Medicare enrollment periods are the defined windows when you can sign up for or change Medicare coverage, and there are five of them: the Initial Enrollment Period (seven months around your 65th birthday), the Special Enrollment Period (eight months after employer coverage ends), the Annual Enrollment Period (October 15 to December 7), the General Enrollment Period (January 1 to March 31), and the Medigap Open Enrollment (six months after Part B starts). Once a window closes, you usually cannot act in it again, which is why understanding how Medicare enrollment periods work matters more than any single plan choice you make later.
This guide was last reviewed in October 2026. Rules and dates can change, so verify your own deadlines with the Social Security Administration (SSA) at SSA.gov or Medicare.gov, or with a free State Health Insurance Assistance Program (SHIP) counselor. Nothing here is individual financial or insurance advice.
Table of Contents
- How Medicare Enrollment Periods Work
- What Is the Initial Medicare Enrollment Period?
- Worked examples by birth month
- How Medicare enrollment periods work day to day
- Do you get Part A and Part B automatically?
- Who else gets an IEP?
- How Does Medicare Part D Enrollment Work?
- What Happens During the Annual Medicare Enrollment Period?
- Can You Change Medicare Coverage Outside Annual Enrollment?
- What does not open a SEP
- What Are the Medicare Enrollment Period Deadlines?
- What Happens If You Miss a Medicare Enrollment Deadline?
- The mistakes that cause the most damage
- How Do You Choose the Right Medicare Plan?
- What Should You Do Before Your Enrollment Deadline?
- Frequently Asked Questions
- What is the three-month rule for Medicare?
- Why is there a waiting period to qualify for Medicare?
- Does COBRA coverage qualify for a Special Enrollment Period?
- Can I choose my start date for Medicare?
- Can I enroll in Medicare Part A only?
- Do I automatically get Medicare Part B at 65?
- What to Do First
How Medicare Enrollment Periods Work
Each enrollment period is a countdown attached to a specific event in your life: turning 65, losing a job, or simply the start of a new calendar year. Inside the window, you have real choices. Outside it, the government generally closes the door, and that closure is what produces late enrollment penalties.
Two different dates matter and people mix them up constantly. The deadline is the last day you can submit an enrollment. The effective date is the day your coverage actually begins, and it is set by rule rather than by the day you happen to submit, so signing up inside your window does not mean coverage starts that day.
Here is the whole system in one place.
| Enrollment period | Dates | Who it applies to | What you can change | If you miss it |
|---|---|---|---|---|
| Initial Enrollment Period (IEP) | 7 months: 3 before your 65th birthday month, the birthday month itself, and 3 after | People turning 65 in the next 3 months, and people eligible due to disability or ESRD | Enroll in Part A, Part B, or both; add Part D or a Medicare Advantage plan | Wait for the General Enrollment Period and pay a possible late penalty |
| Special Enrollment Period (SEP) | 8 months from the month employer coverage ends; 2 months in some Part D situations | People who had employer or retiree drug coverage, or who moved | Enroll in Part B and/or Part D after losing creditable coverage | Same penalty exposure as above |
| Annual Enrollment Period (AEP) | October 15 to December 7 | Anyone already on Medicare | Switch Medicare Advantage plans, switch Part D plans, join, drop, or change Medigap carriers | You stay with your current plan until you can act in a later period |
| General Enrollment Period (GEP) | January 1 to March 31 | People already on Medicare, and anyone who missed an earlier window | Enroll in Part A, Part B, or a Medicare Advantage plan | Coverage gap until the next qualifying period; possible late penalties |
| Medigap Open Enrollment | 6 months from the day Part B starts | Anyone with Part B who is under 65, or who had Part A before turning 65 | Buy any Medigap policy from any carrier without medical underwriting | Later purchases can face underwriting based on health conditions |
What Is the Initial Medicare Enrollment Period?
The Initial Enrollment Period is a seven-month window built around the month you turn 65. It includes the three months before your 65th birthday, the month of your birthday, and the three months after it.
