Read the word exempt in context
Tax-exempt interest can be excluded from federal AGI yet still matter for Medicare. SSA's basic IRMAA income measure adds tax-exempt interest to AGI. A reader who compares only the AGI line with a threshold can therefore miss an important part of the calculation. The word exempt describes a particular tax treatment; it is not a general statement that the income is ignored by every government program or every income definition.
Suppose an invented individual filer, June, has $106,000 of AGI and $5,000 of tax-exempt interest on the relevant return. Her Medicare MAGI would be $111,000. For the ordinary individual schedule in 2026, that is above the first $109,000 boundary. The example uses the 2024 income year and assumes the stated return amounts are correct. It illustrates the add-back rather than recommending a bond purchase, sale or particular asset allocation.
Reconcile the return instead of adding every statement
Financial institutions may issue multiple forms, and a portfolio can contain taxable interest, tax-exempt interest, distributions and gains. The preparer should establish the correct return treatment for each item. Once taxable interest is already included in AGI, adding it again to Medicare MAGI would count it twice. The separate IRMAA add-back is tax-exempt interest, not every item in a statement with “interest” somewhere in its description.
June can begin with the filed return or a carefully prepared estimate and identify the two inputs separately. On recent Form 1040 layouts, tax-exempt interest appears on line 2a and AGI on line 11, but the applicable year's actual form and instructions should be checked. A correction to a brokerage form may change the return-level figures. Entering an old statement total alongside a corrected AGI can create an inconsistent pair of inputs even when each number once appeared in a real document.
Keep a joint return distinct from separate returns
When a married couple filed jointly, the joint income measure is used to determine the applicable joint-return tier for each affected beneficiary. It is not ordinarily divided by two because the assets or accounts have different owners. Two spouses with Medicare can then each owe an individual surcharge based on that shared tier, subject to their own enrollment. Dividing joint income and applying the individual threshold is a different calculation and can misstate the result.
For married filing separately, each spouse's return supplies that person's income figures. The accelerated schedule and the lived-apart exception require their own review. June should not copy a spouse's separate-return MAGI into her own income field or assume that all marriage-related cases use the joint threshold. The MFS guide explains why filing classification and living arrangement need to be established before comparing the income total with a band.
Do not confuse Medicare MAGI with other modified incomes
The phrase modified adjusted gross income appears in many tax and benefit rules. Its definition varies by purpose. A worksheet for an IRA contribution, an insurance subsidy or another deduction should not be imported into an IRMAA calculation merely because it also says MAGI. The relevant Medicare definition is the one supported by SSA's instructions for this purpose. The calculator deliberately asks for AGI and tax-exempt interest separately to make that distinction visible.
Likewise, the standard deduction and other deductions after AGI should not be subtracted from the Medicare measure without a specific applicable rule. A reader can owe little federal income tax and still have a Medicare income-related surcharge. That can feel surprising, but the two systems are asking different questions. A complete return review should identify where an adjustment appears and whether it changes AGI, rather than relying on a general description such as “this reduces my taxes.”
Measure the premium difference with coverage facts
Using June's invented $111,000 Medicare MAGI in the supported 2026 individual schedule puts her in the first surcharge band. If she has both Part B and Part D for twelve months, the surcharge components are $81.20 and $14.50 per month, respectively. Their combined annual amount is $1,148.40. Those figures exclude her ordinary Part B premium and actual drug-plan premium, which remain separate charges. Different coverage months produce a different annual surcharge total.
The example does not mean that $5,000 of tax-exempt interest always costs that amount. A person already within the same band may see no tier change, while someone near another boundary could face a different difference. The cliff and headroom guide focuses on incremental cost. A calculation should show both the starting and ending tiers and identify whose coverage is included before anyone attaches a percentage label to the result.
Evaluate investments on their full merits
A Medicare add-back does not by itself establish that a municipal bond is a poor investment. Credit risk, interest-rate risk, federal and state tax treatment, liquidity, diversification and expected returns all deserve consideration. A taxable alternative may produce different income and different risks. This portal does not compare investments or tell a reader to sell assets merely to remain below a surcharge boundary. Any proposed change needs a broader financial review.
For a useful adviser conversation, bring the relevant return, corrected forms if any, the expected tax-exempt interest for the planning year and the actual Medicare coverage calendar. Ask which figures are confirmed, which can change and which premium year is involved. Keep those documents privately. The calculator can explain the arithmetic using the two return-level inputs; it does not need account numbers, individual securities or an uploaded statement to reproduce that part of the estimate.