When to sign up for Medicare and what a late start costs

Your Initial Enrollment Period runs seven months: the three months before the month you turn 65, that month, and the three months after. That is the answer for most people, not for everyone.

Three situations change it:

• Turning 65 with no job coverage — the seven-month window applies. Start with Section 2.
• Working past 65 on employer coverage — you may delay Part B penalty-free. Section 3 has the test.
• Already past your window — a fallback opens each January. Section 4 has the cost.

Timing is a money decision here. Getting it wrong adds a permanent surcharge to what Medicare actually costs each month.

ℹ️ Financial Disclaimer: Educational information only, not personalized insurance, tax, credit, investment, or debt-relief advice. Medicare figures change annually. Confirm your own dates with the Social Security Administration, your employer’s benefits administrator, a CPA for tax questions, or a qualified attorney before you act.

Your Initial Enrollment Period: the 7-month window

When the window opens and closes

Your Initial Enrollment Period lasts seven months. It opens three months before the month you turn 65 and closes three months after that month.

When your coverage actually starts

Source: Medicare.gov, “When does Medicare coverage start?”

Do you get enrolled automatically?

If you already draw Social Security or Railroad Retirement benefits, the Social Security Administration enrolls you in Part A and Part B when you first qualify. Everyone else has to file.

About 99% of beneficiaries owe no Part A premium, per the CMS 2026 premiums fact sheet. Part B is the decision that costs money.

Still working at 65? When you can delay Part B

Delaying Part B is safe only under specific conditions. Two facts decide it.

The 20-employee rule

CMS Medicare Secondary Payer rules turn on employer size. With 20 or more employees, the group health plan pays first and Medicare pays second, so delaying works. Under 20 employees, Medicare pays first, and staying off Part B can leave most of your bills unpaid.

Your 8-month Special Enrollment Period

The coverage has to come from current employment, yours or a spouse’s. Once that employment or coverage ends, whichever happens first, a Special Enrollment Period gives you eight months to enroll without a penalty.

✅ Action Step: Ask your benefits administrator two questions before your 65th birthday: does this plan cover 20 or more employees, and does it pay primary or secondary once I turn 65?

Missed your window? What the General Enrollment Period costs

If no Special Enrollment Period applies, you can still sign up between January 1 and March 31. Coverage starts the month after you file.

🔍 How It Works: The Part B late enrollment penalty adds 10% of the standard premium for each full 12-month period you could have enrolled and didn’t. Part D works differently: 1% of the national base beneficiary premium per uncovered month, rounded to the nearest $0.10.

📊 Data Point: 2026 standard Part B premium: $202.90 a month. 2026 Part D national base beneficiary premium: $38.99 a month — Source: CMS 2026 Parts A & B fact sheet, November 14, 2025; Medicare.gov.

Two years late on Part B costs 20% of $202.90, or $40.58 a month, for as long as you hold Part B. Medicare’s published Part D example: 14 uncovered months adds $5.50 a month.

No honest source can quote you a lifetime total, because both base figures reset every January. How the Part B penalty is calculated and the Part D version show the full math.

The HSA trap: stop contributing six months before you apply

Sign up for premium-free Part A after 65 and Medicare backdates it up to six months, never earlier than the month you turned 65.

🔍 How It Works: IRS Publication 969 states that from the first month you are enrolled in Medicare your HSA contribution limit is zero, and that this applies to retroactive coverage. Contributions made during those backdated months become excess contributions, taxed at 6% for every year they stay in the account.

Illustration using 2026 limits: self-only coverage at $4,400 plus the $1,000 catch-up is $5,400 a year, or $450 a month. Apply in October at 66, Part A backdates to April, and $2,700 becomes excess. The excise runs $162 a year until corrected.

✅ Action Step: If you contributed during a backdated period, ask a CPA this before you file: should I request a return of excess contributions, which is a different transaction from an ordinary withdrawal?

Three windows that are not sign-up windows

Medicare Open Enrollment, October 15 to December 7, changes plans. It does not join you to Medicare. Coverage from changes made in it starts January 1.

The Medicare Advantage Open Enrollment Period, January 1 to March 31, is only for people already in a Medicare Advantage plan.

Your Medigap Open Enrollment Period is the one that closes permanently: six months, beginning the first month you are both 65 or older and enrolled in Part B. Inside it, insurers cannot deny you or price you on your health history.

💡 Expert Note: Large sites still tell readers who missed their Initial Enrollment Period to sign up between October 15 and December 7. Medicare’s guidance is clear that this is a plan-change window. Acting on that can cost a full year.

Two timing mistakes that cost the most

⚠️ Costly Mistake: COBRA is not group health plan coverage for Medicare purposes. Taking COBRA does not extend your eight-month Special Enrollment Period, which started when the job ended.

Two clocks run from that same event. Eight months for Part A and Part B, but only two months to join a Part D or Medicare Advantage plan.

✅ Action Step: Call your free State Health Insurance Assistance Program counselor and ask: my employer coverage ended on [date], what are my remaining deadlines for Part B and for Part D?

Common questions about Medicare enrollment timing

Your next step

Write down two dates: the month your window opens and the month it closes. If you are working past 65, add the month your employer coverage ends and count eight months forward.

Then file with Social Security online, by phone, or on form CMS-40B.

Related Articles

Medicare

Medicare plan discontinued and the key dates that follow

September 11, 2026

Medicare

Checking the Right Medicare Extra Help Income Limits for 2026

September 11, 2026

Medicare

How to switch Medicare Advantage plans without a costly gap

September 11, 2026

Medicare

What health insurance really costs when retiring before 65

September 8, 2026

Medicare

Medigap Plan G vs Plan N and the Key Costs That Differ

September 9, 2026

Medicare

Why COBRA Isn’t Creditable Coverage, and the Part B Risk

September 7, 2026

Medicare

The real cost behind your Medicare Part B deductible

August 29, 2026

Medicare

How the IRMAA income lookback sets your first Medicare premium

September 4, 2026

Medicare

The Medicare Part B late enrollment penalty rule people overlook

September 4, 2026

Editorial process

About this content

This content is prepared through a structured publishing workflow with dedicated writing, financial review and editorial checks.

1 contributor

Written byView profile

Sameer Patel — Founder & Editor

Research, drafting and original analysis

Contributor at Finance Authority Hub.

Every page on Finance Authority Hub is reviewed for accuracy, clarity and editorial consistency before publication or update.