In this guide 7 sections
Medicare has five enrollment periods with confusingly similar names, two permanent penalties, and one rule about working past 65 that is responsible for most of the expensive mistakes. Here they are in the order they'll matter to you.
Your Initial Enrollment Period
Seven months long, centred on your 65th birthday: the three months before your birthday month, your birthday month, and the three months after.
CMS says people receiving Social Security or Railroad Retirement Board benefits at least four months before Medicare eligibility, and residing in the United States except Puerto Rico, are automatically enrolled in premium-free Part A and Part B. Puerto Rico residents in that situation are automatically enrolled in Part A but must actively enroll in Part B. Confirm your own status and effective dates with SSA or RRB; do not assume that receiving a payment at 65 alone completes enrollment.
The two permanent penalties
Most deadlines in life produce a one-off cost. These produce a recurring one, for the rest of your life.
Late enrollment penalties
Part B
- How it's calculated
- 10% added to your monthly premium for each full 12-month period you could have had Part B and didn't
- How long it lasts
- For as long as you have Part B — permanently
Part D
- How it's calculated
- 1% of the national base premium for each month without creditable drug coverage, once you've gone 63+ days
- How long it lasts
- For as long as you have Part D — permanently
Part A (rare)
- How it's calculated
- Applies only to the minority who must buy Part A; 10% surcharge
- How long it lasts
- Twice the number of years you delayed
| Penalty | How it's calculated | How long it lasts |
|---|---|---|
| Part B | 10% added to your monthly premium for each full 12-month period you could have had Part B and didn't | For as long as you have Part B — permanently |
| Part D | 1% of the national base premium for each month without creditable drug coverage, once you've gone 63+ days | For as long as you have Part D — permanently |
| Part A (rare) | Applies only to the minority who must buy Part A; 10% surcharge | Twice the number of years you delayed |
A Part B penalty for someone who delayed three years without qualifying coverage is 30% on top of the standard premium, every month, forever. Over a twenty-year retirement that's a substantial sum for a piece of paperwork.
If you're still working at 65
This is the rule that matters most and is most often misunderstood. Whether you can safely delay Part B depends on the size of the employer providing your coverage.
Working past 65: what to do
Employer with 20+ employees, you or your spouse actively working
- Part A
- Usually enroll — it's premium-free and pays secondary
- Part B
- Can safely delay; you get a Special Enrollment Period when work or coverage ends
- Part D
- Delay if the drug coverage is creditable — check the annual notice
Employer with fewer than 20 employees
- Part A
- Enroll at 65
- Part B
- Enroll at 65 — Medicare becomes primary and the group plan may pay almost nothing otherwise
- Part D
- Enroll unless coverage is creditable
Contributing to an HSA
- Part A
- Enrolling in any part of Medicare ends HSA contributions; stop 6 months before enrolling
- Part B
- Same
- Part D
- Same
COBRA, retiree coverage, or marketplace coverage
- Part A
- Enroll at 65
- Part B
- Enroll at 65 — none of these count as active employer coverage
- Part D
- Enroll at 65
| Your situation | Part A | Part B | Part D |
|---|---|---|---|
| Employer with 20+ employees, you or your spouse actively working | Usually enroll — it's premium-free and pays secondary | Can safely delay; you get a Special Enrollment Period when work or coverage ends | Delay if the drug coverage is creditable — check the annual notice |
| Employer with fewer than 20 employees | Enroll at 65 | Enroll at 65 — Medicare becomes primary and the group plan may pay almost nothing otherwise | Enroll unless coverage is creditable |
| Contributing to an HSA | Enrolling in any part of Medicare ends HSA contributions; stop 6 months before enrolling | Same | Same |
| COBRA, retiree coverage, or marketplace coverage | Enroll at 65 | Enroll at 65 — none of these count as active employer coverage | Enroll at 65 |
The small-employer rule is equally punishing in the other direction. If the company has fewer than 20 employees, Medicare becomes the primary payer at 65 whether or not you've enrolled — and the group plan pays as though Medicare already paid its share. Someone who skips Part B in that situation can be left personally owing the portion Medicare would have covered.
The Special Enrollment Period
When employment or the employer coverage ends, whichever comes first, an eight-month Special Enrollment Period opens for Part B, with no penalty. Part D has a shorter window — two months — so the two don't run in step, and people occasionally clear the Part B deadline while quietly missing the Part D one.
Other events also open Special Enrollment Periods: moving out of a plan's service area, losing Medicaid eligibility, a plan leaving Medicare, moving into or out of a nursing facility, and several others. Most run two months from the event.
The annual windows
Once you're in Medicare, three recurring periods matter.
The yearly calendar
Medicare Open Enrollment
- Dates
- October 15 – December 7
- What you can do
- Switch between Original Medicare and Medicare Advantage, change Advantage plans, join, switch or drop Part D. Effective January 1.
