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Medicare Part B Enrollment

Medicare Medical Insurance Enrollment

Medium ~15 min MedicareHealthcareInsuranceRetirement

/ What is this form?

Medicare Part B provides medical insurance covering doctor's office visits, outpatient services, preventive care (flu shots, cancer screenings, annual wellness visits), medical equipment, and certain home health services. The standard monthly premium in 2025 is $185, though higher-income beneficiaries pay Income-Related Monthly Adjustment Amounts (IRMAA) surcharges that can more than double the premium.

Approximately 4 million Americans turn 65 each year. While Medicare Part A (hospital insurance) is generally automatic for those who paid Medicare taxes for 40 quarters, Part B requires active enrollment. Failing to enroll during the correct window results in a permanent late enrollment penalty of 10% per year of delay, added to the premium for as long as you have Part B.

CMS-40B is the paper enrollment form used for Part B enrollment outside the Initial Enrollment Period — specifically during the General Enrollment Period (January–March) or a Special Enrollment Period (following loss of qualifying employer coverage). Most people enrolling during IEP can apply online at ssa.gov without a paper form.

/ Who needs this form?

  • Adults turning 65 who are not automatically enrolled in Part B through Social Security
  • People who delayed Part B because they had employer coverage and are now losing that coverage
  • Anyone enrolling during the General Enrollment Period (Jan 1–Mar 31) who missed their Initial Enrollment Period
  • Individuals with Medicare due to disability whose 24-month waiting period has ended
  • People who were enrolled in Part B, voluntarily disenrolled, and now want to re-enroll

/ What you need before you start

Medicare claim number (from your Medicare card) or Social Security Number
Date of birth and full legal name
Permanent address
Reason for enrollment (IEP, GEP, or SEP)
If SEP: employer information and CMS-L564 completed by your employer
Date employer coverage ended (if enrolling via SEP)

/ Step-by-step guide

1 Determine Your Enrollment Period
Initial Enrollment Period (IEP): 7-month window around your 65th birthday (3 months before, the month of, and 3 months after). General Enrollment Period (GEP): Jan 1–Mar 31 each year if you missed IEP — coverage starts July 1 and a late penalty applies. Special Enrollment Period (SEP): 8-month window after employer coverage ends — no penalty.
2 Determine If You Need CMS-40B
Most people get Part A automatically (if they receive Social Security or Railroad Retirement benefits). Part B is NOT automatic — you must actively enroll. If you are turning 65 and not receiving Social Security, you must actively enroll in both Parts A and B. CMS-40B is the form for Part B enrollment during GEP or SEP.
3 Complete the CMS-40B Form
Enter your Medicare claim number (if you have a Medicare card) or Social Security Number, full name, date of birth, sex, and permanent address. Indicate the reason for enrollment: Initial Enrollment Period, General Enrollment Period, or Special Enrollment Period (and the reason for the SEP).
4 If Using SEP, Submit CMS-L564 Proof of Coverage
If you are enrolling under a Special Enrollment Period (because you had employer coverage at 65), you must also submit CMS-L564, Request for Employment Information, signed by your employer. This proves you had qualifying coverage and avoids the late enrollment penalty.
5 Submit to SSA and Begin Paying Premiums
Submit to your local Social Security Administration office, by mail, or online at ssa.gov. The standard Part B premium in 2025 is $185/month (higher-income beneficiaries pay IRMAA surcharges). Premium is typically deducted from your Social Security benefit if you receive one.

/ Key fields explained

Field What to enter Common mistake
Medicare Claim Number Your 11-digit Medicare Beneficiary Identifier (MBI) from your red, white, and blue Medicare card. Format: 1 letter, 10 alphanumeric characters (e.g. 1EG4-TE5-MK72). If you don't have a card yet, use your SSN. Using your old SSN-based claim number if you still have an old red Medicare card — these have been replaced with new MBI cards for all beneficiaries since 2019.
Reason for Enrollment Check the appropriate box: Initial Enrollment Period, General Enrollment Period, or Special Enrollment Period. For SEP, specify the reason: employer/union coverage ended, loss of coverage due to divorce/death, etc. Checking 'General Enrollment Period' when you should be claiming a Special Enrollment Period — GEP triggers a late penalty, while SEP does not. Always claim SEP if you had qualifying employer coverage.
Date Coverage Ended (SEP) The exact date your qualifying employer or union group health coverage ended. This determines the 8-month SEP window. Using the date you stopped working rather than the date insurance coverage actually ended — employer coverage often continues through the end of the month of termination.

/ Common mistakes to avoid

Confusing the 8-month SEP window with a 60-day window — unlike many insurance situations, the Medicare SEP for employer coverage is 8 months from when coverage ends, giving more time.
Assuming COBRA extends the SEP — COBRA is not considered employer coverage for SEP purposes. Enroll in Part B before COBRA expires to avoid a penalty.
Not applying for Part B while still working past 65 with employer coverage — if employer coverage is through a small employer (fewer than 20 employees), Medicare is primary and not having Part B can leave you underinsured.
Forgetting to factor in IRMAA — if your income is above certain thresholds ($103,000 individual, $206,000 married for 2025), you pay more than the standard premium. Plan accordingly.

/ Frequently asked questions

What does Medicare Part B cover?

Part B covers medically necessary services: doctor visits, outpatient hospital care, lab tests, X-rays, mental health services, durable medical equipment, ambulance services, and preventive services (flu shots, mammograms, colonoscopies, annual wellness visits). Part B covers 80% of approved costs after the annual deductible ($257 in 2025).

What is the Medicare Part B premium in 2025?

$185/month is the standard premium. IRMAA surcharges apply to individuals with income above $103,000 (or $206,000 for couples), ranging up to $628.90/month at the highest income tier.

Can I get help paying my Part B premium?

Yes. The Medicare Savings Program (MSP) can help pay the Part B premium, deductible, and copays for people with limited income and resources. Contact your state Medicaid office to apply.

What is Medigap and do I need it?

Medigap (Medicare Supplement Insurance) is private insurance that covers the 20% coinsurance and other costs that Part B doesn't pay. Enrollment in Medigap is easiest during the 6-month Medigap Open Enrollment Period starting when you turn 65 and enroll in Part B. After this period, you may be denied or charged more based on health.