You should sign up for Medicare Part B during your Initial Enrollment Period (IEP), which starts three months before the month you turn 65 and ends three months after your birthday month. If you miss this window and don’t qualify for a Special Enrollment Period, you could face a late enrollment penalty that lasts for the rest of your life.
Under 42 U.S.C. § 1395r, Medicare imposes a 10% premium surcharge for each full 12-month period you were eligible for Part B but didn’t enroll. This penalty gets added to your monthly premium permanently. With the 2026 standard Part B premium at $202.90 per month, even a two-year delay means paying an extra $40.58 every single month — for life.
As of 2025, 68.9 million Americans are enrolled in Medicare, and roughly 62.8 million have both Parts A and B. Missing a deadline is more common than most people think.
What you’ll learn in this article:
- 🗓️ The exact enrollment windows — IEP, SEP, and GEP — and how each one works
- 💰 How the Part B late enrollment penalty is calculated, with real dollar amounts
- 🏢 What to do if you’re still working past 65 with employer coverage (and the small employer trap)
- 📋 How to fill out forms CMS-40B and CMS-L564 step by step
- ⚠️ The most expensive mistakes people make and how to avoid them
What Medicare Part B Covers and Why It Matters
Medicare Part B is the medical insurance portion of Original Medicare. It covers doctor visits, outpatient care, lab tests, preventive screenings, durable medical equipment, and some home health services. Without Part B, you’re responsible for 100% of these costs out of pocket.
Part B is not free. The standard monthly premium is $202.90 in 2026, with an annual deductible of $283. After you meet the deductible, you pay 20% coinsurance for most covered services. These costs are set by provisions of the Social Security Act and change each year.
Unlike Part A — which most people get premium-free — Part B requires active enrollment. If you don’t sign up during the right window, the consequences can follow you for decades.
The Three Enrollment Periods You Need to Know
Your Initial Enrollment Period (IEP): The 7-Month Window
The IEP is a 7-month window centered around your 65th birthday. It begins 3 months before your birthday month, includes your birthday month, and extends 3 months after. This is the primary window for signing up.
When your Part B coverage actually starts depends on which month within the IEP you enroll. If you sign up during the first three months (before your birthday month), coverage begins on the first day of your birthday month. If you sign up during your birthday month or after, coverage is delayed by one to three months.
| When You Sign Up During IEP | When Part B Coverage Starts |
|---|---|
| 1–3 months before birthday month | First day of your birthday month |
| During your birthday month | One month after enrollment |
| 1 month after birthday month | Two months after enrollment |
| 2–3 months after birthday month | Three months after enrollment |
Example: Maria turns 65 on June 15, 2026. Her IEP runs from March 2026 through September 2026. If Maria signs up in April (two months before her birthday month), her Part B coverage starts on June 1. If she waits until August (two months after her birthday month), her coverage won’t begin until November 1 — leaving a five-month gap.
The Special Enrollment Period (SEP): For Workers With Employer Coverage
The SEP exists for people who delayed Part B because they had group health plan (GHP) coverage through their own or a spouse’s current employment. Under the SEP provisions, you can sign up for Part B at any time while you still have employer coverage, or during the 8-month period after the employment or coverage ends — whichever comes first.
The SEP protects you from the late enrollment penalty. As long as you were continuously covered by a group health plan based on current employment (or by Part B) since first becoming eligible for Medicare, you qualify. If you’ve had more than 8 consecutive months without coverage from either current work or Part B, you lose your SEP eligibility.
Example: David turns 65 but keeps working at a company with 200 employees. He stays on his employer’s group health plan and skips Part B. Three years later, at 68, David retires. His 8-month SEP starts the month after his employer coverage ends. He signs up in month two and pays no late enrollment penalty.
| SEP Enrollment Timing | When Part B Coverage Starts |
|---|---|
| While still covered by employer GHP | First day of the month you choose (up to 3-month delay allowed) |
| Month 1 after employer coverage ends | First day of the month after enrollment |
| Months 2–8 after employer coverage ends | First day of the month after enrollment |
The General Enrollment Period (GEP): The Last Resort
If you missed your IEP and don’t qualify for a SEP, the General Enrollment Period is your only option. The GEP runs January 1 through March 31 each year. Coverage doesn’t begin until the month after you enroll — which could mean months without any coverage.
