How to Enroll in Medicare Part B: Application, Timing, and Effective Dates
How to Enroll in Medicare Part B: Application, Timing, and Effective Dates
Part B covers outpatient care, doctor visits, lab tests, durable medical equipment, and preventive screenings — the coverage you'll use most often. Unlike Part A, which is automatic for most people, Part B requires an active decision. And the timing of that decision controls both when coverage starts and whether you'll pay a permanent penalty.
Three Ways to Enroll in Part B
Online (fastest): Apply at ssa.gov/benefits/medicare. The Medicare-only application takes about 15 minutes. This works whether you're signing up during your Initial Enrollment Period or a Special Enrollment Period.
By phone: Call SSA at 1-800-772-1213 (TTY: 1-800-325-0778), Monday through Friday, 8 AM to 7 PM local time.
In person: Visit your local SSA field office. Schedule an appointment first — walk-ins face long waits.
If you're enrolling through a Special Enrollment Period because you're leaving employer coverage, you'll also need to submit Form CMS-40B (Application for Enrollment in Medicare Part B) and Form CMS-L564 (Request for Employment Information) with your employer's certification. These can be faxed or mailed to your local SSA office.
When Does Part B Start? Understanding Effective Dates
Your Part B effective date depends entirely on when during your enrollment window you sign up:
During the Initial Enrollment Period (IEP):
| When you enroll | Coverage starts |
|---|---|
| 3 months before birthday month | 1st of birthday month |
| During birthday month | 1st of the following month |
| 1 month after birthday month | 2 months later |
| 2 months after birthday month | 3 months later |
| 3 months after birthday month | 3 months later |
The message is clear: enroll in the first three months of your IEP for the earliest possible start date.
During a Special Enrollment Period (employer coverage): Coverage can start as early as the first day of the month you enroll, if you sign up while still actively working. If you enroll after employment ends, coverage starts the first day of the month after your enrollment month.
During the General Enrollment Period (January–March): Coverage starts the first day of the month after SSA receives your completed application. If you enroll in February, coverage begins in March.
The Sign-Up Deadline Most People Don't Know About
Your IEP is a strict 7-month window. If you miss it without qualifying employer coverage, the next chance is the annual General Enrollment Period — January 1 through March 31. That means you could wait up to 14 months for coverage to start, with no Part B protection during the gap.
And the penalty follows you: a 10% surcharge on your Part B premium for every full 12-month period you were eligible but didn't enroll. At the 2026 standard premium of $202.90, a three-year delay adds $60.87 to your monthly premium — permanently.
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Can You Sign Up for Part B Online?
Yes. The SSA online application at ssa.gov/benefits/medicare handles Part B enrollment during your Initial Enrollment Period. For Special Enrollment Periods, you'll need to submit the CMS-40B and CMS-L564 forms separately (these aren't available through the online portal), but you can fax them to your local SSA office.
Should You Delay Part B?
Delaying Part B makes sense only if you have qualifying group health plan coverage through active employment — either your own job or your spouse's, at a company with 20 or more employees. COBRA and retiree health plans do not count.
If your employer coverage qualifies, you get an 8-month Special Enrollment Period starting when either the employment or the coverage ends (whichever comes first). No penalty, no gap.
The Medicare Enrollment Guide includes the complete decision framework for delaying Part B, the exact forms needed for each enrollment scenario, and penalty calculation worksheets so you know exactly what's at stake.
Get Your Free Medicare Enrollment Guide: When and How to Sign Up — Quick-Start Checklist
Download the Medicare Enrollment Guide: When and How to Sign Up — Quick-Start Checklist — a printable guide with checklists, scripts, and action plans you can start using today.