What Is Medicare Part B? Coverage, Costs, and Enrollment Explained

Medicare Part B is one of two parts that make up what most people call “Original Medicare.” If you’re approaching 65, already enrolled, or helping a parent navigate health coverage, understanding Part B is essential — it covers the medical services you’re most likely to use.

This guide explains exactly what Medicare Part B covers, how much it costs in 2025, and the enrollment rules you must follow to avoid costly late-enrollment penalties.

What Is Medicare Part B?

Medicare Part B is the medical insurance component of Original Medicare. It covers outpatient services — everything that doesn’t require you to be admitted to a hospital. The other component, Medicare Part A, covers inpatient hospital stays, skilled nursing facility care, and hospice.

Together, Parts A and B form the foundation of Medicare coverage. Most people get both parts simultaneously, though you can have one without the other in certain situations.

Part B is administered by the federal government through the Centers for Medicare & Medicaid Services (CMS). Coverage is standardized — Part B covers the same services for everyone enrolled, regardless of where you live.

What Does Medicare Part B Cover?

Medicare Part B covers two main categories of services:

Medically Necessary Services

“Medically necessary” means services or supplies that are needed to diagnose or treat your medical condition and meet accepted standards of medical practice. Part B covers:

• Doctor visits — office visits with your primary care physician, specialists, surgeons, and other licensed practitioners
• Outpatient hospital care — emergency department visits, observation stays, and same-day surgeries where you aren’t formally admitted
• Ambulance services — emergency transportation when other transport would endanger your health
• Durable medical equipment (DME) — wheelchairs, walkers, oxygen equipment, and blood sugar monitors prescribed by a doctor
• Mental health services — outpatient therapy, psychiatric evaluation, and substance use disorder treatment
• Dialysis — kidney dialysis treatment at a facility or at home
• Second surgical opinions — Part B encourages getting a second opinion before elective surgery
• Lab tests and X-rays — when ordered by your doctor as part of diagnosing or treating a condition
• Outpatient physical, occupational, and speech therapy — to help restore function after injury or illness

Preventive Services

One of the most valuable aspects of Part B is its coverage of preventive care — often at no cost to you. Preventive services covered include:

• Annual Wellness Visit — a yearly check-in with your doctor to review your health and create a personalized prevention plan
• Cardiovascular screenings — cholesterol, lipid, and triglyceride tests
• Cancer screenings — mammograms, Pap smears, colorectal cancer screenings, prostate cancer screenings, and lung cancer screenings (for qualifying smokers)
• Bone density measurements — to screen for osteoporosis
• Depression screening — once per year in a primary care setting
• Diabetes screening — for people at risk
• Flu shots, Pneumococcal vaccines, and Hepatitis B vaccines
• Obesity screening and counseling
• Tobacco cessation counseling — up to 8 sessions per year

Most preventive services are covered at 100% (no cost to you) when provided by a Medicare-participating provider.

What Part B Does NOT Cover

Understanding the limits of Part B is equally important. Part B does not cover:

• Prescription drugs — these are covered under Medicare Part D, a separate plan
• Dental care — routine dental exams, fillings, dentures, and extractions are generally not covered
• Vision care — routine eye exams and glasses are not covered (though cataract surgery is)
• Hearing aids — hearing exams and hearing aids are not covered
• Long-term custodial care — help with daily activities like bathing and dressing in a nursing home or at home
• Cosmetic surgery — unless required after an accidental injury or to repair a defect

These coverage gaps are why many people purchase a Medicare Supplement (Medigap) plan or choose Medicare Advantage instead of Original Medicare.

How Much Does Medicare Part B Cost in 2025?

Medicare Part B has several cost components:

Monthly Premium

The standard Part B monthly premium for 2025 is $185.00 per month. Most people pay this amount directly from their Social Security benefit.

However, higher-income beneficiaries pay more. The Income-Related Monthly Adjustment Amount (IRMAA) adds a surcharge for individuals whose modified adjusted gross income (MAGI) exceeds certain thresholds.

IRMAA is based on your income from two years prior. If your income drops significantly (due to retirement, for example), you can appeal the surcharge using Form SSA-44.

Annual Deductible

Before Medicare pays for most services, you must first meet the Part B deductible. In 2025, the deductible is $257 per year.

Coinsurance

After meeting your deductible, Medicare Part B pays 80% of approved charges for most services. You pay the remaining 20% — and there is no out-of-pocket maximum in Original Medicare alone.

This 20% coinsurance is one of the primary reasons people buy Medigap coverage. A major surgery or extended outpatient treatment can result in substantial bills if you have no supplemental coverage.

When to Enroll in Medicare Part B

Enrollment timing matters enormously. Miss the right window and you’ll pay a permanent premium penalty.

Initial Enrollment Period (IEP)

Your first chance to enroll in Part B is the 7-month Initial Enrollment Period surrounding your 65th birthday:

• 3 months before the month you turn 65
• The month you turn 65
• 3 months after the month you turn 65

Enrolling during the first 3 months of your IEP means your coverage starts the month you turn 65. Enrolling in month 4 or later delays your start date by one to three months.

Special Enrollment Period (SEP)

If you’re still working at 65 and covered by an employer group health plan (through your own job or a spouse’s), you can delay Part B enrollment without penalty. You’ll have a Special Enrollment Period — 8 months after your employment or group coverage ends — to sign up.

Important: COBRA and retiree health coverage do not count as active employment coverage for this purpose. Relying on COBRA to delay Part B enrollment will trigger the late penalty.

General Enrollment Period

If you miss both the IEP and any applicable SEP, you can enroll during the General Enrollment Period: January 1 through March 31 each year, with coverage starting July 1.

Late Enrollment Penalty

If you don’t have qualifying coverage and delay Part B enrollment beyond your IEP, you’ll pay a 10% premium surcharge for each full 12-month period you were eligible but not enrolled. This penalty is permanent — it doesn’t go away after a few years.

For example, delaying enrollment by two years adds 20% to your monthly premium for life. At $185/month, that’s an extra $37/month, or $444/year, forever.

How Part B Works With Other Coverage

With Part A: Together, Parts A and B cover a wide range of medical needs, but leave gaps — the 20% coinsurance, the deductible, and excluded services. Most people fill these gaps with either a Medigap plan or a Medicare Advantage plan.

With Medicare Advantage (Part C): If you choose Medicare Advantage, your Part A and B coverage is bundled into a private insurance plan. You still pay the Part B premium, but the Advantage plan takes over coordination of your benefits.

With Medigap: If you keep Original Medicare (Parts A and B), a Medigap plan covers much of what Part B doesn’t — often including the 20% coinsurance, the deductible, and sometimes coverage during foreign travel.

With employer coverage: If you have retiree health coverage from a former employer, it typically works alongside Medicare rather than replacing it. The coordination of benefits rules determine which pays first.

Key Takeaways

• Medicare Part B covers outpatient medical care, doctor visits, preventive services, and durable medical equipment
• The standard 2025 premium is $185/month; higher earners pay more via IRMAA
• After the $257 annual deductible, Part B pays 80% — you pay 20% with no cap
• Enroll during your Initial Enrollment Period (the 7 months around your 65th birthday) or face a permanent late penalty
• Delaying is penalty-free only if you’re covered by an active employer group health plan through your own or a spouse’s current job

Understanding Part B is the foundation for every other Medicare decision you’ll make. Next, explore how Medicare Advantage compares to Original Medicare to decide which path makes more sense for your situation.