What IRMAA is and why it surprises people
Medicare Part B is funded roughly three-quarters from general revenue and one quarter from beneficiary premiums. The standard premium is set to cover that 25% share. Since 2007 for Part B, and 2011 for Part D, higher-income beneficiaries have been charged a larger share instead — 35%, 50%, 65%, 80% or 85% of programme cost — and the difference between that and the standard premium is the income-related monthly adjustment amount, or IRMAA.
Three features make it catch people out.
It is a cliff, not a phase-in. There is no gradual taper between brackets. One dollar of MAGI over a threshold moves you into the next bracket for the entire year, and both the Part B and Part D surcharges step up together. A single dollar can therefore cost a married couple several thousand dollars, which is an effective marginal rate on that dollar of many thousand percent.
It looks backward two years. The premium you pay in 2025 is set from the tax return you filed for 2023. That lag means the year you sell a business, convert an IRA to a Roth, or realise a large capital gain is not the year you feel it — the bill arrives two years later, often after you have retired and your income has fallen. Social Security will send a notice explaining the determination, and it will refer to a return you filed a long time ago.
Each spouse pays it. A married couple filing jointly is assessed on their joint MAGI, and then each enrolled spouse pays the surcharge separately. So the household cost of a bracket is twice the per-person figure, which is why the cliff is larger for couples than the tables suggest.
The surcharge is not optional and it is not something a plan sets. Part B IRMAA is deducted from your Social Security payment or billed directly; Part D IRMAA is paid to Medicare rather than to your drug plan, in addition to whatever the plan itself charges.
How each surcharge is actually derived
Both surcharges come from the same statutory idea: your tier determines what percentage of programme cost you pay, and the surcharge is the gap between that and the default share.
Part B. The standard premium represents 25% of the cost. At the 50% tier you pay twice that, so the premium is the standard premium × (50 ÷ 25) = 2.0 × standard. At the 85% tier it is 3.4 × standard. The surcharge itself is (σ ÷ 25 − 1) × standard, where σ is your tier's share. With a $185.00 standard premium, the 50% tier gives a surcharge of $185.00 and a total of $370.00.
Part D. Part D beneficiaries are expected to fund 25.5% of the cost of basic drug coverage, and the national base beneficiary premium that CMS publishes each year represents that share. The surcharge is the base premium scaled up by how far your tier exceeds 25.5%: B × (σ − 25.5) ÷ 25.5, rounded to the nearest ten cents. With a $36.78 base, the 50% tier gives $36.78 × 24.5 ÷ 25.5 = $35.34, which rounds to $35.30 — exactly the amount CMS published for 2025.
MAGI for IRMAA purposes is adjusted gross income plus tax-exempt interest. That definition is narrower than several other MAGI definitions in the tax code, and the tax-exempt interest add-back is the part that catches municipal bond investors: income you deliberately arranged not to be taxable still counts here.
The thresholds are indexed annually, and they are the same for single filers and for heads of household. Married filing jointly thresholds are double the single ones except at the top. Married filing separately, where the spouses lived together at any point in the year, has only two brackets and the first of them starts at the 80% share — a punitive structure that makes separate filing an expensive choice for Medicare purposes.
Worked example: a single filer at $150,000
You file as a single taxpayer. Two years ago your adjusted gross income was $150,000 with no tax-exempt interest. You are pricing a 2025 premium, so the standard Part B premium is $185.00 and the Part D base beneficiary premium is $36.78.
- MAGI. $150,000 + $0 = $150,000.
- Bracket. For a single filer the 2025 thresholds are $106,000, $133,000, $167,000, $200,000 and $500,000. $150,000 is above $133,000 and at or below $167,000, so you are in tier 2, charged 50% of Part B cost.
- Part B surcharge. (50 ÷ 25 − 1) × $185.00 = 1.0 × $185.00 = $185.00 a month.
- Part B premium. $185.00 + $185.00 = $370.00 a month.
- Part D surcharge. $36.78 × (50 − 25.5) ÷ 25.5 = $36.78 × 0.960784 = $35.34, rounded to $35.30 a month.
- Total surcharge. $185.00 + $35.30 = $220.30 a month, or $220.30 × 12 = $2,643.60 for the year.
- Headroom. $167,000 − $150,000 = $17,000 before the next threshold.
- Cost of crossing it. At the 65% tier the Part B surcharge becomes 1.6 × $185.00 = $296.00 and the Part D surcharge becomes $57.00, so the monthly total rises from $220.30 to $353.00. The step is $132.70 a month, or $1,592.40 a year.
Now suppose the same person is married and the couple's joint MAGI is $300,000. The joint thresholds are exactly double, so $300,000 sits in the same 50% tier and each spouse pays the same $220.30 a month. But both are enrolled, so the household pays $220.30 × 12 × 2 = $5,287.20, and crossing the $334,000 threshold would cost $132.70 × 12 × 2 = $3,184.80 a year. Every figure in a bracket table is per person; the household number is the one that should drive a planning decision.
Using the headroom figure to plan income
The headroom number is the practical output on this page. It is the size of the income event you can take without triggering the next bracket, and it should be checked before any of the following: a Roth conversion, realising capital gains, selling a rental property, taking a lump-sum distribution, exercising options, or harvesting gains inside a taxable account.
