Medicare enrollment is time-gated, and signing up late without other coverage means a penalty that's added to your premium for life. Most people get one clean shot at it: a seven-month window around their 65th birthday. But the right window depends on your situation, and they don't all work the same way. This guide walks through each enrollment period, when your coverage actually starts, and exactly what a late penalty costs, so you can make the call with confidence.
The windows, at a glance
Medicare has one window for first-time sign-up at 65, a special path for people still working, a backup window for people who miss their first chance, and two windows that only let you change coverage you already have. Here's how they line up.
| Window | When | What it's for |
|---|---|---|
| Initial Enrollment Period (IEP) | 7 months around your 65th birthday | First-time sign-up for Parts A and B |
| Special Enrollment Period (SEP) | While you have current-employer coverage, plus 8 months after it ends | Signing up after 65 if you delayed because you were working |
| General Enrollment Period (GEP) | January 1 – March 31 | Backup if you missed your IEP and have no SEP |
| Fall Open Enrollment | October 15 – December 7 | Changing coverage you already have |
| Medicare Advantage Open Enrollment | January 1 – March 31 | One change for people already in a Medicare Advantage plan |
The rest of this guide takes them one at a time, starting with the one most people use.
Your Initial Enrollment Period: the 7-month window
Your first chance to sign up is your Initial Enrollment Period, and for most people it's the only window they'll ever need. It runs for seven months: the three months before the month you turn 65, the month you turn 65, and the three months after. So if your birthday is in June, your window opens March 1 and closes September 30.
Whether you have to do anything depends on Social Security. If you're already receiving Social Security or Railroad Retirement benefits when you turn 65, you're enrolled in Part A and Part B automatically, and your card arrives in the mail. If you're not yet collecting those benefits, no one enrolls you. You have to sign up yourself through the Social Security Administration, online or by phone, during your window.
A bit of timing detail worth knowing: if you sign up in the three months before your birthday month, coverage starts the first day of your birthday month. If you sign up during or after your birthday month, coverage starts the first of the month after you enroll. Signing up early is the way to have coverage ready the day you turn 65.
If you don't have other qualifying coverage and you let this window close, you move into late-penalty territory. The one situation where you can safely skip it is the next section.
Still working at 65? You may be able to wait
Plenty of people are still working at 65 and covered by a job's health plan, either their own or a spouse's. If that coverage comes from a current employer, you can usually delay Part B and Part D without any penalty and sign up later, through a Special Enrollment Period.
Here's how the timing works, per Medicare's working-past-65 rules. While you still have the current-employer coverage, you can enroll any time you like with no penalty. Once the job ends or that coverage ends, whichever comes first, you get an eight-month Special Enrollment Period to sign up for Part B. Many people still take premium-free Part A at 65 even while working, since it usually costs nothing.
One detail trips people up more than any other, so be clear on it: COBRA and retiree coverage do not count as current-employer coverage for this purpose. They're real insurance, but Medicare doesn't treat them as the kind of active employment coverage that protects you. They do not extend your eight-month window. If you retire and go on COBRA, your eight months still start ticking from when your employment ended, not from when COBRA runs out. People who wait for COBRA to expire before enrolling routinely blow past the SEP and land a lifelong Part B penalty. When your job or job-based coverage ends, treat the eight-month clock as already running.
How big is the employer you're working for? The size matters for whether the employer plan or Medicare pays first, and that affects whether delaying Part B is the smart move at all. The rules around employer coverage are specific enough that it's worth confirming yours before you decide to wait on anything.
The General Enrollment Period: your backup window
If you miss your Initial Enrollment Period and you don't qualify for a Special Enrollment Period, you're not locked out forever. There's a backup: the General Enrollment Period, which runs January 1 through March 31 every year. Anyone who needs Part A or Part B and missed their earlier windows can use it.
There's good news on timing here. Under the Beneficiary Enrollment Notification and Eligibility Simplification (BENES) Act of 2020, for any GEP sign-up dated January 1, 2023 or later, coverage starts the first day of the month after you enroll. That's a real improvement over the old rule, which used to leave people waiting until July 1, sometimes several months bare. The Social Security Administration confirms the change applies to all current GEP enrollments.
One thing to keep in mind: using the GEP doesn't erase a late penalty. If you went without coverage during the months you could have had it, the penalty still attaches when you finally enroll. The GEP gets you in; it doesn't undo the cost of waiting.
Changing coverage later: the two "open enrollment" windows
Two more windows get lumped in with the others, but they do something different. Neither is for signing up the first time. Both are for changing coverage you already have, and confusing them with the IEP is a common mistake.
