Missing your Medicare enrollment window is one of the most expensive mistakes a new retiree can make. The late-enrollment penalty for Part B is 10% added to your premium for every 12-month period you were eligible but didn't enroll — and unlike most penalties, it never goes away.
Here's how to make sure that doesn't happen to you.
Your Initial Enrollment Period (IEP)
Most Americans become eligible for Medicare at age 65. Your Initial Enrollment Period is a 7-month window centered on your 65th birthday month:
- 3 months before your birthday month
- Your birthday month
- 3 months after your birthday month
If you turn 65 in July, your window runs from April 1 through October 31.
Important: If you enroll in the 3 months after your birthday month, your coverage start date is delayed. For the least disruption, enroll during the first 4 months of your window.
What If I'm Still Working at 65?
If you or your spouse are still working and covered by an employer health plan with 20 or more employees, you can delay Medicare enrollment without penalty. You have a Special Enrollment Period (SEP) that begins when your employer coverage ends.
You have 8 months from the date employment or coverage ends to enroll in Medicare Part B without penalty.
⚠️ COBRA and retiree health coverage do not count as "active employer coverage" for this rule. If you leave your job at 65 and go on COBRA, your 8-month SEP clock starts on your last day of employment, not when COBRA ends.
The Part B Late Enrollment Penalty
If you miss your IEP and don't qualify for an SEP, you can only enroll during the General Enrollment Period: January 1 – March 31 each year, with coverage starting July 1.
And here's the sting: the late enrollment penalty is permanent.
For every 12-month period you were eligible but didn't enroll, your Part B premium increases by 10%. Two years late = 20% higher premium. For the rest of your life.
In 2026, the standard Part B premium is $185/month. Two years late means you'd pay $222/month — an extra $444 per year, forever.
Part A: Usually Automatic
Most people don't pay a premium for Medicare Part A (hospital insurance) because they've worked and paid Medicare taxes for at least 10 years (40 quarters). If you're collecting Social Security benefits when you turn 65, you'll be automatically enrolled in Part A and Part B.
If you're not yet collecting Social Security, you need to actively enroll.
Quick Reference
| Situation | Your Move |
|---|---|
| Turning 65, not working | Enroll during your 7-month IEP |
| Still working at 65, employer plan with 20+ employees | Delay is penalty-free; enroll within 8 months of leaving |
| Still working at 65, employer plan with fewer than 20 employees | Enroll at 65 — small employers use Medicare as primary coverage |
| On COBRA at 65 | Your SEP has already started — enroll now |
| Missed your window | Enroll Jan 1–Mar 31; expect a permanent premium penalty |
Use Our Free Medicare Penalty Calculator
If you're unsure whether you'd owe a penalty — or want to see exactly how much it would cost you — use our free Medicare Late Enrollment Penalty Calculator. Enter your birthday and your expected enrollment date and we'll calculate your lifetime penalty cost in seconds.
This article is for informational purposes only and does not constitute insurance or legal advice. Medicare rules are subject to change. Visit medicare.gov for the most current official information.