Medicare Parts A, B, C, and D Explained: What Every Senior Needs to Know

Medicare is the federal health insurance program for Americans aged 65 and older, and for certain younger individuals with disabilities or End-Stage Renal Disease. Despite being one of the most important financial decisions a retiree makes, millions of Americans enroll without fully understanding what each part covers. Getting it wrong can mean paying thousands of dollars in unexpected bills, or missing a critical enrollment window that triggers lifelong penalties. This guide walks through every part clearly, so you can enroll with confidence.

Part A covers inpatient hospital stays, skilled nursing facility care, hospice, and some home health care. Most people pay $0 in monthly premiums for Part A if they or their spouse paid Medicare taxes for at least 10 years. Part B covers outpatient services: doctor visits, preventive care, lab tests, mental health services, and durable medical equipment. The standard Part B premium in 2024 is $174.70 per month, though higher earners pay more via the Income-Related Monthly Adjustment Amount (IRMAA). Together, Parts A and B are called Original Medicare, a fee-for-service system with no out-of-pocket maximum, which is why many people add supplemental coverage.

Key Facts About Each Medicare Part

• Part A: Hospital Insurance

  Covers inpatient stays after a $1,632 deductible (2024). Days 1–60 of a hospital stay have no coinsurance; days 61–90 cost $408/day. No monthly premium if you have 40+ work credits.

• Part B: Medical Insurance

  Covers 80% of approved outpatient costs after a $240 annual deductible (2024). You pay the remaining 20% with no cap, which is why Medigap plans exist. Late enrollment triggers a 10% premium penalty per year you delayed.

• Part C: Medicare Advantage

  Private insurers bundle Parts A and B, usually including Part D and extras like dental, vision, and gym memberships. Plans use provider networks; monthly premiums can be as low as $0 but copays apply at visits.

• Part D: Prescription Drug Coverage

  Standalone drug plans sold by private insurers to complement Original Medicare. Each plan has its own formulary (drug list) and tier pricing. The maximum deductible in 2024 is $545. Late enrollment causes a permanent premium penalty.

• Medigap / Medicare Supplement

  Not an official 'part' but critical, these private policies cover the gaps in Original Medicare like the Part B 20% coinsurance. Plan G is the most comprehensive plan available to new enrollees since 2020.

Your Initial Enrollment Period (IEP) is a 7-month window: the 3 months before your 65th birthday month, the birthday month itself, and the 3 months after. Enrolling in month 1, 2, or 3 of your IEP means coverage starts before or on your birthday month. Waiting until months 5, 6, or 7 delays your start date. If you miss your IEP and aren't covered by employer insurance, you'll pay a 10% Part B premium penalty for every 12-month period you were eligible but didn't enroll, and that penalty lasts for life. For Part D, the penalty is 1% of the national base beneficiary premium for each uncovered month.

Choosing between Original Medicare with a Medigap plan and a Medicare Advantage plan comes down to your health needs, budget, and how often you travel. Original Medicare with Plan G offers maximum flexibility, any doctor who accepts Medicare, anywhere in the country. Medicare Advantage plans often have lower premiums but restrict you to networks and require referrals. Review the Annual Notice of Change sent every September before your plan's October 15 – December 7 Open Enrollment window to make sure your drugs and doctors are still covered at the same rates.

Medicare Part A covers inpatient hospital stays, skilled nursing facility care, home health care, and hospice care. Most people do not pay a monthly premium for Part A because they or their spouse paid Medicare taxes for at least 40 quarters (10 years) during their working career. However, Part A does have cost-sharing requirements that can add up quickly. For 2026, the Part A hospital deductible is over $1,600 per benefit period, meaning you pay this amount each time you are admitted to a hospital after a 60-day break from inpatient care. After the deductible, Part A covers the first 60 days of a hospital stay at no additional cost. Days 61 through 90 require a daily coinsurance payment, and days beyond 90 draw from your 60 lifetime reserve days with an even higher daily coinsurance. Skilled nursing facility coverage requires a qualifying 3-day hospital stay and covers the first 20 days fully, with coinsurance required for days 21 through 100. Understanding these cost structures helps you plan for potential out-of-pocket expenses and evaluate whether supplemental coverage is worth the investment.

Medicare Part B covers outpatient medical services including doctor visits, diagnostic tests, laboratory work, outpatient surgery, durable medical equipment, mental health services, and preventive care. The standard Part B premium for 2026 is approximately $185 per month, with higher-income beneficiaries paying more through the Income-Related Monthly Adjustment Amount (IRMAA). After meeting the annual Part B deductible (approximately $240), you typically pay 20 percent coinsurance for most Part B covered services with no annual out-of-pocket maximum, which is a significant gap that makes supplemental coverage important for many beneficiaries. Part B covers a wide range of preventive services at no cost, including annual wellness visits, cardiovascular disease screening, diabetes screening, cancer screenings (mammograms, colonoscopies, prostate and lung cancer screenings), flu shots, pneumonia vaccines, and depression screening. Taking advantage of these no-cost preventive services helps detect health problems early when treatment is most effective and least expensive.

Medicare Part C (Medicare Advantage) combines Part A, Part B, and usually Part D into a single plan offered by private insurance companies approved by Medicare. These plans must cover everything Original Medicare covers but can also include additional benefits like dental, vision, hearing, fitness programs, and transportation to medical appointments. Most Medicare Advantage plans use managed care networks that require you to use in-network providers, and some plans require referrals from a primary care physician to see specialists. Part C plans have annual out-of-pocket maximums that limit your total spending, providing financial protection that Original Medicare does not offer. Choosing between Original Medicare and Medicare Advantage is one of the most important healthcare decisions for beneficiaries, and the right choice depends on your health needs, preferred doctors, budget, and lifestyle.