Worked examples by birth month
If you turn 65 in June, your IEP runs from March 1 through September 30. Signing up in April, May, or June means coverage starts on the first day of June. Signing up in July, August, or September pushes your start date back to the first of the following month. If you turn 65 in January, your window runs the previous October through the following April.
Enrolling in the first three months is worth planning around when your birthday falls early in the month. Wait until June 15 and your coverage still starts June 1, which can feel odd but works in your favor. Wait until July 2 and it starts August 1 instead.
How Medicare enrollment periods work day to day
Search for your birth month rather than doing the math yourself. Getting these dates right is the practical half of how Medicare enrollment periods work, and the rule never changes, so the table below holds in every year.
| Month you turn 65 | Your IEP opens | Your IEP closes |
|---|---|---|
| January | October, the year before | April 30 |
| February | November, the year before | May 31 |
| March | December, the year before | June 30 |
| April | January | July 31 |
| May | February | August 31 |
| June | March | September 30 |
| July | April | October 31 |
| August | May | November 30 |
| September | June | December 31 |
| October | July | January 31, the following year |
| November | August | February 28 or 29, the following year |
| December | September | March 31, the following year |
Do you get Part A and Part B automatically?
If you are already getting Social Security or Railroad Retirement Board benefits when you turn 65, you are generally enrolled automatically in both Part A and Part B. Several people discover this only when the premium comes out of their Social Security payment, and you can opt out with a signed request to your Social Security office.
If you are not receiving Social Security, nothing happens automatically. You apply yourself.
Who else gets an IEP?
People under 65 with certain disabilities qualify, and so do people with end-stage renal disease, though the rules differ. There is generally a 30-month waiting period for ESRD eligibility, and the clock does not start until the condition is expected to require dialysis or a kidney transplant. Disability eligibility has its own rule tied to how long you have received disability benefits. If you are in either category, contact SSA directly instead of counting seven months around a birthday.
How Does Medicare Part D Enrollment Work?
Part D is the prescription drug benefit, and it travels with Part A in two ways. First, if you buy a Medicare Advantage plan that includes drug coverage, Part D is bundled in. Second, if you have Original Medicare and you have creditable coverage through an employer or retiree plan, you can keep that coverage and delay enrolling in Part D without penalty.
What does not count as creditable coverage: COBRA, retiree plans that do not meet the actuarial test, the ACA marketplace, short-term plans, and most state pharmaceutical assistance programs. If none of those apply to you, enrolling in Part A triggers a Part D too, and you get a short special enrollment period to add a standalone plan if you want one. That window is two months, which is genuinely tight, and missing it leaves you exposed to the Part D late penalty.
Part D late enrollment adds roughly 10 percent to your monthly premium for every full 12-month period you could have had coverage but did not. The penalty stays with you for as long as you keep Part D, which surprises people more than the amount does.
What Happens During the Annual Medicare Enrollment Period?
The Annual Enrollment Period runs from October 15 to December 7 every year. Anything you decide during that window takes effect on January 1 of the following year. The election is always forward-looking; you cannot retroactively switch into a plan you wanted last March.
During AEP you can move between Medicare Advantage plans, switch Part D plans, join or drop a plan, or change Medigap carriers. What you cannot do is start Part B for the first time if you have been declining it while covered by your employer, because Part B enrollment is governed by the IEP and SEP rules rather than by AEP.
One confusion worth clearing up: Medicare’s Annual Enrollment Period is not the same as the health insurance marketplace open enrollment that runs in the fall for everyone else. Different program, different dates, different penalties. Signing up through the marketplace does not enroll you in Medicare.
Can You Change Medicare Coverage Outside Annual Enrollment?
Yes, and the most common reason is a change in your job. If you had employer-sponsored medical coverage that counted as creditable and it ends, you get a Special Enrollment Period that runs eight months, starting the month before the coverage ends if you tell SSA in advance, or the month it ends. Two separate employment-based mistakes are common here: one is assuming the window is measured from the last day of work, and the other is counting backwards instead of forwards from the first month without coverage.
