Part A covers inpatient hospital stays where the beneficiary has been formally admitted to the hospital, including semi-private room, food, and tests. As of January 1, 2018, Medicare Part A had an inpatient hospital deductible of $1340, coinsurance per day as $335 after 61 days confinement within one "spell of illness", coinsurance for "lifetime reserve days" (essentially, days 91-150 of one or more stay of more than 60 days) of $670 per day. The structure of coinsurance in a Skilled Nursing Facility (following a medically necessary hospital confinement of 3 nights in row or more) is different: zero for days 1-20; $167.50 per day for days 21-100. Many medical services provided under Part A (e.g., some surgery in an acute care hospital, some physical therapy in a skilled nursing facility) is covered under Part B. These coverage amounts increase or decrease yearly on 1st day of the year.

The maximum length of stay that Medicare Part A covers in a hospital admitted inpatient stay or series of stays is typically 90 days. The first 60 days would be paid by Medicare in full, except one copay (also and more commonly referred to as a "deductible") at the beginning of the 60 days of $1340 as of 2018. Days 61–90 require a co-payment of $335 per day as of 2018. The beneficiary is also allocated "lifetime reserve days" that can be used after 90 days. These lifetime reserve days require a copayment of $670 per day as of 2018, and the beneficiary can only use a total of 60 of these days throughout their lifetime. A new pool of 90 hospital days, with new copays of $1340 in 2018 and $335 per day for days 61–90, starts only after the beneficiary has 60 days continuously with no payment from Medicare for hospital or Skilled Nursing Facility confinement.

Some "hospital services" are provided as inpatient services, which would be reimbursed under Part A; or as outpatient services, which would be reimbursed, not under Part A, but under Part B instead. The "Two-Midnight Rule" decides which is which. In August 2013, the Centers for Medicare and Medicaid Services announced a final rule concerning eligibility for hospital inpatient services effective October 1, 2013. Under the new rule, if a physician admits a Medicare beneficiary as an inpatient with an expectation that the patient will require hospital care that "crosses two midnights," Medicare Part A payment is "generally appropriate." However, if it is anticipated that the patient will require hospital care for less than two midnights, Medicare Part A payment is generally not appropriate; payment such as is approved will be paid under Part B.[28] The time a patient spends in the hospital before an inpatient admission is formally ordered is considered outpatient time. But, hospitals and physicians can take into consideration the pre-inpatient admission time when determining if a patient's care will reasonably be expected to cross two midnights to be covered under Part A. In addition to deciding which trust fund is used to pay for these various outpatient vs. inpatient charges, the number of days for which a person is formally considered an admitted patient affects eligibility for Part A skilled nursing services.