Medicare Enrollment Periods: When and How to Sign Up

Getting Medicare enrollment timing right is one of the most financially consequential decisions you’ll make around retirement. Enroll at the right time and your coverage starts smoothly. Miss the wrong window and you could face a permanent premium penalty — and a gap in coverage that exposes you to significant out-of-pocket risk.

The cost of getting it wrong is concrete: miss your seven-month Initial Enrollment Period by two full years and Part B costs 20% more for the rest of your life — about $487 extra per year at the 2026 premium, recalculated upward every year the base premium rises. Which of the seven Medicare enrollment periods applies to you, and when, decides whether that penalty ever attaches.

Medicare Enrollment Windows at a Glance

Medicare has more enrollment windows than most people realize, and they do different jobs. Some are for signing up the first time; others are only for changing coverage you already have. Confusing the two is the single most common — and most expensive — enrollment mistake. Here is the full map:

The rest of this guide walks through each of these in turn, then covers the penalties, the under-65 disability path, the high-income wrinkle, and a pre-65 checklist.

The Four Core Medicare Enrollment Periods

1. Initial Enrollment Period (IEP)

Your first opportunity to enroll in Medicare is the Initial Enrollment Period (IEP) — a 7-month window surrounding your 65th birthday:

• 3 months before the month you turn 65
• The month you turn 65
• 3 months after the month you turn 65

When your coverage starts depends on when you enroll within this window:

Enrolling early — in the first three months of your IEP — gets you the fastest start. Waiting until the month of or after your birthday delays coverage.

What you can enroll in during the IEP: Medicare Part A, Part B, a standalone Part D plan, and Medicare Advantage (Part C) plans.

2. General Enrollment Period (GEP)

If you miss your IEP without qualifying for a Special Enrollment Period, your next opportunity is the General Enrollment Period (GEP): January 1 through March 31 each year, with coverage starting July 1 of that year.

The GEP comes with a downside: if you needed Part B and missed your IEP without a qualifying exception, you’ll owe the Part B late-enrollment penalty. This permanent surcharge adds 10% to your Part B premium for each full 12-month period you were eligible but didn’t enroll.

Example: You were eligible at 65 in 2024 but didn’t enroll. You sign up during the January–March 2026 GEP. That’s two full years of delay — your Part B premium increases by 20% permanently.

At $202.90/month (2026 standard premium), a 20% increase means paying an extra $40.58/month for life — about $487/year, and it compounds: the penalty is recalculated as a percentage of the standard premium every year, so as the base premium rises, so does the dollar penalty. Over a 20-year retirement that single missed window can cost well over $10,000.

Since 2023, GEP coverage no longer waits until July. Under the BENES Act simplification, coverage now starts the first day of the month after you enroll during the GEP — a meaningful improvement over the old rule that left late enrollees uninsured for up to six months.

3. Special Enrollment Period (SEP)

A Special Enrollment Period lets you enroll in Medicare outside the IEP and GEP without triggering a late-enrollment penalty. The most common qualifying event is continued employer coverage.

Active employer coverage SEP: If you’re working past 65 and covered by a group health plan through your own active employment (or a spouse’s active employment), you can delay Part B and Part D without penalty. You have an 8-month SEP starting the month after your employment ends or your group coverage ends — whichever comes first.

Critical rules for the employer coverage SEP:

• Coverage must be from active employment — not COBRA, retiree coverage, or marketplace coverage
• COBRA does not qualify you for an SEP — if you rely on COBRA to bridge to Medicare, you may face a late penalty
• The SEP is 8 months — not until the next General Enrollment Period

Other qualifying SEP events include:

• Moving to a new service area (relevant if you split the year between states — see the Medicare snowbird guide)
• Losing Medicaid or Extra Help eligibility (if your income changes, check whether you still qualify for a Medicare Savings Program)
• Gaining creditable drug coverage
• Certain changes in Medicare Advantage or Part D plan availability

The 2023 “exceptional conditions” SEPs: As of January 1, 2023, CMS added several new Special Enrollment Periods for Part B that didn’t exist before. These give a penalty-free path to enroll if you were knocked off course by circumstances outside your control:

• Disaster or emergency SEP — if a federal, state, or local emergency declaration prevented you from enrolling during another valid window.
• Health plan or employer error SEP — if an employer or health plan gave you misleading information that caused you to miss enrollment.
• Formerly incarcerated SEP — a window to enroll after release from a correctional facility.
• Termination of Medicaid SEP — a six-month window to enroll in Part B after losing Medicaid eligibility (important during the post-pandemic Medicaid “unwinding”).

