If Part A is the hospital insurance side of Medicare, Part B is where your everyday medical life lives. Doctor visits, specialist appointments, outpatient surgery, lab work, imaging, preventive screenings, mental health services, durable medical equipment, and outpatient drugs administered in a clinical setting — all of that falls under Part B. For most Medicare beneficiaries, the bulk of their claims activity runs through Part B, not Part A. You’ll interact with your Part B benefits far more often in a typical year than you’ll interact with Part A.
Part B also comes with a monthly premium that everyone enrolled must pay, regardless of their income or work history. In 2025, the standard Part B premium is $185 per month. That’s not a small number, and it’s in addition to any other coverage you carry. Higher earners pay more — significantly more — through income-related monthly adjustment amounts (IRMAA). At the highest income tier, Part B IRMAA adds nearly $444 per month to the standard premium, bringing the total Part B premium to over $628 per month for a single individual. Whether you’re at the standard premium or the top of the IRMAA scale, Part B is a real, mandatory cost of Medicare enrollment.
What Part B Covers: The Core Services
Part B covers physician services broadly — your primary care doctor, your cardiologist, your orthopedic surgeon, your endocrinologist, your psychiatrist, and any other licensed medical professional providing covered services on an outpatient basis. It covers outpatient hospital services, which is an important category that confuses many people: if you go to the emergency room and you’re not formally admitted as an inpatient, that’s a Part B claim, not Part A. Same with outpatient surgery, same-day procedures, and emergency visits that end in discharge rather than admission.
Diagnostic services are covered under Part B: laboratory tests, pathology, x-rays, CT scans, MRI, nuclear medicine imaging, ultrasound, and other diagnostic procedures ordered by your physician. Clinical laboratory services — the blood draws and urinalysis and cultures your doctor orders — are actually covered at 100% under Part B with no deductible and no coinsurance when provided by a participating laboratory. That’s a detail most people don’t know. Your doctor’s visit might cost you 20% after the deductible, but the blood work they order is free.
Ambulance transport is covered under Part B for medically necessary transport to a hospital or skilled nursing facility when other transportation would endanger your health. Ground ambulance is covered at 80% after the deductible. Air ambulance is covered under the same rules but involves higher costs and more billing disputes — air ambulance billing has been a source of surprise balance bills, though federal surprise billing protections have improved this situation in recent years. Durable medical equipment — wheelchairs, walkers, hospital beds, home oxygen, CPAP machines, blood glucose monitors, insulin pumps — is a Part B benefit when prescribed by your doctor and provided by a Medicare-enrolled supplier.
Part B Cost-Sharing: The 80/20 Split
After you meet the annual Part B deductible — $257 in 2025 — Medicare pays 80% of the Medicare-approved amount for covered services and you pay the remaining 20%. This applies to doctor visits, specialist care, outpatient hospital services, diagnostic imaging, durable medical equipment, and most other Part B services. The 20% doesn’t sound alarming in isolation, but there’s no annual cap on it under Original Medicare. If you have a year with significant healthcare utilization — a major surgery, cancer treatment, multiple specialist visits and imaging studies — your 20% can accumulate to a very large number.
Consider a surgery with a Medicare-approved amount of $15,000. Medicare pays $12,000. You owe $3,000 as your 20% share. Add the anesthesiologist’s 20%, the assistant surgeon’s 20%, and any follow-up care, and a single surgical event can generate thousands of dollars in uncapped coinsurance under Original Medicare. For healthy beneficiaries who rarely see specialists or need procedures, the exposure may be manageable. For anyone with ongoing health needs, it isn’t. This is the fundamental reason Medigap exists and why it has real value — it covers that 20% for you, replacing unpredictable out-of-pocket costs with a predictable monthly premium.
It’s also worth understanding the Medicare-approved amount concept. Medicare establishes the maximum it will pay for each covered service based on a fee schedule. Providers who accept Medicare assignment agree to accept that approved amount as payment in full. If your doctor’s fee schedule charges $300 for a visit but the Medicare-approved amount is $150, Medicare pays $120 (80% of $150) and you pay $30 (20% of $150). You don’t owe the additional $150 difference because your provider accepted assignment. But providers who don’t accept assignment can charge up to 15% above the Medicare-approved amount — called the limiting charge — which you can be billed for in addition to your 20% coinsurance. The vast majority of providers accept assignment, but verifying this before you establish care is a reasonable step, especially with specialists.
Preventive Services: What’s Actually Free
One of the most genuinely valuable features of Part B is coverage of preventive services at no cost to you. Services that receive an A or B recommendation from the U.S. Preventive Services Task Force are covered with no deductible and no coinsurance. You pay nothing. These aren’t minor services — they include colorectal cancer screenings (colonoscopy for average-risk adults every 10 years, or more frequently for higher-risk individuals), mammograms (annually for women 40 and over), cervical and vaginal cancer screenings, lung cancer screening with low-dose CT for high-risk current and former smokers, abdominal aortic aneurysm screening for men who have ever smoked, bone density testing for people at risk for osteoporosis, and diabetes screening.
Cardiovascular disease risk assessments, depression screening, and HIV screening are also covered at 100%. The “Welcome to Medicare” preventive visit you can access within the first 12 months of enrolling in Part B is covered free of charge. After that, the annual Medicare wellness visit — distinct from a routine physical exam — is covered at no cost each year. The wellness visit lets your doctor develop or update a personalized prevention plan, assess your risk factors, review your medications, and screen for cognitive impairment. It’s not a head-to-toe physical, but it’s a structured, covered annual touchpoint that costs you nothing.
