You should enroll in Medicare Part B during your Initial Enrollment Period, which runs for seven months centered on the month you turn 65

Your Initial Enrollment Period starts three months before the month you turn 65, includes that birth month, and ends three months after. If you were born in June, for example, your seven-month window runs from March through September. You can enroll anytime during those seven months, and coverage will begin on the first day of the month in which you turn 65 — or the first day of the following month if you enroll after your birth month has passed.

The reason timing matters is that Medicare Part B carries a late enrollment penalty if you miss your Initial Enrollment Period and do not have other may have access to coverage. That penalty is a permanent percentage increase to your Part B premium, calculated for each full 12-month period you go without Part B when you could have had it. The penalty amount changes each year, but it stays on your premium for as long as you have Medicare. Missing your window by even one month can cost you thousands of dollars over your lifetime.

You do not have to enroll in Part B the moment you turn 65 if you have coverage through an employer or union that is still active. If you or your spouse are still working and covered by a group health plan, you can delay Part B without penalty — but you must enroll within eight months after that employment ends or the group coverage stops, whichever comes first.

Key Takeaways

  • Your Initial Enrollment Period is seven months long, centered on your 65th birthday, and you can enroll anytime during it without penalty.
  • If you miss your Initial Enrollment Period and do not have employer or union coverage, you will pay a permanent penalty on top of your regular Part B premium for as long as you have Medicare.
  • If you are still working and covered by your employer's health plan, you can delay Part B and enroll within eight months after you leave that job or lose the coverage.
  • You enroll in Part B through Social Security, Medicare.gov, or by calling 1-800-MEDICARE; the same enrollment covers you for both Part A and Part B.

Who must enroll during the Initial Enrollment Period

If you are turning 65 and you do not have health coverage through an active employer or union, you should enroll in Part B during your Initial Enrollment Period. This includes people who are retired, self-employed, or covered only by Medicaid or a spouse's retiree health plan.

You also need to enroll if you have coverage through a former employer's retiree plan. Unlike active group health plans, retiree plans do not count as may have access to coverage that lets you delay Part B. If you are on a retiree plan and miss your Initial Enrollment Period, you will face the late enrollment penalty.

People on Medicare Advantage or Medigap plans already have Part B as part of their coverage, so they do not need to take a separate enrollment step — but they must have enrolled in Part B during their Initial Enrollment Period to get those plans in the first place.

When you can delay Part B without penalty

You can put off enrolling in Part B if you have creditable coverage — health insurance through your current job or your spouse's current job. The coverage must be from an employer or union with 20 or more employees. If you work for a small business or are self-employed, that does not count as creditable coverage.

Your eight-month Special Enrollment Period begins the month after your employment ends or your group coverage stops, whichever happens first. You must enroll during those eight months to avoid the late enrollment penalty. If you wait until month nine, the penalty applies.

If your spouse is still working and you are covered under their group health plan, the same rule applies: you can delay Part B, but you must enroll within eight months after your spouse retires or loses that coverage. You do not get an extra eight months because you are the spouse — the clock starts when the coverage ends, not when you turn 65.

How to enroll in Part B

You can enroll through three main routes: online at Medicare.gov, by phone at 1-800-MEDICARE, or in person at your local Social Security office. You will need your Social Security number and information about any current health coverage you have.

If you are already receiving Social Security benefits, you may be enrolled in Part B automatically when you turn 65. Check your Social Security statement or call 1-800-MEDICARE to confirm. If you are not yet receiving benefits, you will need to enroll yourself.

Enrollment through Social Security is the same process whether you are signing up for Part A, Part B, or both. You do not fill out separate forms for each part. Once you enroll, you will receive your Medicare card in the mail within two to three weeks, though your coverage can begin on the first day of the month you enrolled or the first day of the following month, depending on when you submitted your enrollment.

What happens if you miss your Initial Enrollment Period

If you miss your seven-month Initial Enrollment Period and you do not have creditable coverage through an employer or union, you will face a late enrollment penalty when you do enroll. The penalty is a percentage of the standard Part B premium, and it increases for each full 12-month period you went without Part B when you could have had it.

The penalty does not go away. It stays on your premium for as long as you have Medicare, even if you later switch to a different plan or move to a different state. The only way to avoid it is to enroll during your Initial Enrollment Period or within eight months after losing creditable coverage.

You can still enroll in Part B after your Initial Enrollment Period ends, but only during the General Enrollment Period, which runs from January 1 through March 31 each year. Coverage would not begin until July 1 of that year, so there is a gap during which you have no Part B coverage.

Special situations that extend your enrollment window

If you are a federal employee or a member of the military, you may have different enrollment rules. Federal employees covered under the Federal Employees Health Benefits Program (FEHB) can delay Part B without penalty as long as they are enrolled in FEHB, even after they retire — but they must enroll in Part B within 12 months after FEHB coverage ends.

If you are a veteran with VA health coverage, you can delay Part B without penalty as long as you are using VA benefits. However, you should still enroll in Part B during your Initial Enrollment Period or within eight months after you stop using VA coverage, because the late enrollment penalty can explore if you wait longer.

If you are a U.S. citizen or permanent resident living outside the United States, your Initial Enrollment Period still applies, but you cannot enroll online through Medicare.gov. You will need to call 1-800-MEDICARE or contact a U.S. embassy or consulate to enroll.

Coordinating Part B with other coverage

If you have Medicaid, you should still enroll in Part B during your Initial Enrollment Period. Medicaid can work alongside Medicare and may help pay your Part B premium and out-of-pocket costs. Enrolling in Part B does not affect your Medicaid coverage.

If you have a Medigap policy or Medicare Advantage plan, you must have Part B to use either one. If you are thinking about switching from one plan to another, check whether your new plan requires you to be enrolled in Part B first — most do.

If you are still working and covered by your employer's plan, you can stay on that plan and delay Part B. When you do enroll in Part B later, your employer plan and Part B will coordinate, with one plan paying first and the other paying second based on federal coordination rules.

Frequently Asked Questions

What if I turn 65 while I'm still working?

If you are covered by your employer's health plan, you can delay Part B without penalty. You must enroll within eight months after you leave that job or the coverage ends. If you retire before turning 65, your eight-month window starts when you retire, not when you turn 65.

Can I enroll in Part B after my Initial Enrollment Period ends?

Yes, but only during the General Enrollment Period from January 1 through March 31 each year, and coverage would not start until July 1. You would also owe a late enrollment penalty. Enrolling during your Initial Enrollment Period is always the better choice if you can.

Do I have to enroll in Part B if I have Medicare Advantage?

You must have Part B to enroll in Medicare Advantage. If you already have Medicare Advantage, you are already enrolled in Part B. You cannot have Advantage without it.

What if my employer says I don't need Part B because they have good coverage?

Your employer's opinion does not change the enrollment rules. If your group health plan is active and you work for an employer with 20 or more employees, you can delay Part B. But if that coverage ends, you must enroll in Part B within eight months or face a penalty. Get the enrollment important date in writing from your employer's benefits office.

Will I owe back premiums if I enroll late?

No. You will not owe premiums for the months you were not enrolled. You will owe the late enrollment penalty added to your regular premium going forward, but not retroactive premiums for the past.