Each of these grants up to six months to enroll and waives the late penalty. If you think one applies to you, contact Social Security and ask specifically for the exceptional-conditions SEP — front-line staff don’t always volunteer it.

4. Annual Enrollment Period (AEP)

The Annual Enrollment Period runs from October 15 to December 7 each year. This is not an enrollment period for first-time Medicare enrollees — it’s for people who are already on Medicare and want to change their coverage.

During the AEP, you can:

• Switch from Original Medicare to Medicare Advantage
• Switch from Medicare Advantage to Original Medicare
• Switch between Medicare Advantage plans
• Join a Part D plan (if you don’t have one)
• Switch between Part D plans
• Drop Part D coverage

Changes made during AEP take effect January 1 of the following year.

The AEP is your annual opportunity to review your coverage. Part D drug formularies and premiums change each year, as do Medicare Advantage plan networks and benefits. Even if you’ve been happy with your current plan, it’s worth comparing options each fall.

Medicare Advantage-Specific Enrollment Periods

Medicare Advantage Open Enrollment Period (OEP)

From January 1 to March 31 each year, Medicare Advantage enrollees have an additional window to:

• Switch from one Medicare Advantage plan to another
• Drop Medicare Advantage and return to Original Medicare (adding a Part D plan if desired)

This period is only available if you’re already enrolled in a Medicare Advantage plan. You cannot use it to enroll in Medicare Advantage for the first time — you must use the IEP or AEP for initial Advantage enrollment.

Initial Coverage Election Period for Medicare Advantage

When you first become eligible for Medicare, you can enroll in a Medicare Advantage plan during your IEP. If you’re already enrolled in Original Medicare, you can switch to Medicare Advantage during the AEP or if you have a qualifying SEP. For a full breakdown of the fall enrollment season, see the Medicare annual enrollment guide.

The 5-Star Special Enrollment Period

Medicare rates every Advantage and Part D plan on a 1-to-5 star scale for quality and service. If a 5-star plan is available in your area, you get a special once-per-year window — running December 8 through November 30 — to switch into it, outside the normal enrollment calendar. It’s a small but genuinely useful escape hatch: if you’re stuck in an underperforming plan and a 5-star option exists where you live, you don’t have to wait for the AEP. You can use this SEP only once per year, and only to move into a 5-star plan.

Deciding between Original Medicare plus Medigap and an Advantage plan in the first place is a separate question — compare the two approaches here and see how the total costs stack up.

The Part D Late-Enrollment Penalty

The Part D late-enrollment penalty applies separately from the Part B penalty and is calculated differently.

The formula: 1% of the national base beneficiary premium × number of full months you were eligible for Part D but didn’t have creditable coverage.

In 2026, the national base beneficiary premium is $38.99. So each month of uncovered delay costs you about $0.39/month — permanently added to your Part D premium, and recalculated each year against the current base premium.

Example: You delay Part D for 30 months (2.5 years). Your penalty is 30% × $38.99 = $11.70/month — added to your Part D premium forever. For a deeper look at how the drug benefit itself works, see Medicare Part D explained.

What Is Creditable Coverage?

You avoid the Part D penalty if you have “creditable” prescription drug coverage — coverage at least as good as standard Part D. This includes:

• Employer or union group health plan drug coverage (must notify you annually if it’s creditable)
• TRICARE
• VA prescription drug benefits
• Retiree drug coverage from a former employer (if certified as creditable)
• CHIP drug coverage

Keep documentation showing you had creditable coverage. If you’re ever asked to prove it, you’ll need the annual “creditable coverage” notice your insurer should have sent you.

Enrolling Before 65: Disability and ESRD

Medicare isn’t only for people turning 65. Roughly one in seven beneficiaries qualifies earlier, and the enrollment timing works differently:

Social Security Disability (SSDI): If you’ve received SSDI cash benefits for 24 months, you’re automatically enrolled in Medicare Parts A and B starting the 25th month — no application needed. Your card arrives in the mail. This 24-month waiting period is the rule most people don’t expect. (See Social Security disability benefits for how the underlying benefit works.)

ALS (Lou Gehrig’s disease): The 24-month wait is waived entirely. Medicare begins the same month your SSDI benefits start.

End-Stage Renal Disease (ESRD): People who need dialysis or a kidney transplant can qualify for Medicare at any age. Coverage generally begins the fourth month of dialysis, though it can start sooner with a home-dialysis training program or a transplant. ESRD enrollment has its own rules and is worth confirming directly with Social Security.

