Your Initial Enrollment Period: When to Sign Up for Medicare

For most people, the first real Medicare decision is about timing. Sign up late and you can face a gap in coverage plus a late enrollment penalty that is added to your Part B premium for as long as you have Part B. Sign up for Part B when you don't need it yet and you may pay a premium you could have postponed.

Medicare calls your first chance to sign up the Initial Enrollment Period. This guide explains how that window is counted, who is enrolled automatically, when coverage begins, and when waiting is allowed, using Medicare.gov and the official Medicare & You 2027 handbook (both published in 2026).

This site is independent and is not affiliated with Medicare, CMS, Social Security or any insurer. What follows is general information, not insurance, financial, tax, legal or medical advice.

How the Initial Enrollment Period is counted

Medicare.gov's 2026 guidance describes the Initial Enrollment Period as a 7-month window that starts 3 months before the month you turn 65, includes your birthday month, and ends 3 months after it. You can sign up for Part A, Part B or both during this window.

The Medicare & You 2027 handbook gives a worked example: if you turn 65 on June 2, your window runs from March through September.

There is one quirk worth knowing. The same 2027 handbook says that if your birthday falls on the first day of a month, your window starts 4 months before your birthday month and ends 2 months after it, so someone turning 65 on June 1 has a February-through-August window.

Some people qualify before 65. According to the 2027 handbook, people getting Social Security or certain Railroad Retirement Board (RRB) disability benefits get Part A and Part B automatically after 24 months of benefits, and people with ALS get them when their disability benefits begin.

Who is signed up automatically

The Medicare & You 2027 handbook says that if you already get Social Security or RRB benefits, you will automatically get Part A and Part B starting the first day of the month you turn 65. Medicare.gov's 2026 sign-up tool spells out the condition behind this: your benefits need to start at least 4 months before you turn 65.

If you are enrolled automatically, the 2027 handbook says your Medicare card arrives in the mail 3 months before your 65th birthday or your 25th month of disability benefits. You can decline Part B, but the handbook says you need to say so before the coverage start date shown on your card. If you do nothing, you keep Part B and the premium is taken from your benefit payment.

There is a geographic exception. According to the 2027 handbook, people living in Puerto Rico who get Social Security or RRB benefits get Part A automatically but must sign up for Part B themselves.

If you are not getting Social Security or RRB benefits yet, you need to apply. The 2027 handbook points to Social Security's online application and says you can also contact Social Security 3 months before you turn 65 to set up an appointment. If you worked for a railroad, you apply through the RRB.

When your coverage starts

Coverage always begins on the first day of a month, and the start date depends on when in the window you act. Medicare.gov's 2026 coverage-start page lays it out this way:

• If you sign up before the month you turn 65, Part B starts the month you turn 65 (Medicare.gov, 2026). The 2027 handbook notes that if your birthday is on the first of the month, coverage starts the month before.
• If you sign up in the month you turn 65 or during the 3 months after, Part B starts the next month (Medicare.gov, 2026).
• If you qualify for premium-free Part A, it starts the month you turn 65. Medicare.gov (2026) says that if you sign up for it later, Part A is backdated up to 6 months, but never earlier than the month you turned 65.

If other coverage ends around your birthday, signing up early in the window is the simplest way to avoid a gap.

When waiting on Part B can make sense

Medicare.gov (2026) notes that most people sign up for both Part A and Part B when first eligible, but that in some cases it might make sense to wait. The most common reason is health coverage from a current job.

Medicare.gov's 2026 sign-up tool says that if you or your spouse are still working and the employer providing your coverage has 20 or more employees, you can wait and sign up for Part B within 8 months after the job or the coverage ends, whichever happens first, without a late penalty. Because most people don't pay a premium for Part A, the tool says you may still want Part A at 65.

If the employer has fewer than 20 employees, the same 2026 tool warns that your job-based coverage might not pay for services if you don't have both Part A and Part B. The 2027 handbook's chart on which insurance pays first shows why: at an employer that size, Medicare generally pays first. Ask the employer's benefits office before skipping enrollment.

According to the 2027 handbook, these do not count as coverage based on current employment:

• COBRA or similar continuation coverage after employment ends
• Retiree coverage
• VA coverage
• Individual coverage, such as a Health Insurance Marketplace plan
• Former employer coverage you get through severance or a layoff

If one of these is your only coverage, plan to enroll during your Initial Enrollment Period.

Two more situations need care. Medicare.gov (2026) says you need both Part A and Part B to keep TRICARE as a retiree or to keep CHAMPVA; active-duty service members and their families follow different rules. And if you contribute to a health savings account, the 2027 handbook says you can't contribute after you have Medicare and that premium-free Part A can be backdated, so the timing of your last contribution matters. A tax professional can help.

Common mistakes to avoid

• Assuming COBRA buys you time. Medicare.gov (2026) says COBRA isn't considered group health plan coverage for Medicare's employment-based Special Enrollment Period, and getting it doesn't change when that window ends.
• Keeping Marketplace subsidies too long. The 2027 handbook says that once you're eligible for premium-free Part A you won't qualify for Marketplace help with premiums, and you may have to pay back help you received when you file your taxes.
• Letting the window close. Medicare.gov (2026) says that if you miss your Initial Enrollment Period and don't qualify for a Special Enrollment Period, you'll have to wait for the yearly General Enrollment Period and may pay a monthly penalty that grows the longer you wait. Our guide to General and Special Enrollment Periods covers those options.
• Sharing your Medicare Number with a caller. The 2027 handbook advises against giving your Medicare Number or other personal information to anyone who contacts you by phone, email or in person unless you've given them permission in advance.

Getting personal help

Your situation may not fit neatly into these rules, especially with a mix of employer, retiree, military or Marketplace coverage. Medicare.gov's interactive sign-up questions are a good starting point. For one-on-one help at no cost, contact your State Health Insurance Assistance Program (SHIP); Medicare.gov describes SHIPs as not connected to any insurance company or health plan. Social Security handles the sign-up itself, and 1-800-MEDICARE can answer general questions.

Key takeaways

• Medicare.gov (2026) describes the Initial Enrollment Period as 7 months: 3 months before your 65th-birthday month, that month, and 3 months after.
• If you already get Social Security or RRB benefits, you are usually enrolled in Part A and Part B automatically; otherwise you apply through Social Security.
• Signing up before your birthday month generally means coverage starts the month you turn 65.
• Active coverage from your or your spouse's current job can let you delay Part B; COBRA, retiree, VA and Marketplace coverage do not.
• Confirm your plan with your employer's benefits office and a SHIP counselor before you decline Part B.

Sources

• Medicare.gov (CMS), When does Medicare coverage start?
• Medicare.gov (CMS), When can I sign up for Medicare?
• Medicare.gov (CMS), How do I sign up for Medicare?
• Medicare.gov (CMS), Working past 65
• Centers for Medicare & Medicaid Services, Medicare & You 2027 (official handbook)
• Medicare.gov (CMS), Joining a plan