Missing the Medicare Part B enrollment window has a price. Here is what it actually costs.

Federal retirees who miss the Medicare Part B enrollment window without a valid SEP usually discover the consequences in three layers, each worse than the one above it. The first layer is a permanent premium surcharge. The second is a coverage gap that lasts until the next General Enrollment Period. The third is the lifetime dollar cost of the surcharge compounded across a normal retirement. Together, the three layers make a missed Part B window one of the most expensive mistakes in federal retirement.

The penalty is not theoretical. It is published by CMS, calculated by Social Security, and applied for the rest of the retiree’s life. The penalty applies whether the missed enrollment was an oversight, a misunderstanding, or a deliberate choice that turned out to be wrong.

The first consequence: a permanent premium surcharge.

The Part B late-enrollment penalty is 10% of the standard Part B premium for every full 12 months the beneficiary could have been enrolled in Part B but was not. The penalty is applied to the standard premium for the year of coverage, and it continues for as long as the beneficiary has Part B.

The penalty applies on top of any IRMAA surcharge the beneficiary may also owe. A higher-income beneficiary who also missed the enrollment window can end up paying the standard premium plus the late-enrollment surcharge plus the IRMAA surcharge. None of the three are mutually exclusive.

The penalty is calculated in full 12-month increments. Delaying Part B by 11 months does not trigger the penalty. Delaying by 13 months triggers 10%. Delaying by 25 months triggers 20%. The increments are strict, and the rounding does not bend.

The second consequence: a coverage gap.

Outside of an IEP or a valid SEP, the only time a beneficiary can enroll in Part B is during the General Enrollment Period (GEP), which runs January 1 through March 31 each year. Coverage under a GEP enrollment does not begin until the month following the enrollment.

The practical effect is that a retiree who misses the IEP or SEP and waits for the next GEP may go several months without Part B coverage. During those months, FEHB remains the primary payer, the retiree pays the FEHB deductible and coinsurance on every claim, and there is no Medicare coordination to reduce the out-of-pocket cost.

For a retiree with a serious health event during the coverage gap, the financial exposure can be substantial. For a retiree without a serious health event, the gap is still uncomfortable — the FEHB-only arrangement costs more than the Medicare-primary arrangement almost every time.

The third consequence: the lifetime dollar cost.

The dollar cost of the penalty compounds across a normal retirement. On 2026 numbers, the standard Part B premium is $202.90. A retiree who delayed Part B for 24 months without a valid SEP pays a 20% surcharge, or about $40 per month, on top of the standard premium. That is roughly $481 per year, and it continues for as long as the retiree has Part B.

A retiree who lives another 25 years after enrolling in Part B pays roughly $12,000 in lifetime late- enrollment penalties, in addition to the standard premiums they would have paid anyway. A retiree who delayed for 36 months pays 30%, or roughly $18,000 over 25 years. The surcharge also rises over time, because the standard premium rises over time, and the penalty is calculated as a percentage of the standard premium.

The lifetime dollar figure is the one most retirees underestimate. The monthly surcharge looks small enough that retirees absorb it and move on. The lifetime figure is the one that would have changed the decision if the retiree had seen it earlier.

The situations where the penalty does not apply.

The Part B late-enrollment penalty is not applied in every missed-enrollment scenario. The most important exception is the Special Enrollment Period triggered by loss of EGHP coverage based on current employment. A federal employee who retires at 67 with continuous FEHB coverage based on active employment can use the SEP to enroll in Part B without triggering the penalty, even though they declined Part B at 65.

Other exceptions exist for certain international volunteers, for certain disabled individuals, and for beneficiaries who lose Medicaid. The exceptions are narrow, and the documentation requirements are strict. Federal retirees should not assume an exception applies without confirming it with Social Security directly.

How to avoid the consequences.

The cleanest way to avoid the penalty is to enroll in Part B during the IEP at 65, even if you are still working and covered by FEHB. The premium is the same as the SEP premium. The Medicare-primary coordination starts immediately. The risk of a coverage gap or a missed deadline disappears.

The second-cleanest way is to delay Part B while working, with active-employment EGHP coverage, and enroll during the SEP at retirement. This path works but requires that the documentation be in order and the eight-month window be respected. It is the path most federal retirees use, and it works — when it is used correctly.

The path that almost always fails is delaying Part B beyond the SEP without enrolling during the next GEP, on the assumption that FEHB alone will be sufficient. FEHB alone is sufficient for many retirees, but it does not protect against the late-enrollment penalty when Part B is eventually enrolled.

For more on the mechanics of the SEP, read how the Special Enrollment Period for Medicare Part B works.