When a disability-based beneficiary later turns 65, they get a second Initial Enrollment Period around their 65th birthday — a fresh chance to enroll in or change coverage, including a new Medigap open-enrollment window, regardless of what happened earlier.

High Earners: Enrollment Timing and IRMAA

If your income is above a threshold, you’ll pay more than the standard premium for Parts B and D through the Income-Related Monthly Adjustment Amount (IRMAA). This doesn’t change when you enroll, but it changes what enrollment costs — and the timing of your income matters.

IRMAA is based on your tax return from two years prior (your 2026 premium uses your 2024 income). That two-year lookback means a one-time income spike — selling a house, a large Roth conversion, realizing capital gains — in the year before you enroll can inflate your Medicare premiums for a full year once you’re on Medicare. Planning the timing of those events around your enrollment year can save thousands. For the full bracket tables and appeal options, see IRMAA Medicare surcharges.

The practical enrollment takeaway: don’t be surprised if your first Part B bill is higher than the standard $202.90. If a life-changing event (retirement, loss of income, divorce, death of a spouse) has since lowered your income, you can ask Social Security to use your current income instead of the two-year-old figure.

Common Enrollment Mistakes and How to Avoid Them

Mistake 1: Relying on COBRA to Delay Medicare

COBRA continuation coverage extends your employer health plan temporarily after you leave a job — but it doesn’t count as “active employer coverage” for Medicare purposes. If you retire at 65, take COBRA, and skip Medicare, you’ll face the Part B (and possibly Part D) late-enrollment penalty when your COBRA runs out.

Solution: Enroll in Medicare Parts A and B when you retire, even if you elect COBRA for other coverage.

Mistake 2: Missing the 8-Month Window After Leaving Work

If you work past 65 with active employer coverage and then retire, you have only 8 months to enroll in Part B without penalty. Many people believe they can wait until the next annual enrollment period — they cannot. Missing the 8-month SEP window means waiting for the General Enrollment Period (January–March) with a penalty attached.

Solution: Enroll in Part B within 8 months of your employment or employer coverage ending.

Mistake 3: Confusing Observation Status With an Inpatient Stay

This affects SNF coverage, not enrollment — but it’s a costly mistake that often arises during the Part A benefit period. See Medicare Part A coverage for details.

Mistake 4: Not Enrolling in Part A When It’s Free

Most people are entitled to premium-free Part A (if they’ve worked 40+ quarters). There’s no reason to delay. Even if you have employer coverage, enrolling in Part A early means it can act as secondary coverage.

One exception: If you contribute to a Health Savings Account (HSA), enrolling in Part A (or Part B) makes you ineligible for future HSA contributions — and Part A can apply retroactively up to six months, so you must stop HSA contributions before that lookback bites. If you’re actively contributing to an HSA and have good employer coverage, delay Medicare enrollment until you stop contributing. See HSAs and Medicare for the timing rules.

Mistake 5: Forgetting to Compare Plans Each October

Your Medicare coverage doesn’t automatically optimize itself. Drug formularies change, plan premiums increase, and your health needs evolve. The AEP (October 15–December 7) is your annual window to check whether a different plan serves you better.

Solution: Each fall, use Medicare’s Plan Finder at Medicare.gov to compare your current plan against alternatives based on your current medications and providers.

Mistake 6: Not Understanding Medigap Open Enrollment

Medigap has its own enrollment window: a 6-month period starting when you turn 65 and are enrolled in Part B. During this window, insurers cannot deny you a Medigap policy or charge more due to pre-existing conditions.

After this window, you generally have no guaranteed right to buy Medigap. If you develop health conditions later and want to switch, you may find coverage unavailable or unaffordable.

Solution: If you want Medigap coverage, purchase it during your Medigap open enrollment window. See Medigap plans compared for help choosing the right plan.

Medicare and Employer Coverage: Working Past 65

If you continue working past 65, the interaction between your employer health plan and Medicare depends on your employer’s size:

Employer with 20+ employees: Your employer health plan is the primary payer; Medicare is secondary. You can often delay Part B without penalty as long as you have active employer coverage.

Employer with fewer than 20 employees: Medicare is the primary payer. If you don’t enroll in Part B, your employer plan may deny claims because it expects Medicare to pay first. Enrolling in Part B is important in this situation.

Self-employed: Your self-employed health coverage (marketplace plan, small group plan) typically doesn’t count as employer group coverage for Medicare purposes. Delaying Medicare enrollment while on a marketplace plan may trigger a late penalty.

