The Medicare Part B Special Enrollment Period (SEP) gives you up to eight months to sign up for Part B after you lose employer-based health coverage, without paying a late enrollment penalty. Most people who search for this have delayed enrolling in Part B because they had insurance through their own or a spouse’s job, and now need to know exactly how the process works and how long they have.
Who Qualifies for the SEP
The most common path to a Special Enrollment Period is straightforward: you turned 65 (or older), had health coverage through an active employer group health plan, and either you or your spouse is still working or recently stopped. As long as that employer coverage was active and based on current employment, you qualify for an SEP when it ends.
The key word is “active.” COBRA coverage, retiree health plans, and individual marketplace plans do not count. The coverage must be tied to current employment, either yours or your spouse’s. If you retired at 66 and kept COBRA for 18 months, that entire COBRA period does not protect you from late enrollment penalties.
Employer Size Matters
For the employer plan to serve as your primary insurance (meaning it pays before Medicare), the employer generally needs at least 20 full-time or part-time employees. If your employer has fewer than 20 employees, Medicare is typically your primary payer, and the group plan becomes secondary. In that situation, delaying Part B enrollment can leave you with significant gaps in coverage, because the employer plan may not cover what Medicare would have paid first. If your small employer participates in a multi-employer plan where at least one employer has 20 or more employees, the standard rules apply and the plan remains primary.
How the 8-Month Window Works
You can enroll in Part B at any time while you’re still covered under the employer group plan based on current employment. Once that coverage ends, or once the employment itself ends (whichever happens first), an 8-month enrollment window opens. The clock starts the month after the triggering event. If your last day of work is June 15 and your employer coverage also ends in June, your SEP runs from July through February of the following year.
This is a hard deadline. If you miss the 8-month window, your next opportunity to enroll is during the General Enrollment Period, which runs January 1 through March 31 each year, with coverage not starting until July. And you’ll face a late enrollment penalty that sticks with you permanently.
What the Late Enrollment Penalty Looks Like
Without an SEP, Medicare adds 10% to your monthly Part B premium for every full 12-month period you could have been enrolled but weren’t. That penalty is permanent. It never goes away and gets recalculated each year as the standard premium changes.
For example, if you went two full years without Part B and didn’t qualify for an SEP, you’d pay a 20% surcharge on top of the standard premium. With the 2026 standard premium at $202.90 per month, that’s roughly an extra $40.58 every month for the rest of your life on Medicare. Enrolling during a valid SEP eliminates this penalty entirely.
When Your Coverage Starts
If you sign up for Part B while you or your spouse are still working and covered, or within the first full month after employer coverage ends, your coverage generally begins the first month after you sign up. You also have the option to delay your Part B start date by up to three months in this scenario, which can help you avoid overlapping coverage and double-paying premiums.
If you enroll later in your 8-month window (after that first full month), coverage typically starts the month after you sign up. There’s no retroactive coverage, so the sooner you act, the smaller any gap between your old coverage and Medicare will be.
Forms You’ll Need to Submit
Two forms are required. The first is CMS-40B, which is your formal request to enroll in Part B. The second is CMS-L564, the Request for Employment Information. You fill out Section A of the CMS-L564 yourself, and your employer (or former employer) completes and signs Section B, confirming the dates of your group health plan coverage and employment.
Both forms get submitted together to your local Social Security office by mail, fax, or in person. You can also start the process by calling Social Security at 1-800-772-1213. Getting the employer section signed promptly matters, because processing delays can push your coverage start date back.
SEP for International Volunteers
A separate, less common SEP exists for people who served as volunteers outside the United States. If you volunteered through a program lasting at least 12 months, sponsored by a tax-exempt nonprofit organization, and either skipped enrolling in Part B when first eligible or dropped your enrollment during your service, you get a 6-month SEP. You don’t need to have actually served the full 12 months; the program itself just needs to cover that duration.
This 6-month window begins the first day of the month when you no longer meet the volunteer criteria. For instance, if you returned from volunteer service on October 25, your SEP runs from October 1 through March 31 of the following year.
SEPs for Exceptional Circumstances
Medicare also grants Special Enrollment Periods for exceptional situations like natural disasters or emergencies that prevented you from enrolling on time. If you sign up for Part A or Part B through one of these circumstance-based SEPs, you also get a two-month window to join a Medicare Advantage Plan or a Part D drug plan. Coverage through those plans starts the first day of the month after the plan receives your enrollment request.
These SEPs are handled case by case. If a hurricane, wildfire, or other disaster disrupted your ability to enroll during your normal window, contact Social Security to explain the situation and request relief. Documentation of the event and how it affected your enrollment helps.
Common Mistakes That Cost People Money
The most expensive mistake is assuming any health coverage protects you. COBRA, retiree plans, health sharing ministries, and ACA marketplace plans do not qualify you for an SEP. If you rely on any of these after turning 65 without enrolling in Part B, every month that passes adds to your penalty calculation.
Another common error is waiting until employer coverage actually ends to start thinking about enrollment. You can enroll while still working and covered, which gives you the smoothest transition with no gap. Many people also forget that the 8-month window is tied to the earlier of two events: the end of employment or the end of coverage. If you stop working in March but your employer plan continues through May, your 8-month clock still starts in April, based on when employment ended.
Finally, if your employer’s HR department gave you incorrect information about how Medicare coordinates with your plan, that bad advice doesn’t automatically extend your enrollment window. Documenting any misinformation may help if you need to appeal, but it’s not a guaranteed remedy.