Medicare is not one program -- it is four parts, five enrollment windows, and a handful of deadlines that carry permanent penalties if you miss them. The rules are written in government language, but the decisions underneath them are straightforward once someone translates. That is what this guide does.
The Four Parts, in Plain English
Part A is hospital insurance. It covers inpatient hospital stays, skilled nursing facility care after a hospital stay, hospice, and some home health care. If you or your spouse worked and paid Medicare taxes for 40 quarters -- roughly ten years -- Part A comes with no monthly premium. Most people qualify, which is why Part A enrollment is usually the easy part.
Part B is medical insurance. It covers doctor visits, outpatient care, preventive services, lab work, and durable medical equipment. Unlike Part A, everyone pays a monthly premium for Part B, and it is the part where the enrollment deadlines carry real teeth. Most of the penalty stories you hear about Medicare are Part B stories.
Part C, better known as Medicare Advantage, is not an add-on -- it is a replacement. A private insurance company bundles your Part A and Part B coverage (and usually drug coverage) into a single plan, often with extras like dental or vision, typically in exchange for a provider network and plan rules like prior authorization. You still must be enrolled in Parts A and B to join one.
Part D is prescription drug coverage, sold by private insurers. You either buy a standalone Part D plan alongside Original Medicare or get drug coverage bundled inside a Medicare Advantage plan. Even if you take no medications today, Part D has its own late penalty, so skipping it entirely is rarely the money-saver it appears to be.
The Enrollment Windows That Matter
The Initial Enrollment Period (IEP) is the one that matters most. It is a seven-month window built around your 65th birthday: the three months before your birthday month, your birthday month itself, and the three months after. Enroll during the early months and coverage starts the month you turn 65. This is the clean, penalty-free on-ramp, and for most people it is the window to use.
The General Enrollment Period (January 1 through March 31 each year) is the catch-up window for people who missed their IEP and have no special circumstances. Coverage begins July 1, which means missing your IEP can leave you with months of no coverage -- on top of any late penalty you have accrued.
Open Enrollment (October 15 through December 7) is the annual window when anyone already on Medicare can switch plans: change Part D plans, move from Original Medicare into a Medicare Advantage plan, or move back. Changes take effect January 1.
Medicare Advantage Open Enrollment (January 1 through March 31) is a narrower window just for people already in a Medicare Advantage plan. You can switch to a different Advantage plan or drop back to Original Medicare -- one change per year.
Special Enrollment Periods (SEPs) exist for life events, and the biggest one covers working past 65. If you are still working at 65 and covered by an employer group health plan from an employer with 20 or more employees, you can delay Part B without penalty. When that employment or coverage ends, you get an eight-month SEP to enroll in Part B penalty-free. Two warnings come with this rule. First, the HSA conflict: Medicare enrollment makes you ineligible to contribute to a Health Savings Account, so you and your employer must stop HSA contributions before your Medicare coverage begins. Second, retroactive backdating: if you enroll in premium-free Part A after 65, coverage can be backdated up to six months. Those backdated months count as Medicare coverage, which means HSA contributions made during them can create a tax problem you did not see coming. If you are working past 65 with an HSA, plan the stop date before you file for Medicare or Social Security.
Late Penalties Are Permanent
The Part B late enrollment penalty adds 10 percent to your monthly premium for every full 12-month period you could have had Part B but did not sign up. Wait three years past your window and your premium is 30 percent higher -- not for three years, but for the rest of your life. The penalty is recalculated against each year's premium, so it grows in dollar terms as premiums rise.
The Part D late enrollment penalty works on a finer scale: roughly 1 percent of the national base premium for every month you went without Part D or other creditable drug coverage after becoming eligible. It is smaller per month than the Part B penalty, but it compounds the same way -- monthly, and permanently. If you have drug coverage from an employer or union that is at least as good as Medicare's (the official term is "creditable coverage"), keep the annual notice that says so. It is your proof against the penalty later.
What Medicare Costs in 2026
These are the 2026 figures. Medicare costs change every year, so treat these as this year's snapshot and check medicare.gov or the annual CMS announcement for the current numbers.