Fall Open Enrollment runs October 15 through December 7. During it, you can switch between Original Medicare and Medicare Advantage, join or change a Part D drug plan, or drop one for another. Any change you make takes effect January 1. This is the once-a-year window where most people review whether their current plan still fits, and it's worth using even if you don't expect to switch, because plans change their costs and drug lists from year to year.
Medicare Advantage Open Enrollment runs January 1 through March 31. It's narrower: it's only for people already enrolled in a Medicare Advantage plan, and it allows one change. You can switch to a different Medicare Advantage plan, or drop back to Original Medicare and add a standalone Part D plan. If you're on Original Medicare, this window isn't for you.
Neither of these lets a first-timer enroll in Medicare. If you've never signed up and you missed your IEP, the GEP above is your path, not these.
What a late penalty actually costs
This is the part that makes the windows matter. Sign up late without other creditable coverage and you don't just pay back-premiums, you pay a surcharge added to your premium that, for Part B and Part D, lasts as long as you have the coverage. Each part calculates it differently.
Part B. Your monthly premium rises 10% for each full 12-month period you could have had Part B but didn't, and the penalty lasts as long as you have Part B. A two-year gap is a 20% surcharge, every month, for the rest of your life.
Part D. For each full month you went without creditable drug coverage, Medicare adds 1% of the national base beneficiary premium to your Part D premium. The Part D late enrollment penalty is keyed to a national figure that changes each year, so your dollar penalty can shift slightly from year to year even though your percentage doesn't. It also lasts as long as you have Part D.
Part A. Most people get Part A premium-free and never face this one. But if you have to buy Part A and you sign up late, the premium penalty is 10%, and you pay it for twice the number of years you delayed. Delay two years, pay the higher premium for four.
The percentages stay abstract until you put a dollar figure on them, so here's the Part D math worked through.
Worked example #1: a Part D late penalty
The figures below are hypothetical and shown only to illustrate how the calculation works. They are not a real case and not a prediction of your own result.
Say someone went 24 full months without creditable drug coverage after their Initial Enrollment Period ended, then enrolled in a Part D plan. Their penalty is 1% per month, so 24%. In 2026 the national base beneficiary premium that CMS sets for this calculation is $38.99. So the math is $38.99 × 0.24 = $9.36, which Medicare rounds to the nearest $0.10, landing at about $9.40 a month. That gets added on top of whatever their chosen drug plan charges, and it sticks for as long as they keep Part D. The base figure is reset annually by CMS, so the exact dollar amount can drift year to year while the 24% stays fixed.
A penalty under $10 a month may not sound like much in isolation. Over a 20-year retirement, it's more than $2,000 of avoidable cost, for a few months of paperwork skipped at 65.
Frequently asked questions
For most people, during their Initial Enrollment Period: the seven months around your 65th birthday. Sign up in the three months before your birthday month to have coverage ready the day you turn 65. The exception is if you're still working at 65 with coverage from a current employer, in which case you can usually delay and use a Special Enrollment Period later.
Not necessarily. If you (or a spouse) are still working and you have health coverage from that current employer, you can usually delay Part B and Part D without a penalty and enroll later, during the eight-month Special Enrollment Period that starts when the job or coverage ends. Confirm your employer's size and how its plan coordinates with Medicare before you decide to wait.
No. COBRA and retiree coverage are not treated as current-employer coverage, so they do not let you delay Part B penalty-free and do not extend your Special Enrollment Period. Your eight-month window starts when your employment ends, not when COBRA runs out, so enroll in Part B around the time the job ends rather than waiting for COBRA to expire.
For Part B and Part D, as long as you keep that coverage, meaning for life in most cases. The Part B penalty is 10% of the premium for each full year you delayed; the Part D penalty is 1% of the national base premium for each month you went without creditable drug coverage. The Part A penalty, which applies only if you buy Part A, lasts for twice the number of years you delayed.
If you don't qualify for a Special Enrollment Period, you can sign up during the General Enrollment Period, January 1 through March 31. Coverage starts the first of the month after you enroll. A late penalty may still apply for the time you went without coverage.
Learn More
- What is Medicare? Parts A, B, C, and D explained
- Medicare late enrollment penalties: Parts A, B, and D
- Dual-eligible plans (FIDE-SNPs): Medicare and Medicaid together
- Medicare plans and coverage in California
- Medicare plans and coverage in Texas
- Medicare plans and coverage in Florida
Find personalized help figuring out when to sign up for Medicare and which enrollment window fits your situation at brevy.com.
The information on Brevy.com is for educational purposes only and is not a substitute for professional legal, financial, or medical advice. Rules vary by state and program and change frequently. Always verify with the relevant agency or a qualified professional. Brevy is not a law firm, financial advisor, or healthcare provider.