Anyone who enrolls during the GEP will almost certainly face the Part B late enrollment penalty. That penalty is added to your monthly premium for as long as you have Part B. The GEP is a safety net, but it comes at a steep cost.
Example: Susan missed her IEP at age 65 and didn’t have employer coverage. She’s now 67. Susan must wait until the next GEP (January 1 – March 31) to enroll. If she signs up in February, coverage starts March 1. She’ll also pay a 20% penalty on top of her premium — permanently.
How the Part B Late Enrollment Penalty Works
The penalty is 10% of the standard Part B premium for each full 12-month period you could have been enrolled but weren’t. This penalty applies for as long as you have Part B — meaning it follows you for life.
The calculation uses the current year’s standard premium, not the premium from the year you should have enrolled. So the dollar amount of your penalty increases every time CMS raises the standard premium.
Real Dollar Examples of the Penalty
Using the 2026 standard premium of $202.90:
| Years You Delayed | Penalty Percentage | Monthly Penalty Amount | Total Monthly Premium (2026) |
|---|---|---|---|
| 1 year | 10% | $20.29 | $223.19 → rounded to $223.20 |
| 2 years | 20% | $40.58 | $243.48 → rounded to $243.50 |
| 3 years | 30% | $60.87 | $263.77 → rounded to $263.80 |
| 5 years | 50% | $101.45 | $304.35 → rounded to $304.40 |
A person who delayed Part B by just two years would pay an extra $243.50 per month instead of $202.90. Over 20 years, that $40.58 monthly penalty adds up to $9,739.20 in unnecessary costs.
The penalty amount gets recalculated each year when the standard premium changes. As premiums rise over time, the penalty’s dollar impact grows with them.
The Small Employer Trap: When Employer Coverage Isn’t Enough
Not all employer coverage qualifies you for a SEP. The rules depend on the size of the employer. For companies with 20 or more employees, the employer group health plan is the primary payer and Medicare is secondary. You can safely delay Part B.
For employers with fewer than 20 employees, the rules flip. Medicare becomes the primary payer, and the employer plan is secondary. This means your employer plan will pay very little on its own — and if you don’t have Part B, you could be stuck with massive medical bills despite having insurance.
| Employer Size | Primary Payer | Can You Delay Part B? |
|---|---|---|
| 20+ employees | Employer group health plan | Yes — SEP protects you |
| Fewer than 20 employees | Medicare | No — you likely need Part B immediately |
Example: Greg, age 65, works for a small accounting firm with 12 employees. He assumes his employer plan is enough and skips Part B. When Greg has surgery, the hospital bills $50,000. His employer plan only pays as a secondary payer — but there’s no primary payer because Greg never enrolled in Part B. Greg owes tens of thousands of dollars. He should have checked with his employer’s plan administrator before deciding to delay.
IRMAA: When Your Income Raises Your Part B Premium
Higher-income beneficiaries pay more for Part B through the Income-Related Monthly Adjustment Amount (IRMAA). The Social Security Administration reviews your Modified Adjusted Gross Income (MAGI) from two years prior — so your 2024 income determines your 2026 IRMAA.
If your individual MAGI exceeds $109,000 (or $218,000 for married filing jointly), you’ll pay a surcharge on top of the standard premium. The 2026 IRMAA surcharges for Part B range from $81.20 to $487.00 per month, depending on income.
| Single Filer MAGI | Joint Filer MAGI | Monthly Part B Premium (2026) |
|---|---|---|
| $109,000 or less | $218,000 or less | $202.90 (standard) |
| $109,001 – $137,000 | $218,001 – $274,000 | $284.10 |
| $137,001 – $171,000 | $274,001 – $342,000 | $406.90 (approx.) |
| $171,001 – $205,000 | $342,001 – $410,000 | $528.90 (approx.) |
| $205,001 – $499,999 | $410,001 – $749,999 | $649.20 |
| $500,000+ | $750,000+ | $689.90 |
The IRMAA combines with the late enrollment penalty. A high-income earner who also delayed Part B could face a monthly premium of $689.90 plus a penalty surcharge. This makes timing your enrollment even more important if you expect higher income in your 60s.
