Missed Medicare Part B at 65? The First Sign-Up Window Carries No Penalty, the Second Carries 10% for Life (Birth Months January to September)

The Medicare Part B penalty is described as “10% for each year you waited”, and that is true but not the whole picture: only full 12-month periods count, the months are counted from the end of your Initial Enrollment Period to the end of the month you sign up, and after that period ends you can sign up only during a three-month window each year. Put together, your birth month decides which window carries a penalty. The numbers below come from the engine behind the Medicare late enrollment penalty calculator, which reproduces the examples Medicare prints.

The rule, in four sentences

• Your Initial Enrollment Period is the 7 months from 3 months before to 3 months after the month you turn 65. Medicare counts you as 65 the day before your birthday, so a birthday on the 1st moves everything a month earlier.
• If you do not sign up then and have no Special Enrollment Period, you can sign up only in the General Enrollment Period, January 1 to March 31, and coverage starts the month after you sign up (CMS).
• The Part B premium rises 10% for each full 12 months between the close of your Initial Enrollment Period and the close of the month you sign up (42 CFR 408.22, 408.24(b)).
• The penalty lasts as long as you have Part B, and because it is 10% of the standard premium, it moves when the premium does.

What each January-to-March window carries, by birth month

Read across: someone born in June turns 65 in June, the Initial Enrollment Period runs March to September, and the first window after it is January to March of the next year. Signing up in January counts 4 months (October to January), in March 6: no penalty. The window a year later counts 16 to 18 months and carries 10%; the one after that 28 to 30 months and 20%. The same is true for every birth month from January to September, because the first window always comes before the 12th counted month.

October to December birthdays end the Initial Enrollment Period in January to March, so the first window overlaps it: only the months after it ends can be used (a sign-up in January for an October birthday is still inside the Initial Enrollment Period). The boundary shows in the table: for November the second window is Jan 0%, Feb–Mar 10%, and for December the first is Jan–Feb 0%, Mar 10%, because the 12th counted month falls in February or March. If your birthday is on the 1st of a month, use the row of the month before.

What the 10% costs

One step is 10% of the 2026 standard Part B premium of $202.90: $20.30 a month, $243.60 a year. Over 20 years of Part B at that premium it is $4,872; the real total will differ because the premium changes every year (CMS had not announced the 2027 amount on October 22, 2026), and the penalty is on top of any income-related surcharge. A second step is twice that, a third three times. Medicare.gov's own example is a 24-month wait: $202.90 + 20% = $243.48, which it rounds to $243.50 a month.

Part D works differently: one month at a time

Part D has no 12-month steps. The penalty is 1% of the national base beneficiary premium ($38.99 in 2026) for each full month you could have had Part D and went without it or other creditable drug coverage, once the gap reaches 63 days, rounded to the nearest 10 cents (CMS). So a year of delay costs under a quarter as much per month as on Part B:

The Part D figure for one year ($4.70) is CMS's own example of Kim, who went 12 months without coverage. Part D is also forgiving in two ways Part B is not: gaps shorter than 63 days are not charged, and months before you qualified for Extra Help do not count. Both penalties last as long as you have the coverage.

If you are still working: the eight-month clock

Coverage through your own or your spouse's current work gives a Special Enrollment Period: you can sign up for Part B while you are covered and for eight months after the coverage or the job ends, whichever ends first, with no penalty. Social Security says COBRA and retiree plans do not count, and that coverage which ends while the Initial Enrollment Period is still open gives no such period. Months with employer coverage are left out of the count.

Example: eligible in June 2024, covered at work through December 2026. The Special Enrollment Period runs through August 2027, and a sign-up then carries no penalty. After it closes, the months since the coverage ended all count, and the next sign-up window is January to March 2028: January 2028 counts 13 months (10%), March 15 (10%). The count starts when the coverage ended, not when you turned 65, so the months you were covered at work do not add to the penalty.

What this does not cover

It is the arithmetic of the rule, not advice on when to enroll: waiting means no Part B coverage in the months before it starts, and the right choice depends on the other coverage you have. It does not model disability-based Medicare, the exceptional-conditions Special Enrollment Periods, a Medicare Savings Program (Medicare.gov says you generally owe no Part B penalty if you enroll in one), the Part A penalty (a different rule, for people who must buy Part A), or income-related surcharges. A sign-up month is not the same as the month you are covered: Part B starts the month after you sign up in the General Enrollment Period. Confirm your dates with Social Security, which decides the penalty.