The 7 Medicare Enrollment Mistakes That Cost Real Money

All seven up front, because the list is the point:

1. Assuming Medicare signs you up automatically
2. Missing your 7-month sign-up window
3. Ignoring the Part B late penalty (the expensive one)
4. Assuming any employer coverage lets you delay Part B
5. Assuming your drug coverage is "creditable"
6. Confusing the January window with fall enrollment
7. The IRMAA income surprise from a 2-year-old tax return

Mistake 1: Assuming Medicare signs you up automatically

Sometimes it does. If you're already getting Social Security benefits at least 4 months before you turn 65, you're enrolled automatically in Part A (Hospital Insurance, the part that covers hospital stays) and Part B (Medical Insurance, the part that covers doctors and outpatient care), and you don't have to do anything. If you're not taking Social Security yet, nobody enrolls you. No packet shows up, and no reminder comes. You apply yourself, through Social Security, at SSA.gov or by phone. Source: SSA and Medicare.gov, 2026.

The fix costs nothing. A few months before you turn 65, ask Social Security one plain question: am I being enrolled automatically, or do I need to apply? That single call keeps every penalty on this page from ever touching you.

Mistake 2: Missing your 7-month sign-up window

Your first chance to enroll is the Initial Enrollment Period, a personal 7-month window built around the month you turn 65: the 3 months before your birthday month, your birthday month, and the 3 months after. One quirk moves it. If your birthday falls on the first of a month, Medicare treats you as turning 65 the month before, so your whole window starts one month earlier. Source: Medicare.gov and CMS, Medicare & You 2026.

Sign up inside the window and there's no penalty at all. Miss it without coverage that qualifies you for an exception and two things happen: the penalty meter in the next section starts running, and you usually wait for the General Enrollment Period, January 1 through March 31, with coverage starting the month after you sign up. So put your 7 months on the calendar now, even if 65 is still a ways off. Our guide to when to sign up for Medicare walks through the window month by month. Source: Medicare.gov, 2026.

Mistake 3: Ignoring the Part B late penalty

This is the expensive one. The rule, straight from Medicare: your monthly Part B premium goes up 10% for each full 12-month period you were eligible but didn't sign up, and the increase lasts for as long as you have Part B. Source: Medicare.gov, Part B late enrollment penalty, 2026.

Now the 2026 math. The standard Part B premium is $202.90 a month. Ten percent of that is $20.29. Wait two full years and you're paying 20% more: an extra $40.58 every month, which is $486.96 a year. Carry that across 20 years of retirement at today's rate and it comes to roughly $9,739, for missing a window. And the true cost usually runs higher, because the penalty is a percentage of each year's premium, and premiums tend to rise. Source: derived from the CMS 2026 Medicare Parts A & B premiums and deductibles fact sheet; the 20-year figure holds 2026 rates flat.

The fix is a calendar, not money: know your window from Mistake 2, or make sure you genuinely qualify to delay, which is Mistake 4.

Mistake 4: Assuming any employer coverage lets you delay Part B

If you're working past 65 with health insurance from the job, you may be able to wait on Part B with no penalty. But the rule is narrower than people think, and the test is where the coverage comes from: a group health plan based on current employment, yours or your spouse's. While that coverage lasts, and for 8 months after the job or the coverage ends, you get a Special Enrollment Period, a penalty-free chance to sign up. COBRA, the federal law that lets you temporarily keep employer or union coverage after the job ends, doesn't count. Neither does retiree coverage. In both cases nobody's still working. Source: Medicare.gov, 2026, and CMS, "How Medicare Works with Other Insurance," Pub. 02179, February 2026.

Employer size matters too, just not the way most people assume. It doesn't decide whether you get the Special Enrollment Period; it decides who pays your bills first. At 20 or more employees, the job's plan pays first, so delaying Part B is usually safe. At fewer than 20, Medicare pays first, so delaying Part B is usually a costly mistake even though you have insurance. Source: CMS, "How Medicare Works with Other Insurance," Pub. 02179, February 2026.

Don't guess. Before you delay Part B even one month, call Social Security or 1-800-MEDICARE, confirm your specific coverage qualifies, and write down the date and what they told you. The whole situation is in our guide to working past 65.