You can appeal your IRMAA if you’ve experienced a life-changing event — such as retirement, divorce, or the death of a spouse — that significantly reduced your income. File Form SSA-44 with Social Security to request a reconsideration.
How to Sign Up: Three Methods
Online Through Social Security
You can apply for Part B online at SSA.gov. Click “Sign Up for Part B Only,” follow the prompts, and upload any required documentation. This is the fastest and most convenient method. You do not need to log into a my Social Security account first for this application.
By Phone
Call Social Security’s toll-free number at 1-800-772-1213 (TTY: 1-800-325-0778). A representative can walk you through the application. Have your Medicare number, personal details, and any employer coverage documentation ready before you call.
In Person at a Social Security Office
Visit your local Social Security office with your completed forms and supporting documents. Call ahead to schedule an appointment — walk-ins are possible but waits can be long.
Walking Through Form CMS-40B: Line by Line
Form CMS-40B is the official application to enroll in Medicare Part B. You use this form if you already have Part A and need to add Part B. The form is straightforward, but mistakes on it can delay your enrollment by weeks or months.
Section 1: Basic Information
- Line 1 — Medicare Number: This is the number on your red, white, and blue Medicare card. If you’ve applied for Part A, you can also find it at SSA.gov.
- Lines 2–5 — Name and Address: Enter your full legal name, mailing address, city, state, and zip code. Make sure this matches Social Security’s records exactly.
- Line 6 — Phone Number: Include your area code. Social Security may call you if there’s a problem with your application.
- Line 7 — Signature: You must sign the form by hand. Printed names are not accepted. If you sign with an “X” mark, a witness must also sign.
- Line 8 — Date Signed: Enter the date you’re signing.
The Remarks Section: The Most Important Part
The “Remarks” section is where you specify your desired Part B start date. This is critical. According to Medicare enrollment guides, if your employer coverage ends August 31, you should request a September 1 start date to avoid any gap. Leaving this blank can result in Social Security choosing a start date that doesn’t align with your needs.
When You Also Need Form CMS-L564
If you’re enrolling during a Special Enrollment Period, you must also submit Form CMS-L564 (“Request for Employment Information”). You fill out Section A, and your employer fills out Section B, confirming your group health plan coverage dates.
Common employer mistakes on the CMS-L564 include listing the start date of the current insurance carrier rather than the actual start date of the group health plan. Double-check the dates before submitting. An incorrect CMS-L564 can cause Social Security to deny your SEP, forcing you into the GEP and triggering the penalty.
Three Real-World Scenarios
Scenario 1: Standard Enrollment at 65 — No Employer Coverage
Linda turns 65 on September 20, 2026. She retired at 62 and has been on a marketplace plan. She has no employer coverage.
| Linda’s Situation | What Happens |
|---|---|
| IEP opens June 1, 2026 (3 months before birthday month) | Linda can start enrolling |
| Linda signs up in July 2026 | Coverage begins September 1 (the first day of her birthday month) |
| Linda pays $202.90/month | No penalty — she enrolled on time |
| Linda’s Medigap Open Enrollment starts September 2026 | She has 6 months to buy a supplement at guaranteed-issue rates |
Linda’s best move is to enroll during the first three months of her IEP. If she waits until October or November, her coverage start date gets pushed back and she risks a gap.
