Two penalties, both permanent, both easy to trip
Medicare charges a late enrollment penalty when you delay signing up without having other qualifying coverage. There are two of them, they are computed completely differently, and both last as long as you hold the coverage. That is the feature that makes them expensive: this is not a one-off fee but a permanent increase to a premium you will pay every month for the rest of your life.
The Part B penalty adds 10% to the standard premium for each full twelve-month period you were eligible for Part B and did not enrol. Partial periods count for nothing — eleven months of delay costs nothing at all, and the twelfth month costs 10% forever. It applies for as long as you have Part B, which for most people means until death.
The Part D penalty adds 1% of the national base beneficiary premium for every full month you went without creditable prescription drug coverage after your initial enrollment period ended. Every month counts here, not just whole years. Two features make it awkward. It is calculated on a national benchmark rather than on your own plan's premium, so choosing a cheaper drug plan does not shrink it. And that benchmark is reset every year, so the dollar amount of your penalty moves annually even though the percentage does not.
Both are avoided the same way: by having coverage that Medicare recognises. For Part B that means group health coverage based on current employment, yours or your spouse's. For Part D it means drug coverage certified as creditable, meaning it is expected to pay at least as much as standard Part D coverage on average.
The trap almost everyone falls into is the same one. COBRA and retiree health coverage are real, comprehensive insurance, and neither counts as coverage based on current employment. Someone who retires at 65, takes eighteen months of COBRA and enrols in Part B afterwards has accumulated uncovered months the whole time, and Medicare will say so.
How each penalty is computed
Part B. Count the months between the end of your initial enrollment period and the point you enrolled, excluding any months you had coverage from current employment. Divide by twelve and take the whole number. Multiply by 10% and apply that to the standard premium for the year. With a $185.00 standard premium and thirty uncovered months, the whole-number part is two, so the increase is 20% and the penalty is $37.00 a month — carried alongside the $185.00, for a total of $222.00.
Because the penalty is a percentage of the standard premium and the standard premium rises most years, the dollar penalty rises with it. And because IRMAA is applied separately, a higher-income beneficiary pays both: the income surcharge is not a percentage of the penalised premium, and the penalty is not a percentage of the surcharged one. They are added.
Part D. Count every full month without creditable coverage, multiply by 1%, apply that to the national base beneficiary premium, and round to the nearest ten cents. With a $36.78 benchmark and twenty-six uncovered months, that is 0.26 × $36.78 = $9.5628, which rounds to $9.60 a month. The rounding is done on the final figure rather than on the percentage.
The gap that produces a Part D penalty is not a single missed day. Medicare counts a break of 63 or more consecutive days without creditable coverage; shorter gaps do not trigger it. But once a penalty is assessed it applies for as long as you have Part D coverage, and enrolling does not clear the months already counted.
The lifetime figure on this page is simply the annual cost multiplied by the years to the planning age you enter. It holds both premium parameters constant, which understates the real total, because both the standard Part B premium and the Part D benchmark have historically risen over time.
Worked example: retiring at 65 and taking COBRA
You turn 65 in March and retire the same month. You elect eighteen months of COBRA, assume it counts as coverage, and enrol in Part B and a Part D plan once it runs out. Your initial enrollment period ended at the end of June — three months after the month you turned 65 — and by the time you enrol you have thirty months without Part B and twenty-six full months without creditable drug coverage. You are now 70 and planning to 88. The standard Part B premium is $185.00 and the Part D benchmark is $36.78.
- Complete twelve-month periods without Part B. floor(30 ÷ 12) = 2.
- Part B increase. 2 × 10% = 20%.
- Part B penalty. $185.00 × 20% = $37.00 a month. The other six months add nothing to this penalty.
- Part D penalty. 26 × 1% = 26%, and $36.78 × 0.26 = $9.5628, rounded to $9.60 a month. Here every month counted.
- Combined monthly. $37.00 + $9.60 = $46.60.
- Annual cost. $46.60 × 12 = $559.20.
- Years carried. 88 − 70 = 18.
- Lifetime cost. $559.20 × 18 = $10,065.60, and more than that in reality, because both premium benchmarks rise over time.
Two details are worth pulling out. The Part B penalty here was set by two completed years, so delaying six more months would have cost nothing extra and delaying seven more would have added another 10%. And had the same person enrolled in a standalone Part D plan while on COBRA — which is permitted, and costs the plan premium alone — the $9.60 would never have arisen and the Part B position would have been unchanged.
What to do about a penalty you have already incurred
The first thing to establish is whether it is correct. Both penalties are assessed from records that can be wrong, and both can be appealed. For Part B, Social Security makes the determination and you may request reconsideration if your months of employer coverage were miscounted. For Part D, the plan sends a letter with a reconsideration form and a deadline, and the most common successful ground is proof of creditable coverage that the record does not reflect — the annual notice your employer or retiree plan sends every autumn is exactly that evidence, which is why those letters are worth keeping.