Medicare Advantage Open Enrollment
- Dates
- January 1 – March 31
- What you can do
- If you're already in an Advantage plan: switch to a different one, or return to Original Medicare and add Part D. One change only.
General Enrollment Period
- Dates
- January 1 – March 31
- What you can do
- For people who missed their Initial Enrollment Period and have no Special Enrollment Period. Coverage starts the month after you sign up. Penalties still apply.
| Period | Dates | What you can do |
|---|---|---|
| Medicare Open Enrollment | October 15 – December 7 | Switch between Original Medicare and Medicare Advantage, change Advantage plans, join, switch or drop Part D. Effective January 1. |
| Medicare Advantage Open Enrollment | January 1 – March 31 | If you're already in an Advantage plan: switch to a different one, or return to Original Medicare and add Part D. One change only. |
| General Enrollment Period | January 1 – March 31 | For people who missed their Initial Enrollment Period and have no Special Enrollment Period. Coverage starts the month after you sign up. Penalties still apply. |
Open Enrollment in the autumn is the one to actually use. Medicare Advantage and Part D plans change their formularies, networks, copays and premiums every January, and the plan that was best for your medications last year frequently isn't this year. Every plan sends an Annual Notice of Change in September. Almost nobody reads it, and it's the document that tells you whether your plan just stopped covering your drug.
The Medigap window, again
The federal Medigap Open Enrollment Period lasts six months, starting the first month you are at least 65 and enrolled in Part B. This is different from Medicare’s annual plan-change windows.
During it, any Medigap insurer must sell you any policy they offer at their standard rate, regardless of your health history. After it, in most states, they can medically underwrite — charge more, or decline. Unlike the other windows, this one does not come round again each year.
A checklist for turning 65
- 1.Four months before your birthday: work out whether you'll have active employer coverage from an employer of 20 or more. That single fact determines everything else.
- 2.If contributing to an HSA, stop contributions six months before you plan to enroll in any part of Medicare.
- 3.Three months before: enroll in Part A, and Part B unless you're safely delaying it. Do it online at ssa.gov.
- 4.Confirm in writing whether any drug coverage you're keeping is "creditable" — the plan must tell you each year, and this is what protects you from the Part D penalty.
- 5.Choose your structure: Original Medicare with Medigap and Part D, or Medicare Advantage. Price your actual medications, not just premiums.
- 6.If choosing Original Medicare, buy Medigap inside the six-month window. Do not let it lapse.
- 7.Call your state's SHIP program before deciding. It's free, unbiased, and they do this all day.
- 8.Diarise October 15 every year to review the Annual Notice of Change.
Questions families ask
What happens if I miss my Medicare enrollment deadline?
If you have no qualifying coverage and no Special Enrollment Period, you'll wait for the General Enrollment Period from January 1 to March 31, with coverage starting the following month — so you could be uninsured for months. You'll also pay a Part B late penalty of 10% of the premium for each full year you delayed, added to your premium permanently.
Can I delay Part B if I'm still working at 65?
Usually yes, if you have active coverage through an employer with 20 or more employees, where you or your spouse is still working. When that employment or coverage ends you get an eight-month Special Enrollment Period with no penalty. If the employer has fewer than 20 employees, or the coverage is COBRA, retiree or marketplace coverage, you should enroll at 65.
What does "creditable" drug coverage mean?
Coverage that Medicare considers at least as good as standard Part D. Employers and plans must tell you each year in writing whether your drug coverage is creditable. Keeping creditable coverage protects you from the Part D late penalty; going 63 days or more without it starts the penalty clock. Keep those annual notices.
Is Medicare Open Enrollment the same as Obamacare open enrollment?
No, though they overlap on the calendar and cause endless confusion. Medicare Open Enrollment is October 15 to December 7 and applies to Medicare Advantage and Part D. Marketplace open enrollment is a separate programme with separate dates. Anyone eligible for Medicare should generally not be buying marketplace coverage.
Can I contribute to an HSA once I'm on Medicare?
No. Enrollment in any part of Medicare, including premium-free Part A, ends HSA contribution eligibility. Because Part A enrollment can be backdated up to six months when you sign up after 65, the standard advice is to stop contributing six months before you enroll to avoid a tax penalty. You can still spend an existing HSA balance.
Do I have to do anything if I'm already getting Social Security?
Confirm your individual status. CMS’s automatic-enrollment rule generally applies when you receive Social Security or RRB benefits at least four months before eligibility and reside in the United States. Puerto Rico residents meeting the rule are automatically enrolled only in Part A and must actively enroll in Part B. Drug-plan and other coverage choices require separate attention.
Was this guide helpful?
Your feedback helps us make the information clearer.
Report an error
For help with your own care, request a free introductory call.