The right way to use it is to size the transaction up to the headroom rather than avoiding the bracket entirely. A Roth conversion of $16,000 when you have $17,000 of headroom costs nothing in IRMAA; a conversion of $18,000 costs the whole cliff. The last $2,000 of that conversion carries an implicit IRMAA charge of $1,592.40 for a single filer, on top of the income tax — which is a bad enough deal that the conversion should either shrink or grow enough to justify it.
Sometimes crossing is correct. If a large Roth conversion permanently reduces future required minimum distributions, one year of surcharge may be a small price. The calculation to make is the cliff cost for one year against the tax saved over the remaining lifetime, and the cliff cost is only ever a single year's charge, because the two-year lookback moves on.
Three other things reduce or reverse an IRMAA determination. Tax-exempt interest is added back, so shifting to municipal bonds does not help here even though it helps elsewhere. Qualified charitable distributions from an IRA satisfy a required minimum distribution without appearing in AGI, which is one of the few tools that reduces IRMAA directly. And a life-changing event — retirement or reduced work hours, marriage, divorce, death of a spouse, loss of a pension, or loss of income-producing property — lets you ask Social Security to use a more recent year's income on Form SSA-44, which is the single most useful remedy for a newly retired person whose determination is based on their final working year.
Once you know the surcharge, fold it into the wider retirement healthcare budget with the retirement healthcare cost calculator, and check the interaction with benefit taxation using the Social Security benefit taxation calculator — the same extra dollar of income often triggers both.
2025 IRMAA brackets, based on 2023 modified adjusted gross income
| Single filer MAGI | Married filing jointly MAGI | Share of Part B cost | Part B premium | Part D surcharge |
|---|---|---|---|---|
| $106,000 or less | $212,000 or less | 25% | $185.00 | $0.00 |
| Over $106,000 to $133,000 | Over $212,000 to $266,000 | 35% | $259.00 | $13.70 |
| Over $133,000 to $167,000 | Over $266,000 to $334,000 | 50% | $370.00 | $35.30 |
| Over $167,000 to $200,000 | Over $334,000 to $400,000 | 65% | $480.90 | $57.00 |
| Over $200,000 to under $500,000 | Over $400,000 to under $750,000 | 80% | $591.90 | $78.60 |
| $500,000 and above | $750,000 and above | 85% | $628.90 | $85.80 |
Married filing separately, where the spouses lived together during the year, uses only three bands: $106,000 or less at 25%, over $106,000 to under $394,000 at 80%, and $394,000 and above at 85%. Every amount in the last two columns is per person.
What this calculator does not do
- It does not include your Part D plan premium. The Part D figure here is the surcharge alone, paid to Medicare. Your drug plan bills its own premium separately, and that varies by plan and region.
- It does not model the hold-harmless provision. Most beneficiaries whose Part B premium is deducted from a Social Security payment are protected from an increase larger than their cost-of-living adjustment — but that protection does not apply to IRMAA, which is one of its stated exceptions.
- It does not account for Medicare Advantage plans that give back part of the Part B premium. Some plans offer a Part B premium reduction, which changes the total but not the surcharge itself.
- It uses one year's parameters at a time. The standard premium, the base beneficiary premium and all the thresholds change each year. If you are projecting several years ahead, run each year with its own figures rather than assuming today's.
- It cannot see a life-changing event. If you have filed Form SSA-44 and Social Security has agreed to use a more recent year, enter that year's income instead of the year from two years ago.
- It treats the household as jointly assessed. A couple where only one spouse is enrolled pays the surcharge once, so set the number of enrollees to one even if the return is joint.
Form SSA-44 is the appeal that actually works
If your income has fallen because of a life-changing event, you can ask Social Security to base the determination on a more recent year instead. The qualifying events are specific: marriage, divorce or annulment, death of a spouse, you or your spouse stopping work or reducing hours, loss of income-producing property through a disaster or similar event, loss or reduction of a pension, and receipt of a settlement from a former employer's closure or bankruptcy. Simply having lower income than two years ago is not on the list. File Form SSA-44 with documentation — the year you retire is the classic case, because the determination is otherwise made from your final, highest-earning year.
Where IRMAA fits in the retirement tax landscape
IRMAA is one of several places where an extra dollar of income in retirement costs far more than its marginal tax rate suggests. The others are the taxation of Social Security benefits, where each additional dollar of other income can drag up to 85 cents of benefit into taxable income; the capital-gains rate brackets, where crossing a threshold reprices the whole gain; and the ACA premium tax credit for anyone retiring before 65. Planned together, these produce a coherent picture of which years to realise income in. Planned separately, they collide.
The useful discipline is to think in terms of an annual income target rather than an annual tax bill. Work out the MAGI ceiling that keeps you below the next IRMAA threshold, check it against the Social Security taxation thresholds and the top of your capital-gains bracket, take the lowest of them, and fill that space deliberately with Roth conversions or realised gains in years when it is otherwise empty. The years between retirement and the start of required minimum distributions are usually the ones with the most room.
Two related pages complete the picture. The Medicare late enrollment penalty calculator covers the other permanent premium increase, which is triggered by timing rather than income. And the Medigap versus Medicare Advantage calculator handles the coverage decision that sits alongside the premium one — noting that the Part B premium and any IRMAA are payable on both routes, so the surcharge does not favour either.