Scenario 2: Working Past 65 With Employer Coverage — Using the SEP
Tom turns 65 on March 10, 2026. He works for a company with 500 employees and is covered under the company’s group health plan. He plans to retire at 67.
| Tom’s Situation | What Happens |
|---|---|
| Turns 65, keeps working with employer GHP | Employer plan is primary; he can safely skip Part B |
| Retires at 67 in March 2028 | Employer coverage ends March 31, 2028 |
| 8-month SEP begins April 2028 | Tom has until November 2028 to enroll in Part B |
| Tom signs up in April 2028 | Coverage begins May 1, 2028 — no penalty |
Tom should enroll in Part B one to two months before his retirement date to avoid any gap in coverage. He also needs to submit both Form CMS-40B and Form CMS-L564 with his employer’s verification.
Scenario 3: Missed the Deadline — Facing the GEP and Penalty
Karen turned 65 in 2023. She had no employer coverage, didn’t understand Medicare, and never signed up for Part B. It’s now 2026.
| Karen’s Situation | What Happens |
|---|---|
| Missed IEP in 2023, no employer coverage | No SEP available |
| Must wait for GEP (January 1 – March 31, 2026) | Signs up in February 2026 |
| Coverage starts March 1, 2026 | Gap from mid-2023 to March 2026 |
| Delayed ~2.5 years (2 full 12-month periods count) | 20% permanent penalty |
| Monthly premium: $202.90 + $40.58 penalty | $243.50/month for life |
Karen’s penalty of $40.58 per month will be recalculated every year as the standard premium changes. If she lives another 25 years, that penalty could cost her over $12,000 in total — and the amount grows with each annual premium increase.
Mistakes to Avoid
Mistake 1: Assuming COBRA Counts as Employer Coverage
COBRA continuation coverage does not qualify you for a Special Enrollment Period. COBRA is not coverage based on current employment — it’s coverage that continues after employment ends. If you elect COBRA at 65 and skip Part B, you will not be protected from the penalty. Your 8-month SEP clock starts when your active employment or employer coverage ended — not when COBRA runs out.
Mistake 2: Working for a Small Employer and Skipping Part B
If your employer has fewer than 20 employees, Medicare is the primary payer. Your employer plan expects Medicare to pay first. Without Part B, you’re left with massive uncovered costs. Always verify your employer’s size and check with the plan administrator in writing.
Mistake 3: Not Enrolling Before Employer Coverage Ends
The safest strategy is to enroll in Part B one to two months before your employer coverage ends. If you wait until after the coverage ends, you risk a gap. According to Medicare Interactive, contacting your HR department early and starting the paperwork in advance prevents this gap.
Mistake 4: Filing the CMS-L564 With Wrong Dates
If your employer fills out the CMS-L564 with the wrong coverage start date — for example, listing when the current insurer started instead of when the group health plan began — Social Security may deny your SEP. The consequence is getting pushed into the GEP and paying the lifetime penalty.
Mistake 5: Ignoring the 8-Month SEP Deadline
Your SEP lasts only 8 months after employer coverage ends. If you let that deadline pass, your only option is the GEP (January–March), and you will pay the penalty. Mark the date on your calendar the moment you learn your retirement or coverage end date.
Pros and Cons of Delaying Part B Enrollment
| Pros of Delaying Part B | Cons of Delaying Part B |
|---|---|
| Save $202.90/month while employer plan covers you | Risk of lifetime penalty if you miss the SEP deadline |
| Employer coverage may offer broader benefits than Medicare | Small-employer plans may not cover you as primary — leaving major gaps |
| Delay lets you align Medigap Open Enrollment with retirement | COBRA does not protect your SEP — the clock starts at employment end |
| No penalty if you use the SEP correctly | Paperwork errors on CMS-L564 can disqualify your SEP |
| You stay in one plan instead of coordinating two | Any lapse of 8+ months without Part B or employer coverage permanently disqualifies you from the SEP |
Do’s and Don’ts of Medicare Part B Enrollment
Do’s
- Do sign up during the first three months of your IEP if you have no employer coverage. This gives you the earliest possible coverage start date.
- Do verify your employer has 20 or more employees before deciding to delay Part B. Get confirmation in writing.
- Do submit Forms CMS-40B and CMS-L564 together if using a Special Enrollment Period. Missing the L564 can delay or deny your enrollment.
