The Medicare Part B late enrollment penalty is 10% of the standard premium for each full 12-month period you could have had Part B but didn’t sign up. For most people it lasts as long as you have Part B — meaning for life.
Two 12-month periods is 20%. Five is 50%. It compounds against you every year you wait, and it doesn’t reset when you finally enroll.
Quick answer
| Question | Answer |
|---|---|
| How much | 10% of the standard Part B premium per full 12-month period missed |
| How long you pay it | As long as you have Part B, for most people |
| When it starts counting | After your Initial Enrollment Period ends |
| Who’s exempt | Anyone enrolling during a valid Special Enrollment Period |
| Where the trap is | COBRA and retiree coverage do not count as current employment |
When you were supposed to sign up
Your Initial Enrollment Period lasts 7 months. It starts 3 months before the month you turn 65 and ends 3 months after that month.
Miss it and your next general opportunity is the General Enrollment Period, January 1 through March 31 each year. Coverage then starts the month after you sign up.
That gap is where the penalty accrues. If you turned 65 in June and finally enroll during the following year’s General Enrollment Period, you’ve passed one full 12-month period.
The exemption most people qualify for
If you kept working past 65 and had a group health plan based on current employment, you didn’t need to take Part B on time. You get a Special Enrollment Period instead.
The rule as CMS states it: you can enroll at any time while covered under a group health plan based on current employment, or during the 8-month period that begins the month the employment ends or the group health plan coverage ends — whichever comes first.
Enroll inside that window and there’s no penalty.
The COBRA trap
This is the single most expensive misunderstanding in Medicare enrollment.
COBRA feels like employer coverage. It’s the same plan, the same network, the same card. But Medicare doesn’t classify it that way.
COBRA coverage and retiree health plans aren’t considered coverage based on current employment, and people who have COBRA or retiree coverage aren’t eligible for a Special Enrollment Period when their coverage ends.
Read that carefully, because the timing matters more than it first appears. Your 8-month Special Enrollment Period starts when your employment or group coverage ends — not when your COBRA runs out. You have up to 8 months after you stop working (or lose the insurance, if that happens first) to sign up for Part B without a penalty, whether or not you elect COBRA.
COBRA typically runs 18 months. If you ride it to the end and then go looking for Medicare, your 8-month window closed 10 months earlier. The penalty starts, and it’s permanent.
The same logic applies to retiree health coverage. It’s not current employment coverage, so it doesn’t hold your Special Enrollment Period open.
What to do if you’re in the gap right now
- Find out when your group coverage based on current employment actually ended. Not when COBRA started — when the employment ended. That’s the clock.
- Count 8 months forward. If you’re still inside it, file for Part B now using the Special Enrollment Period. SSA form CMS-40B plus CMS-L564, the employment information form your employer completes.
- If the window has closed, your enrollment path is the General Enrollment Period (January 1 – March 31), with coverage starting the month after you sign up.
- Check whether the penalty was calculated correctly. It’s based on full 12-month periods, not partial ones. Enrolling before the first full 12 months elapse means no penalty at all.
Frequently asked questions
Does the penalty ever go away?
For most people, no. It’s charged for as long as you have Part B.
I had COBRA and nobody told me. Can I appeal?
You can request equitable relief from SSA if you can show you were misinformed by a federal employee or that an administrative error occurred. It’s a specific and narrow request, not a general appeal, but it exists.
Is the penalty a flat dollar amount?
No. It’s a percentage of the standard Part B premium, so it moves when the standard premium changes each year.
Does having Part A protect me?
No. Part A and Part B have separate rules, and most people get Part A premium-free at 65. The late enrollment penalty here applies specifically to Part B.
What if my spouse is still working and I’m on their plan?
A group health plan based on your spouse’s current employment counts. The Special Enrollment Period applies the same way.
Is 8 months counted from the last day of coverage?
The 8-month period begins the month the employment ends or the group health plan coverage ends. Confirm the exact month with your former employer’s benefits administrator before assuming.
Related reading on this site: Medigap Open Enrollment: Your One 6-Month Window covers the supplemental coverage window that opens once Part B starts, and Medicaid Renewal: How to Keep Coverage at Redetermination covers a different deadline that catches people the same way.
Source: Medicare.gov, "Avoid late enrollment penalties" and "COBRA coverage"; CMS, "Original Medicare (Part A and B) Eligibility and Enrollment" (https://www.medicare.gov/basics/costs/medicare-costs/avoid-penalties)