Medicare enrollment seems simple — until you realize that a missed deadline can mean paying higher premiums for the rest of your life. Whether you’re retiring or still working at 65, here’s what you need to know.
Key takeaways
- Your Initial Enrollment Period lasts 7 months: 3 months before your 65th birthday month, that month, and 3 months after.
- Late Part B enrollment generally adds 10% to your premium for each full 12-month period you went without it — usually for life.
- Still working with employer coverage? You can often delay Part B without penalty and use an 8-month Special Enrollment Period later.
- COBRA and retiree coverage don’t count as current employment coverage for delaying Part B.
The parts of Medicare in 30 seconds
- Part A — hospital insurance. Premium-free for most people with 10 years of work history.
- Part B — doctor visits, outpatient care, preventive services. Has a monthly premium.
- Part D — prescription drug coverage, through private plans.
- Part C (Medicare Advantage) — private plans that bundle A, B and usually D, often with extra benefits and a provider network.
- Medigap — supplemental insurance that helps pay Original Medicare’s out-of-pocket costs.
When to enroll
| Your situation | What to do |
|---|---|
| Retired or not covered by a current employer | Enroll in Parts A and B during your Initial Enrollment Period. Sign up in the 3 months before your birthday month so coverage starts on time. |
| Already getting Social Security | You’ll usually be enrolled in A and B automatically. Watch for your Medicare card. |
| Working (or spouse working) with employer coverage | Ask HR whether the employer has 20+ employees. If so, you can usually delay Part B and enroll later without penalty. |
| On a Marketplace plan | Enroll in Medicare during your Initial Enrollment Period, then cancel the Marketplace plan so coverage doesn’t overlap. Premium tax credits generally end once you’re eligible for premium-free Part A. |
The penalties you want to avoid
- Part B: 10% higher premium for each full 12-month period you could have had Part B but didn’t — generally for as long as you have Part B. Wait four years and you could pay 40% more, permanently.
- Part D: if you go 63 days or more without Part D or other creditable drug coverage after your Initial Enrollment Period, a penalty of 1% of the national base premium per month without coverage is added — usually for life.
- Part A (only if you must buy it): a 10% higher premium for twice the number of years you delayed.
The COBRA trap
If you leave a job and keep your old plan on COBRA, you are not covered by a current employer. Your 8-month window to sign up for Part B starts when the employment ends — not when COBRA ends. Many people miss this and end up with a lifetime penalty and a gap in coverage.
If you have an HSA
You can’t contribute to an HSA once you’re enrolled in any part of Medicare. If you sign up for Part A after 65, coverage can be backdated up to 6 months (but not before your 65th birthday month), so many people stop HSA contributions about 6 months before they apply. You can still spend your HSA money tax-free on qualified medical costs, including Medicare premiums. See HSAs explained.
Your Medicare checklist
- 6 months before 65: decide whether you’ll keep working and confirm your employer coverage rules with HR. Stop HSA contributions if you’ll enroll soon.
- 3 months before your birthday month: your Initial Enrollment Period opens. Apply at SSA.gov if you’re not enrolled automatically.
- Choose your path: Original Medicare (A + B) plus a Part D plan and possibly Medigap — or a Medicare Advantage plan.
- Check drug coverage: make sure your medications are on your Part D or Medicare Advantage plan’s formulary.
- Cancel overlapping coverage (such as a Marketplace plan) once Medicare starts.
- Every fall: review your plan during Medicare Open Enrollment (October 15 – December 7).
Free, unbiased help: every state has a State Health Insurance Assistance Program (SHIP) offering free one-on-one Medicare counseling. Find yours through Medicare.gov.
Related: Bridging coverage before 65 · COBRA vs. Marketplace
Sources: Medicare.gov; Social Security Administration; CMS. Smart Premium Guide is not affiliated with Medicare or any government agency. For education only — not insurance advice. Last reviewed October 2026.