The complete guide

Medicare enrollment: every window, every deadline, every penalty

Short answer: most people enroll during the seven-month window around their 65th birthday, and some are enrolled automatically without doing anything. If you are still working with employer coverage, you can delay without penalty and use a Special Enrollment Period later. If you miss your window with no valid excuse, the General Enrollment Period in January through March is your catch-up, and permanent penalties apply. Here is every window, how to tell which one applies to you, how to actually sign up, and what the penalties cost in real dollars.

By the Medicare Decoder team · Updated October 12, 2026 · 4-minute read · Figures are official CMS amounts, checked against CMS

Key takeaways

Your Initial Enrollment Period is seven months: three months before your birthday month, the month itself, and three after.
If you already receive Social Security, Parts A and B start automatically; everyone else has to apply.
Active employer coverage lets you delay Part B penalty-free, and gives you an 8-month Special Enrollment Period when it ends.
COBRA and retiree coverage do not count as active employer coverage for delaying Part B.
The Part B penalty is 10% per 12 months late, permanently; the Part D penalty is 1% of the base premium per month.

Jump to Which window is yours Automatic vs. applying How to sign up If you are still working The penalties FAQ

Which window applies to you

Window	When	Who it is for	Coverage starts
Initial Enrollment Period	7 months around your 65th birthday month	Almost everyone turning 65	First of your birthday month if you enroll early; otherwise the month after you enroll
Special Enrollment Period	8 months after active employer coverage ends	People who worked past 65 with group coverage	Usually the month after you enroll
General Enrollment Period	January 1 to March 31	People who missed their window with no excuse	The month after you enroll, with penalties attached
Annual Enrollment (AEP)	October 15 to December 7	Existing beneficiaries changing Advantage or Part D plans	January 1
Medicare Advantage Open Enrollment	January 1 to March 31	People already in an Advantage plan	First of the month after the change

Other Special Enrollment Periods exist for moving, plan terminations, Medicaid changes, and more; the full catalog is in our SEP article.

Note the difference between the first three rows and the last two. The first three are about getting into Medicare. The last two are about changing private plans once you are already in it, which is what the fall advertising blitz is about (the open enrollment page). Mixing them up is common: you cannot use October’s window to enroll in Part B for the first time.

Automatic enrollment, or applying yourself

You are enrolled automatically in Parts A and B if you are already receiving Social Security or Railroad Retirement benefits when you turn 65, or if you have received disability benefits for 24 months. Your card arrives in the mail about three months before your birthday month. You can decline Part B if you have active employer coverage, by following the instructions that come with the card.

You must apply yourself if you are not yet taking Social Security, which is now most people, since many delay benefits past 65 to grow them. Nobody will remind you, and the lack of a reminder is responsible for a large share of late-enrollment penalties.

How to actually sign up

Online at ssa.gov/medicare, which takes about ten minutes and is the fastest route for most people. Social Security administers Medicare enrollment, not Medicare itself.
By phone at 1-800-772-1213, or at a local Social Security office by appointment.
If you are delaying because you are working: when you do enroll, you will need form CMS-L564, which your employer signs to prove you had creditable coverage, plus form CMS-40B. Request it before you leave the job; chasing a former employer’s HR department later is harder.
Then choose your path: Medicare itself is only Parts A and B. Adding Medigap and Part D, or choosing Advantage, are separate enrollments you make with private insurers or through the Plan Finder.

If you are still working at 65

The rule turns on employer size. If your employer has 20 or more employees, the group plan generally pays first and you may delay Part B penalty-free while actively employed. If your employer has fewer than 20, Medicare usually pays first, and delaying Part B can leave you with very little coverage, so most people in small-employer situations enroll at 65. Either way, take premium-free Part A unless you contribute to an HSA, because Part A enrollment ends HSA eligibility and can apply retroactively up to six months. Full detail in working past 65.

What the penalties actually cost

Penalty	How it is calculated	Example
Part B late enrollment	10% of the standard premium for each full 12 months you were eligible and not enrolled, permanently	Two years late: about 20% added to $202.90, roughly $40 a month for life
Part D late enrollment	1% of the national base beneficiary premium per uncovered month, permanently	Three years late: about 36% of $41.33 in 2027, roughly $15 a month for life
Part A (only if you buy in)	10% added for twice the number of years you were late	Rare; applies only to people without enough work history

Both penalties attach to the national figures, so they grow as those figures rise. Creditable coverage from active employment prevents both.

There is one route out of a Part B penalty worth knowing: if an employer or a government representative gave you bad information that caused the delay, you can request equitable relief from Social Security. It is not automatic and it is not widely advertised, but it exists and is granted. Free help with that, and with everything else on this page, is available through SHIP counselors at no cost.

Questions people also ask

When can I sign up for Medicare?

During your Initial Enrollment Period, the seven months spanning the three months before your 65th birthday month, that month, and the three months after. Other windows exist: an 8-month Special Enrollment Period after active employer coverage ends, and a General Enrollment Period each January 1 to March 31.

Am I automatically enrolled in Medicare at 65?

Only if you are already receiving Social Security or Railroad Retirement benefits, or have received disability benefits for 24 months. If you have delayed Social Security, you must apply yourself, and nobody will remind you.

Can I delay Medicare if I am still working?

Yes, if you have active coverage through an employer with 20 or more employees. You can delay Part B penalty-free and get an 8-month Special Enrollment Period when that employment or coverage ends. COBRA and retiree coverage do not qualify.

How do I sign up for Medicare?

Through Social Security, not Medicare: online at ssa.gov/medicare, by phone at 1-800-772-1213, or at a local office. If you delayed because of employer coverage, you will also need form CMS-L564 signed by the employer and form CMS-40B.

What is the penalty for enrolling in Medicare late?

Part B adds 10% to your premium for each full 12 months you were eligible but not enrolled, permanently. Part D adds 1% of the national base premium for each month without creditable drug coverage, also permanently. Both grow as the underlying figures rise.

Sources for this article

SSA.govSocial Security: Medicare: enrollment windows, how to apply, and the forms involved.
Medicare.govSigning up for Medicare: who is enrolled automatically and when coverage begins.
CMS.gov2026 premiums and deductibles: the premium amounts the penalties are calculated against.

Figures checked against these sources on October 12, 2026. We update within days when CMS publishes new amounts.

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