- Do use the Remarks section on Form CMS-40B to specify your desired Part B start date — never leave it blank.
- Do check if you qualify for a Medicare Savings Program that can pay your Part B premium. Income limits for the QMB program in 2026 are $1,350/month for individuals.
Don’ts
- Don’t assume COBRA counts as current employer coverage. It does not extend your SEP window.
- Don’t wait until your employer coverage ends to start the enrollment process. Begin 1–2 months early to prevent gaps.
- Don’t ignore the IRMAA if your income exceeds $109,000 (single) or $218,000 (joint). Budget for the higher premium.
- Don’t rely on your employer’s HR department to fill out Form CMS-L564 correctly. Review every date before you submit.
- Don’t let the 8-month SEP window expire. Once it’s gone, you’re forced into the GEP and a permanent penalty.
Medicare Savings Programs: State Help With Part B Premiums
If your income is limited, your state may pay some or all of your Part B costs through a Medicare Savings Program (MSP). These programs are administered by state Medicaid offices, but they’re available to anyone who meets the income and resource requirements.
The most comprehensive program is the Qualified Medicare Beneficiary (QMB) program. QMB pays your Part A premium (if applicable), your Part B premium, and all deductibles, coinsurance, and copayments. For 2026, the income limit is $1,350/month for individuals and $1,824/month for married couples, with resource limits of $9,950 and $14,910 respectively.
MSP enrollment also automatically qualifies you for Extra Help, the federal program that reduces Part D prescription drug costs. Even if you think your income is too high, it’s worth applying — some states like New York have expanded their MSP eligibility beyond the federal minimums.
Applications are submitted to your state Medicaid office. You should receive a decision within 45 days. If denied, you have the right to appeal through a fair hearing.
FAQs
Can I sign up for Part B online?
Yes. Visit SSA.gov, click “Sign Up for Part B Only,” complete the application, and upload supporting documents. No in-person visit required.
Does COBRA extend my Special Enrollment Period?
No. COBRA is not coverage based on current employment. Your 8-month SEP clock starts when active employment or employer coverage ends, not when COBRA expires.
Is the Part B penalty permanent?
Yes. The 10%-per-year penalty is added to your premium for as long as you have Part B — potentially for the rest of your life.
Can I enroll in Part B outside of the three enrollment periods?
No. You must use your IEP, a qualifying SEP, or the annual GEP. There is no other enrollment window for Part B.
Do I need Part B if my spouse’s employer covers me?
It depends. If the employer has 20+ employees, you can safely delay Part B with SEP protection. Under 20 employees, Medicare is primary and you likely need Part B.
What is the Part B premium in 2026?
$202.90 per month. Higher-income individuals may pay more due to IRMAA surcharges that range from $284.10 to $689.90 total.
Can I appeal my IRMAA amount?
Yes. File Form SSA-44 with Social Security if a life-changing event like retirement or divorce reduced your income below the IRMAA threshold.
What forms do I need for a Special Enrollment Period?
Two forms. You need Form CMS-40B (your Part B application) and Form CMS-L564 (employer verification of your group health plan coverage).
Will my state pay my Part B premium?
Possibly. If your income is below $1,350/month (individual), you may qualify for the QMB Medicare Savings Program, which covers premiums, deductibles, and copays.
What happens if my employer fills out the CMS-L564 wrong?
Your SEP could be denied. Social Security may reject your application, forcing you into the GEP with a permanent penalty. Always verify the dates before submitting.
Related reading
- Will Social Security Automatically Enroll Me in Medicare? (w/Examples) + FAQs
- What to Do 6 Months Before Retirement? (w/Examples) + FAQs
- When Can Seniors Change Medicare Plans? (w/Examples) + FAQs
- Does Social Security Start Automatically? (w/Examples) + FAQs
- Are Medicare Part B Premiums Paid in Advance? (w/Examples) + FAQs
- Should I Enroll in Medicare Part D? (w/Examples) + FAQs
- Is Nationwide Long-Term Care Insurance Worth It? (w/Examples) + FAQs