The 20-employee rule matters too. Employer plans with 20 or more employees generally count as creditable coverage. Plans from employers with fewer than 20 employees, and many retiree plans, usually do not, though there are exceptions. If your employer plan is small, ask the benefits administrator in writing before you rely on it.
Moving to a different state triggers a SEP, and so does losing Medicaid coverage. Leaving a Medicare Advantage plan gives you a two-month window to return to Original Medicare and buy Part D, and in many states a special Medigap window as well.
What does not open a SEP
COBRA coverage is the big one. People buy COBRA expecting it to protect them, then learn it does not count as creditable coverage for Medicare purposes and they owe penalties. Retiree plans that fail the creditable coverage test, ACA marketplace plans, short-term insurance, and going uninsured by choice all fail the same way, so get your specific case answered in writing by SSA.
What Are the Medicare Enrollment Period Deadlines?
The dates below recur the same way every year unless Congress changes them. Confirm yours before you act.
| Action | Deadline | Coverage starts |
|---|---|---|
| Sign up during your Initial Enrollment Period, before your birthday | Three months before your 65th birthday month | First day of your 65th birthday month |
| Sign up during your IEP, after your birthday | Three months after your 65th birthday month | First day of the following month |
| Use the SEP after employer coverage ends | 8 months from the month coverage ends | Varies: first day of the month you apply or the first of the following month, with protections tied to prior coverage |
| Change Advantage, Part D, or Medigap carriers | December 7 | January 1 |
| Enroll in the General Enrollment Period | March 31 | First day of the month after you apply, or April 1 in some circumstances |
| Buy Medigap with guaranteed issue | 6 months after your Part B starts | First day of the month after you buy |
Application processing takes time. People who apply online through SSA.gov report their Medicare number sometimes taking a couple of days to appear in their online profile, so submit before your window closes rather than on the final day.
What Happens If You Miss a Medicare Enrollment Deadline?
Missing the initial window when you first turn 65 typically means waiting for the General Enrollment Period, which starts January 1. Coverage would then begin no earlier than April 1 in many cases. That stretch can be a real gap during which you are responsible for hospital bills, which is why people panic about a missed deadline, and the panic is proportionate.
The financial consequence is a late enrollment penalty of 10 percent of the Part B standard premium for each full 12-month period you could have been enrolled but were not. Two full years late means roughly 20 percent added, and it applies for as long as you hold Part B. Working the arithmetic across decades is what turns this from an annoyance into a serious decision for some people.
Part D carries its own penalty on the same 10 percent-per-year basis, and it follows you for as long as you keep a Part D plan.
Not every situation is penalised. If you had employer coverage, a SEP may still be available to you even if you missed the IEP, because that clock runs from when the coverage ended rather than from your birthday. People who had Medicaid coverage or another arrangement that counted should tell SSA and ask about the exception, since people with limited resources may also qualify for help with premiums.
The mistakes that cause the most damage
- Waiting for age 65 while uninsured instead of signing up early. The gap has to be paid for, and you were not eligible to buy anything outside a window.
- Assuming COBRA bridges the transition. It does not, and people who rely on it are surprised by a penalty years later.
- Enrolling in Part A only and forgetting that Part B is a separate decision with its own clock.
- Treating the Annual Enrollment Period as the moment to first join. It is for changing plans you already have.
- Letting a broker set the dates. Deadlines are fixed by federal rule, and bad advice about them is common enough that forum threads about it run for years.
- Inheriting the paperwork for a parent late. A family member who joins the task six months into an eight-month SEP is a recurring and avoidable scenario.
How Do You Choose the Right Medicare Plan?
Compare on the things that actually change your bill, not on marketing language. Original Medicare lets you see any doctor who accepts Medicare; Medicare Advantage plans often use networks and may require prior authorization for some services. If you travel often or split your year between states, check whether a plan follows you.
