How to Enroll in Medicare Part B and When You Need to Act

What Medicare Part B Covers and Who Needs It

Medicare Part B is the medical insurance portion of Medicare that covers doctor visits, outpatient care, lab tests, X-rays, and some preventive services. It is separate from Part A (hospital insurance) and requires a separate enrollment step. Most people pay a monthly premium for Part B, and the amount depends on your income — higher earners pay more.

You do not automatically get Part B when you turn 65. You must take action to enroll, and the timing matters. If you miss your enrollment window without a valid reason, you may face a permanent penalty on your premium for as long as you have Medicare.

Part B is optional, but most people need it. If you are still working and covered by an employer health plan, you may be able to delay enrollment without penalty. If you are retired or not covered by a group plan, you should enroll during your initial enrollment period.

Key Takeaways

Your initial enrollment period is the three months before you turn 65, the month you turn 65, and the three months after — a seven-month window total.
You can enroll through Medicare.gov, by phone at 1-800-MEDICARE, by mail, or in person at your local Social Security office.
If you miss your initial window and do not have employer coverage or another valid reason, you will pay a 10 percent premium penalty for life.
Your Part B premium is deducted from your Social Security check each month, or you can pay Medicare directly if you do not receive Social Security.
Part B coverage begins the first day of the month after you enroll, so timing your enrollment affects when your coverage starts.

Your Seven-Month Initial Enrollment Period

The clock starts three months before the month you turn 65. If you turn 65 in June, your enrollment period runs from March 1 through September 30. This is your initial enrollment period, and it is the easiest time to enroll without penalty.

You do not have to wait until the month you turn 65 to enroll. Many people enroll in the months before, which gives Medicare time to process the paperwork and have coverage ready on your birthday month. Enrolling early also means you avoid the rush in the month you turn 65, when Social Security and Medicare offices are busiest.

If you are still working and covered by an employer or union health plan, you may be able to delay Part B enrollment without penalty. This is called creditable coverage. When you eventually leave that job or lose the coverage, you will have an eight-month special enrollment period to sign up for Part B without penalty. You will need to show proof that you had employer coverage when you delayed.

How to Enroll: Four Routes

Online through Medicare.gov: Go to Medicare.gov, click "Sign Up for Medicare," and create or log into your Medicare account. You will answer questions about your citizenship, work history, and current coverage. The process takes about 10 minutes. You can upload documents or mail them later if needed.

By phone: Call 1-800-MEDICARE (1-800-633-4227). Representatives are available seven days a week, 8 a.m. to 8 p.m. Eastern time. Have your Social Security number, birth date, and citizenship information ready. They can enroll you over the phone and answer questions about your specific situation.

In person: Visit your local Social Security office. You can find the nearest office at ssa.gov or call 1-800-772-1213. Bring your Social Security card, proof of citizenship (passport, birth certificate, or naturalization papers), and proof of residency (utility bill or lease). Social Security staff can enroll you on the spot.

By mail: Request Form CMS-40B from Medicare by calling 1-800-MEDICARE or downloading it from Medicare.gov. Fill it out, sign it, and mail it to the address on the form. Mail enrollment takes longer — allow four to six weeks for processing.

Documents You Will Need

The exact documents depend on your citizenship and work history, but start with these basics: your Social Security number, your birth date, and proof of U.S. citizenship or legal residency. A Social Security card, passport, birth certificate, or naturalization papers all work for citizenship.

If you are enrolling online or by phone, you may not need to provide documents when ready. Medicare will tell you if they need additional paperwork. If you are enrolling in person or by mail, bring everything at once to avoid delays.

If you have employer coverage and want to delay Part B, keep records of your enrollment materials from that plan — your employee ID card, a letter from your employer confirming coverage dates, or your plan documents. You will need proof of this coverage later if you want to avoid the late-enrollment penalty.

When Your Coverage Starts and Premium Costs

Part B coverage begins on the first day of the month after you enroll. If you enroll in March, coverage starts April 1. If you enroll in May, coverage starts June 1. This is why timing matters: if you need coverage to start in January, you must enroll by December of the previous year.

Your monthly Part B premium varies based on your income from two years ago. In 2024, the standard premium is $164.90 per month, but higher earners pay more — up to $560.50 per month for the highest income bracket. Medicare uses your tax return from two years prior to calculate your income level. If your income drops significantly (retirement, job loss, divorce), you can request a recalculation.

Your premium is normally deducted from your Social Security check. If you do not receive Social Security, Medicare will bill you directly by mail or automatic bank withdrawal. You can change your payment method anytime by calling 1-800-MEDICARE.

What Happens If You Miss Your Enrollment Window

If you do not enroll during your initial seven-month period and you do not have a valid reason for the delay, you will face a late-enrollment penalty. The penalty is 10 percent of the standard Part B premium for each full 12-month period you were may be able to access but not enrolled. This penalty is permanent — you pay it for the rest of your life, even if you enroll years later.

There are exceptions. If you were covered by an employer or union health plan when you turned 65, you can enroll without penalty during an eight-month special enrollment period after that coverage ends. If you are a federal employee or retiree, different rules may explore. If you have end-stage renal disease or ALS, you may have different enrollment rules.

If you believe you have a valid reason for missing your important date, contact Social Security or Medicare to request a waiver of the penalty. You will need to explain your situation in writing and provide supporting documents. Waivers are rare but possible — for example, if Medicare failed to send you enrollment materials or if you were unable to communicate due to a medical condition.

Part B and Other Medicare Coverage

Part B works alongside Part A (hospital insurance). Most people have both. If you are still working and covered by an employer plan, you can delay both Part A and Part B without penalty if your employer has 20 or more employees. If your employer has fewer than 20 employees, you must enroll in Part A and Part B when you turn 65, even if you are still working.

Some people also enroll in a Medigap policy (supplemental insurance) or a Medicare Advantage plan (Part C) to cover costs that Part B does not pay. Medigap enrollment has its own important date and rules. If you enroll in Part B and then want to buy a Medigap policy, you have a six-month open enrollment period to do so without medical underwriting.

Part D (prescription drug coverage) is separate from Part B and has its own enrollment period. You can enroll in Part B and Part D at the same time or at different times, but delaying Part D without creditable coverage also triggers a penalty.

Frequently Asked Questions

Can I enroll in Part B after I turn 65 if I missed my initial window?

Yes, but you will pay a permanent 10 percent penalty on your premium unless you have a valid reason for the delay, such as employer coverage. The penalty applies for the rest of your life. Contact Social Security or Medicare when ready to enroll and discuss whether you may have access to for a penalty waiver.

What if I am still working and have health insurance through my job?

You can delay Part B enrollment without penalty if your employer has 20 or more employees. When you leave that job or lose coverage, you have eight months to enroll in Part B without penalty. Keep proof of your employer coverage dates — you will need it later.

Do I have to enroll in Part B if I have a Medigap or Medicare Advantage plan?

Yes. Part B is the foundation of Medicare. Medigap and Medicare Advantage both require Part B enrollment. You cannot have either of those plans without Part B.

When does my Part B premium start if I enroll in the middle of the month?

Coverage always begins on the first day of the month after you enroll, regardless of which day in the month you enroll. Your first premium payment is deducted from your Social Security check or billed to you in the month your coverage starts.

Can I change my mind after I enroll in Part B?

You can drop Part B, but you will face a penalty if you drop it and later re-enroll, unless you have employer coverage or another valid reason. Once you drop Part B, you cannot re-enroll until the next general enrollment period (January 1 to March 31), and your coverage will not start until July 1 of that year.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.