Mistake 5: Assuming your drug coverage is "creditable"

Part D, the drug part of Medicare, has its own penalty, and it hinges on one word. Creditable means your drug coverage is expected to pay, on average, at least as much as a standard Part D plan. Go 63 or more days in a row without creditable coverage after your first sign-up window ends, and joining later adds a penalty of 1% of the national base beneficiary premium, which is $38.99 in 2026, for every month you went without, rounded to the nearest ten cents. You generally keep paying it for as long as you have Medicare drug coverage. Two years uncovered works out to about $9.40 a month. Smaller than the Part B penalty, but it doesn't stop. Source: Medicare.gov, Part D late enrollment penalty, 2026; CMS, 2026 Part D bid information.

The fix is a piece of mail. An employer or union plan that offers drug coverage has to tell you in writing, every year and before October 15, whether that coverage is creditable. Source: CMS, Creditable Coverage disclosure requirements, 2026. Read it and keep it. If it says "not creditable," that's your cue to look at Part D now, not later. One vocabulary note, because it trips people up: creditable is a drug-coverage word. The Part B delay in Mistake 4 turns on current employment, not on whether your coverage is creditable. The full penalty math, with Medicare's own example, is in our guide to whether you need Part D if you take no prescriptions. Source: Medicare.gov and CMS, 2026.

Mistake 6: Confusing the January window with fall enrollment

There are two yearly windows, and mixing them up locks people out. The one that matters for everyone is the Annual Enrollment Period, October 15 through December 7, when anyone with Medicare can change coverage for the following year, effective January 1. Source: Medicare.gov, 2026.

Then there's the January window, January 1 through March 31, and it's much narrower than it sounds. It's only for people already in a Medicare Advantage plan (the private all-in-one alternative to Original Medicare, which is Part A and Part B run by the federal government), and it allows exactly one change: switch to a different Advantage plan, or go back to Original Medicare and add a drug plan. You can't use it to move from Original Medicare into Advantage. Sit through the fall planning to handle everything in January, and you may find the door you wanted is closed until next October. Mark October 15. Our AEP game plan lays out that season step by step. Source: Medicare.gov, 2026.

Mistake 7: The IRMAA surprise from a 2-year-old tax return

IRMAA (the Income-Related Monthly Adjustment Amount) is a surcharge added to both your Part B and Part D premiums once your income passes a threshold. For 2026 it starts above $109,000 for a single filer and $218,000 for a couple filing jointly. Source: CMS, 2026 Medicare Parts A & B premiums and deductibles fact sheet.

Two things catch people. First, the lookback: Medicare uses your tax return from two years ago, so your 2026 premiums are set by your 2024 income. A one-time spike, say a large retirement account withdrawal or a Roth conversion, can raise your premiums two years later, right as you enroll. Second, it's a cliff, not a slope. One dollar over a line and you pay that tier's full surcharge. Source: CMS, 2026 fact sheet, and SSA, 2026.

The fix has two parts. Plan any big income moves with the 2-year echo in mind. And if that old return no longer reflects your income because of a life change like retirement, ask Social Security about using more recent income instead; the request form is SSA-44. Source: SSA, Form SSA-44, 2026.

Questions people ask

What happens if I miss my Medicare enrollment window?

You get another chance, but it's slower and can cost more. The General Enrollment Period runs January 1 through March 31 every year, with coverage starting the month after you sign up, and late penalties can apply depending on how long you waited. If job-based coverage from current employment explains the gap, the Special Enrollment Period in Mistake 4 may cover you instead. Source: Medicare.gov, 2026.

Does the Part B late penalty ever go away?

Generally no. You pay it for as long as you have Part B, and the percentage is applied to each year's premium as premiums change. Part A is a gentler story: most people get Part A without a monthly premium, and premium-free Part A has no late penalty at all. Source: Medicare.gov, 2026.

I'm still working at 65. Do I have to sign up for Medicare?

No law forces you. If your health coverage is a group plan from your own or your spouse's current job, you can usually delay Part B penalty-free and use the 8-month Special Enrollment Period when the work ends. The catches, including COBRA, retiree coverage, and the under-20-employee rule, are covered in Mistake 4 above. Source: Medicare.gov, 2026.

Can I switch to a Medicare Advantage plan in January?

Only if you're already in one. The January-through-March window gives people in a Medicare Advantage plan one change, including going back to Original Medicare. Moving from Original Medicare into an Advantage plan generally waits for the fall Annual Enrollment Period, October 15 through December 7. Source: Medicare.gov, 2026.

Nearly every mistake on this page comes down to timing, knowing your dates and your definitions before the deadlines arrive. What a page can't do is tell you what to pick, because that depends on facts only you know. The official sources below can look at your actual situation, they're free, and nobody there earns a commission on your answer.