Vaccines are a category worth calling out specifically. Flu shots, pneumococcal pneumonia vaccines, COVID-19 vaccines, and hepatitis B vaccines are all covered at 100% under Part B. Since 2023, the Inflation Reduction Act expanded zero-cost vaccine coverage for Medicare beneficiaries significantly, ensuring that all recommended adult vaccines are available without cost-sharing. Getting your recommended vaccines through Medicare costs you nothing — no deductible, no coinsurance, no copay. This is one of the clearest free benefits in all of Medicare and one of the easiest to take advantage of.
Outpatient Mental Health Coverage
Part B covers outpatient mental health services at the standard 80% after the deductible. That includes individual therapy, group therapy, family counseling, psychiatric evaluations, psychological testing, and medication management visits with a psychiatrist. Licensed clinical psychologists, clinical social workers, licensed professional counselors, and marriage and family therapists are all recognized Medicare providers for outpatient mental health services. You don’t need a referral to see a mental health provider under Medicare, though your primary care doctor can help coordinate care.
Telehealth for mental health has been a significant development in recent years. Congress has extended telehealth flexibilities introduced during COVID-19 through at least 2026, which means you can receive mental health services via video visit from your home in most circumstances without the geographic restrictions that previously limited Medicare telehealth. For beneficiaries in rural areas, people with mobility limitations, or anyone who simply finds it easier to access care remotely, this is a meaningful improvement in access. The 80/20 cost-sharing applies to telehealth mental health visits just as it does to in-person visits.
Partial hospitalization programs (PHP) for mental health are also a Part B benefit — these are intensive, structured outpatient programs for people who need more support than weekly therapy but don’t require inpatient psychiatric admission. PHP is covered at 80% after the deductible, like other Part B outpatient services. As of 2024, an annual mental health wellness visit is covered as a preventive service without cost-sharing, similar to the annual Medicare wellness visit for physical health.
Durable Medical Equipment and Supplies
Durable medical equipment (DME) is a Part B benefit when your doctor prescribes it and a Medicare-enrolled supplier provides it. DME includes wheelchairs (manual and power), scooters, walkers, crutches, hospital beds, home oxygen equipment, CPAP and BiPAP machines for sleep apnea, blood glucose monitors and test strips for diabetes management, infusion pumps, and prosthetic limbs and orthotics. The equipment must be durable (able to withstand repeated use), primarily medical in nature, appropriate for home use, and necessary for your medical condition.
Part B covers DME at 80% after the deductible — you pay the 20% coinsurance. If you’re on Original Medicare without a Medigap plan, that 20% can be substantial for expensive equipment. A power wheelchair, for instance, can have a Medicare-approved amount in the thousands of dollars, and 20% of that is real money. Medigap plans generally cover the DME coinsurance as part of their standard benefit. Medicare Advantage plans cover DME but may require prior authorization and may have different cost-sharing structures, so check your plan’s specific terms.
IRMAA: The Higher-Earner Premium Surcharge
If your Modified Adjusted Gross Income from two years ago exceeded $106,000 for a single filer or $212,000 for married filing jointly, you pay more than the standard $185 Part B premium. The surcharge is called IRMAA — Income-Related Monthly Adjustment Amount. It’s assessed in tiers based on income, and the additional amount ranges from $74.00 per month at the lowest tier to $443.90 per month at the highest tier for Part B in 2025. Part D has its own IRMAA surcharge applied at the same income thresholds, adding another $12.90 to $81.00 per month depending on the tier.
IRMAA uses income from two years prior because that’s the most recent year for which the IRS has complete tax data when CMS calculates your premium. In 2025, that means your 2023 MAGI determines your IRMAA tier. This lag creates a well-known problem: a one-time income event in a past year — a Roth conversion, a business sale, exercising stock options, a large required minimum distribution — can push you into a high IRMAA tier even though your ongoing income has returned to normal. If your current income is significantly lower than the income used to calculate your IRMAA due to a life-changing event (retirement, divorce, reduction in work hours, loss of income-producing property), you can appeal using Social Security Form SSA-44. Documenting the life change and submitting the appeal can knock you down one or more IRMAA tiers and save real money.
Enrolling in Part B: Timing and the Late Penalty
Your Initial Enrollment Period for Part B spans 7 months: 3 months before your 65th birthday month, your birthday month itself, and the 3 months after. Enrolling before your birthday month means coverage starts on the first of your birthday month. Enrolling in your birthday month means coverage starts the first of the following month. Enrolling in any of the 3 months after your birthday month delays your coverage start by 2 to 3 months.
If you miss your Initial Enrollment Period without having qualifying coverage from a current employer, you’ll face the Part B late enrollment penalty. It’s 10% of the standard premium for each 12-month period you were eligible but not enrolled. That penalty is permanent — it stays with you as long as you have Part B. Miss enrollment for 2 years and you’ll pay an extra 20% on top of the standard premium forever. At $185 per month, a 20% penalty adds $37 per month, which is $444 per year, which over 20 years of retirement is nearly $9,000 in extra premiums for an avoidable mistake.
People still covered by a current employer’s group health plan at 65 can delay Part B without penalty. But “current employer” means you’re actively employed and covered under a plan from that employment. Retiree coverage, COBRA, and coverage through a non-working spouse’s former employer don’t exempt you. Get this wrong and you’ll pay the penalty. If there’s any ambiguity about whether your coverage qualifies, check with Social Security before your Initial Enrollment Period closes — it’s the kind of question worth asking twice.