where the standard calculation runs into limits, and where you can go to estimate your own number. All sources and related pages referenced in this article are listed, in the order they appear, in the Sources section at the end.

What Counts as MAGI for IRMAA Purposes

Social Security Administration guidance defines MAGI for IRMAA purposes as a beneficiary’s adjusted gross income, the figure reported on Line 11 of Form 1040, plus any tax-exempt interest income reported on Line 2a.² That definition is narrower than “gross income” and broader than “taxable income”: it picks up municipal bond interest that never appears on a tax bill, but it does not add back retirement account balances that have not yet been distributed.

Which Tax Year’s Return Actually Counts

IRMAA runs on a two-year lookback. SSA guidance states that the agency uses “the most recent tax return information the IRS is able to provide,” which in practice means your 2026 premium is based on the MAGI reported on your 2024 tax return, or your 2023 return if 2024 data was not yet available when brackets were set. That lag is why an income event from two years ago, not this year’s income, is what is actually determining this year’s premium.

The 2026 IRMAA Brackets for Medicare Part B

CMS publishes separate bracket tables for individual filers, joint filers, and beneficiaries who are married filing separately, each pairing a MAGI range with a total monthly Part B premium.¹ The table below reflects those figures; married-filing-separately beneficiaries face a compressed structure with only three tiers, since that filing status is designed to discourage income-splitting for Medicare purposes.

Source: Centers for Medicare & Medicaid Services, “2026 Medicare Parts B Premiums & Deductibles,” published November 14, 2025 [1].

Because the surcharge is not phased in, someone with 2024 MAGI of $109,001 pays the same $284.10 total premium as someone at $136,999, while a single dollar less, at $108,999, would have kept that person at the standard $202.90 rate. CMS’s bracket structure treats each threshold as a cliff rather than a gradual slope, which is the detail most worth planning around.¹

Part D Carries Its Own Surcharge

Medicare Part D prescription drug plans carry a separate IRMAA surcharge on the same MAGI brackets, added to whatever premium a beneficiary’s specific Part D plan charges. For 2026, that surcharge ranges from $14.50 a month at the lowest bracket to $91.00 a month at the highest.¹ A household near a bracket threshold is effectively deciding both premiums at once, since Part B and Part D use identical MAGI cutoffs.

What Can Push Two-Year-Old MAGI Into a Higher Bracket

Because the number in question comes from a return filed two years ago, the events that move it are rarely last-minute decisions. Required minimum distributions, which generally begin at age 73 under current IRS rules, are one common driver, since they add taxable IRA income that was not previously part of MAGI. Roth conversions and one-time capital gains events work the same way: MWM has examined how IRA withdrawal timing in particular interacts with Medicare premiums in a companion article, which looks at the planning side of this question in more depth than a bracket table can. Contribution limits and income thresholds also shift every year; MWM’s 2026 Retirement Plan Adjustments guide covers the updated Roth IRA and contribution figures that interact with MAGI planning more broadly, and the firm’s guide to tax-efficient retirement withdrawal planning⁷ addresses how account type and sequencing affect the income a retiree reports in a given year.

When You Can Appeal an IRMAA Determination

The Social Security Administration allows beneficiaries to request a reduced IRMAA using Form SSA-44⁶, but only when a specific life-changing event reduced income after the tax year SSA used to set the bracket. SSA recognizes eight such events: marriage, divorce or annulment, death of a spouse, work stoppage, work-hour reduction, loss of income-producing property, loss of pension income, and an employer settlement payment tied to bankruptcy or reorganization, according to SSA’s current Form SSA-44 and its companion guidance on requesting a lower IRMAA.⁶ The form can be filed online, by phone, by mail or fax, or in person at a local Social Security office, and each event requires supporting documentation.

Where the Standard Calculation Falls Short

The two-year lookback and the appeals process both have edges worth knowing before you rely on either one. New Medicare enrollees may not have two full years of tax data on file yet; SSA’s own guidance allows for MAGI as old as three years prior when more recent returns are not available³, so a beneficiary’s first premium can be based on an unexpectedly old snapshot of income. The appeals process, meanwhile, is narrower than it looks: it exists to correct IRMAA when income genuinely dropped due to one of the eight listed events⁶, not to reverse a bracket that a retiree moved into deliberately, such as through a planned Roth conversion or a strategic asset sale. A retiree who intentionally realized income for other financial planning reasons generally has no SSA-44 pathway back to a lower bracket, even if the same income event would qualify for other tax planning treatment. And because the bracket structure is a cliff rather than a slope, an estimate that is off by even a modest amount can put a household in the wrong tier entirely, which is a reason to treat any MAGI projection as an estimate to confirm with a tax preparer, not a final number.

None of this is a substitute for personalized advice. Coordinating a MAGI estimate with your CPA and, where trusts or business income are involved, your estate planning attorney, is a more reliable path than working from a bracket table alone.