The second is whether an equitable relief request applies. Where the delay was caused by error, misrepresentation or inaction by a federal employee or someone acting on Medicare's behalf, Social Security can grant relief that removes the penalty and adjusts the enrollment date. This is a narrow remedy and it is not available simply because you were badly advised by a private broker or an employer.
The third is whether the penalty is worth avoiding by declining coverage, and the answer is almost always no. Part B pays for physician services, outpatient care, durable equipment and much of the rest of ordinary medicine; going without it to avoid a 20% surcharge is a false economy on a scale that dwarfs the penalty. The same holds for Part D: a beneficiary taking no medication today who declines Part D to save a modest premium accumulates 1% a month against a future in which they almost certainly will take medication.
Where the penalty genuinely does change a decision is in choosing between routes to drug coverage. If a Medicare Advantage plan includes drug coverage, enrolling in it stops the Part D clock just as a standalone plan does. Compare the two routes on total cost with the Medigap versus Medicare Advantage calculator, and model what your drugs will actually cost under either with the Part D drug cost calculator.
Penalty by length of delay at 2025 premium parameters
| Months uncovered | Part B increase | Part B penalty | Part D penalty | Combined per year |
|---|---|---|---|---|
| 12 | 10% | $18.50 | $4.40 | $274.80 |
| 24 | 20% | $37.00 | $8.80 | $549.60 |
| 36 | 30% | $55.50 | $13.20 | $824.40 |
| 48 | 40% | $74.00 | $17.70 | $1,100.40 |
| 60 | 50% | $92.50 | $22.10 | $1,375.20 |
| 72 | 60% | $111.00 | $26.50 | $1,650.00 |
Part D penalties are rounded to the nearest ten cents, which is why 48 months gives $17.70 rather than the $17.65 the unrounded arithmetic produces. Multiply the last column by the years you expect to hold the coverage to get the lifetime figure.
The situations that create uncovered months
- Taking COBRA and treating it as coverage. COBRA is not coverage based on current employment, so it neither protects you from the Part B penalty nor opens a special enrollment period when it ends. This is the single most common cause of a Part B penalty. Its drug component may still be creditable for Part D, which is a separate question with a separate answer.
- Retiree health coverage. Same problem, same reason. Retiree plans are frequently comprehensive and frequently drug-creditable, and they are still not current employment.
- Working for a small employer. Where the employer has fewer than 20 employees, Medicare is generally the primary payer once you are 65 and eligible, and the group plan pays second. Staying out of Part B in that situation can leave large bills unpaid as well as accumulating penalty months.
- Marketplace coverage after 65. An ACA exchange plan is not creditable coverage for Part B purposes, and the premium tax credit generally ends once you are eligible for premium-free Part A. Staying on an exchange plan past 65 is an expensive mistake in two directions at once.
- Health savings account contributions. Enrolling in any part of Medicare, including premium-free Part A, ends HSA eligibility. Part A enrollment can be backdated up to six months when you claim Social Security after 65, so contributions in that window can become excess contributions.
- Assuming Part D does not matter because you take no drugs. The clock runs whether or not you need the coverage, and it never stops running backwards. The cheapest available standalone plan is normally far less than 1% a month compounding into a permanent surcharge.
The eight-month special enrollment period runs from employment, not from COBRA
When coverage based on current employment ends, you get an eight-month special enrollment period to sign up for Part B without penalty. It starts the month after the employment ends or the group coverage ends, whichever comes first — and COBRA does not extend it. Someone who retires, takes eighteen months of COBRA and then applies for Part B has been outside the special enrollment period for ten months by the time they file. The Part D special enrollment period is different and shorter, generally two months after creditable coverage ends. Diarise both dates from the last day of employment, not from the last day of COBRA.
How the penalty interacts with the rest of the Medicare bill
The late enrollment penalty is one of three things that can push a Medicare premium above the standard figure, and they stack rather than substitute.
The income-related surcharge, IRMAA, is added for higher-income beneficiaries based on income from two years earlier. It is calculated independently of the penalty, so a beneficiary with both pays the standard premium, plus the income surcharge, plus the penalty percentage of the standard premium. Work out the surcharge separately with the Medicare IRMAA calculator.
The plan premiums you choose sit on top: a Part D plan's own premium, a Medigap policy's premium, or a Medicare Advantage plan's premium where it charges one. None of these changes the penalty, because the penalty is computed from national benchmarks rather than from what you pay.
There is also a Part A penalty, which this calculator does not cover because it affects very few people. Most beneficiaries get Part A premium-free on their own or a spouse's work record. Those who must buy it and enrol late pay 10% more for twice the number of years they delayed — a different structure from Part B's permanent increase, and one that does eventually end.
Finally, build the penalty into the whole retirement health budget rather than looking at it alone. A $559.20 annual penalty carried for eighteen years is real money, and it sits alongside premiums, deductibles, drug costs and the possibility of long-term care. The retirement healthcare cost calculator assembles the whole figure, and the long-term care cost projection calculator sizes the part Medicare does not cover at all.