Look at prescriptions next. Note which drugs you take regularly, then check each plan’s formulary and what you would pay at your usual pharmacy. Formulary changes between years are common, so the plan that suited you last fall may not be the same one next fall.
Then work out the total yearly cost: premiums, a Part D deductible, coinsurance, and for Advantage plans an annual out-of-pocket maximum. Part B and Part D premiums can also increase if your modified adjusted gross income is higher than the threshold, a surcharge commonly called IRMAA, and it lasts a couple of years after income drops rather than resetting immediately.
Medigap fills Part A and Part B gaps but not prescriptions and not Part C costs. Buy it inside your six-month window and the underwriting disappears; buy later and insurers can ask about your health history.
What Should You Do Before Your Enrollment Deadline?
- Write down which window you are actually in, using the table above, and put its closing date on a calendar with a two-week warning.
- Check whether any coverage you have now counts as creditable, and get that in writing from the employer or plan administrator.
- List your doctors, regular medications, and travel pattern before you look at any plan.
- Estimate the yearly total, not just the monthly premium, using the plan’s own official information.
- Apply through SSA.gov, by phone at 800-772-1213, or at a local Social Security office, before the deadline rather than on it.
- Confirm you actually want both Part A and Part B. Enrolling in Part A only is legal, but adding Part B later can trigger a penalty unless you had continuous equivalent coverage.
- Save your confirmation and Medicare number somewhere you will find them again in six months.
If you want someone neutral to walk through this with you, a SHIP counselor provides free help and does not sell plans.
Frequently Asked Questions
What is the three-month rule for Medicare?
The three-month rule is the structure of your Initial Enrollment Period. Your window runs three months before the month of your 65th birthday, the month itself, and three months after. Enroll in the first three months and coverage begins the first day of your birthday month. Enroll later and it starts the first of the following month.
Why is there a waiting period to qualify for Medicare?
The wait is not about age, it is about eligibility status. People under 65 with ESRD generally cannot qualify until the condition has persisted for 30 months, the point at which dialysis or a transplant is expected. Disability-based eligibility has its own waiting rules tied to the length of time you have received disability benefits. Neither wait applies to people who qualify simply by turning 65.
Does COBRA coverage qualify for a Special Enrollment Period?
No. COBRA does not count as creditable coverage for Medicare purposes, and relying on it instead of Part B or Part D is one of the costliest mistakes people make at 65. Retiree plans, marketplace plans, and short-term insurance usually fail the same test. Small employer plans under 20 employees often do not count either, though exceptions exist. Ask SSA to confirm your specific situation.
Can I choose my start date for Medicare?
Only within limits. Inside your Initial Enrollment Period, choosing a date in the first three months locks your start to the first day of your birthday month. Choosing a later month moves your start to the first day of the following month. In the General Enrollment Period, coverage normally begins the first day of the month after you apply. You cannot pick an arbitrary date outside those rules.
Can I enroll in Medicare Part A only?
You can. Part A is hospital insurance, and many people with employer coverage enroll in it alone while delaying Part B. The catch is that enrolling in Part A also starts Part D eligibility, so you may want a standalone Part D plan. If you later add Part B and never had Part A or equivalent coverage, a late penalty can apply for the months you skipped.
Do I automatically get Medicare Part B at 65?
If you receive Social Security or Railroad Retirement Board benefits, you are generally enrolled automatically in Part A and Part B. Nothing happens automatically if you are not receiving those benefits, so you must apply yourself. If you are covered by employer insurance and do not want Part B yet, submit a signed opt-out request, and be deliberate about it.
What to Do First
Find your window before you find a plan. Look at the table above, identify which of the five Medicare enrollment periods applies to you, and write its closing date somewhere you will see it every week. Then confirm the details with SSA at SSA.gov, Medicare.gov, or a free SHIP counselor, and ask about any coverage you currently have before assuming it protects you.