The standard Part B premium is $202.90 per month in 2026, and the Part B deductible is $283 for the year. Part A is premium-free for most people, but it has an inpatient deductible of $1,736 per benefit period in 2026 -- and note that this is per benefit period, not per year, so two hospital stays separated by enough time can each trigger it.
Higher earners pay more through IRMAA -- the income-related monthly adjustment amount. In 2026, surcharges begin above $109,000 of modified adjusted gross income for a single filer and $218,000 for a joint return, rising through several tiers above that. The catch most people miss: IRMAA is based on your tax return from two years earlier, so your 2026 premiums are set by your 2024 return. If your income has dropped since then because of a life-changing event -- retirement is the most common one -- you can file Form SSA-44 with Social Security and ask them to use your more recent, lower income instead. Many new retirees overpay simply because no one told them this form exists.
Part D in 2026: The Donut Hole Is Gone
Prescription drug coverage looks meaningfully better than it did a few years ago. In 2026, your out-of-pocket spending on covered Part D drugs is capped at $2,100 for the year. Once you hit the cap, you pay nothing more for covered prescriptions that year. And the infamous coverage gap -- the "donut hole" that older guides still warn about -- no longer exists. It was eliminated in 2025. If someone is still explaining Medicare drug coverage to you with a donut-hole diagram, they are working from an outdated map.
One more tool worth knowing: the Medicare Prescription Payment Plan lets you spread your out-of-pocket drug costs across the months of the year in even installments instead of paying large sums at the pharmacy counter. It does not reduce what you owe, but it smooths the cash flow, which matters most for people facing expensive medications early in the year.
The Choice That Sticks: Medigap vs. Medicare Advantage
This is the section that saves people real money, because it is the one decision in Medicare that is genuinely hard to undo. Original Medicare pays most of your costs but leaves gaps -- deductibles, coinsurance, and no annual out-of-pocket cap. You can fill those gaps with a Medigap (Medicare Supplement) policy, which costs a monthly premium but lets you see any doctor who accepts Medicare, with little paperwork and few surprises. Or you can choose Medicare Advantage, which often has low or zero additional premium and extra benefits, in exchange for networks, plan rules, and cost-sharing that varies by plan and by year.
Here is the part the brochures underplay. Your Medigap open enrollment period -- the six months starting when you are 65 or older and first enrolled in Part B -- is the one time insurers must sell you any Medigap policy they offer with no medical underwriting. No health questions, no exclusions, no rate-up for your conditions. In most states, that window happens once in your life. Outside it, insurers in most states can review your health history and decline you or charge you more. A handful of states have broader rules, but you should not plan around being in one.
The practical consequence: choosing Medicare Advantage at 65 can quietly close the Medigap door. If you develop health problems later and want to move back to Original Medicare with a Medigap policy, you may not pass underwriting. Federal law does give you one escape hatch -- the 12-month trial right. If you joined a Medicare Advantage plan at 65 as your very first Medicare enrollment, you can switch to Original Medicare within your first 12 months and buy a Medigap policy on a guaranteed-issue basis, no underwriting allowed. Treat that first year as a genuine trial: if the network feels tight, the referrals slow, or the prior authorizations frustrating, the exit is only guaranteed while the trial right lasts.
Neither path is wrong. Advantage plans work well for many people, especially those who value the lower premiums and extras and are comfortable inside a network. Medigap works well for people who want maximum flexibility and predictable costs, and who can afford the premium. The mistake is not the choice itself -- it is making the choice without knowing which doors it closes.
Free Help Exists -- Use It
Two resources cost nothing and have no sales agenda. The plan finder at medicare.gov lets you compare every Advantage and Part D plan in your ZIP code against your actual medication list. And every state has a SHIP -- State Health Insurance Assistance Program -- with trained counselors who will walk through your options one-on-one, for free, without selling you anything.
And when a rule in this guide meets the particulars of your own life -- a spouse's employer plan, an HSA balance, an income spike two years before retirement -- ask Jaya on WithDave. Translating government programs into plain English, for your exact situation, is what she is there for.