For a deeper treatment of the working-past-65 decision, see working past 65 and Medicare and Medicare vs. employer insurance at 65.

Enrolling in Medicare: The Mechanics

To enroll in Medicare:

• Online: Visit SSA.gov and sign in to my Social Security to enroll during your IEP
• By phone: Call 1-800-772-1213 (SSA) — available Monday through Friday
• In person: Visit your local Social Security Administration office

If you’re already receiving Social Security retirement benefits, you’re typically enrolled in Medicare Parts A and B automatically at 65. You’ll receive your Medicare card in the mail about 3 months before your birthday. (Whether to start Social Security that early is a separate decision — see when to claim Social Security.)

If you’re not yet collecting Social Security, you need to actively enroll during your IEP. It’s not automatic.

Your 6-Months-Before-65 Checklist

Enrollment goes smoothly when you start early. Here’s a simple timeline:

• 6 months before 65 — Stop HSA contributions if you’ll enroll in any part of Medicare at 65 (Part A’s retroactive start can claw back the last six months of contributions). Confirm whether your employer coverage is “creditable” and whether you’ll keep working.
• 3 months before 65 — Your IEP opens. If you’re not auto-enrolled, apply online at SSA.gov. Enrolling in these first three months gets coverage to start the first day of your birthday month.
• At 65 — If you’re delaying because of active employer coverage with 20+ employees, get written confirmation that the coverage is primary, and keep proof for your future SEP.
• When you stop working — Your 8-month Part B SEP clock starts. Don’t rely on COBRA; enroll in Part B before COBRA begins.
• Within 6 months of starting Part B — If you want Medigap, buy it now during guaranteed-issue open enrollment. After this window, insurers can underwrite or decline you.

Frequently Asked Questions

Does Social Security automatically enroll me in Medicare? Only if you’re already collecting Social Security benefits before 65 (or have received SSDI for 24 months). Otherwise enrollment is not automatic — you must actively sign up during your IEP, or risk a late penalty.

Can I delay Part B if I have a marketplace (ACA) plan? No. A marketplace plan is not “active employer coverage,” so it does not give you a penalty-free SEP. If you stay on a marketplace plan past 65 instead of enrolling, you’ll likely owe the Part B late penalty later — and marketplace subsidies generally end once you’re eligible for premium-free Part A.

What happens if I miss my Initial Enrollment Period entirely? Unless you qualify for an SEP, you’ll have to wait for the General Enrollment Period (January 1 – March 31), with coverage starting the first of the following month, and you’ll likely owe the permanent Part B late-enrollment penalty.

Do I need Part D if I take no prescriptions? Usually yes, unless you have other creditable drug coverage. Part D is insurance against future drug costs, and skipping it without creditable coverage means a permanent late penalty when you do enroll. A low-premium plan keeps the penalty clock from running.

Can I switch out of a Medicare Advantage plan mid-year? Yes, during the Medicare Advantage Open Enrollment Period (January 1 – March 31) you can switch MA plans once or return to Original Medicare. Outside that window you generally need the AEP, a qualifying SEP, or a 5-star plan availability to make a change.

Key Takeaways

• Your Initial Enrollment Period is a 7-month window around your 65th birthday — enroll during the first 3 months for the fastest coverage start
• The Part B late-enrollment penalty is 10% per year of delay, permanent — about $40.58/month on a 20% penalty at 2026 premiums
• The Part D late-enrollment penalty is 1% per full month of delay (≈$0.39/month each), permanent
• COBRA does not count as active employer coverage — don’t rely on it to delay Medicare
• After leaving employer coverage, you have 8 months to enroll in Part B penalty-free
• New 2023 “exceptional conditions” SEPs offer a penalty-free path if disaster, employer error, incarceration, or loss of Medicaid caused you to miss enrollment
• Disability beneficiaries get Medicare after 24 months of SSDI (immediately for ALS); ESRD qualifies at any age
• The Annual Enrollment Period (October 15–December 7) is your window to review and change plans each year

For help understanding what each part of Medicare covers, see Medicare Part A and Medicare Part B. If you’re deciding between Original Medicare and Medicare Advantage, compare the two approaches here.

Sources

• Medicare.gov — Get started with Medicare
• SSA.gov — Medicare (Publication EN-05-10043, PDF)
• Medicare.gov — Medicare costs
• Medicare.gov — Get free personalized Medicare counseling (SHIP)

All sources are official government or nonprofit consumer resources, verified July 2026. Medicare and Social Security rules and dollar amounts change annually — confirm current figures at the links above before making decisions.