The Hutchins Center Explains: Federal budget basics

About two-thirds of the budget goes to mandatory spending, programs for which the government does not set a spending cap in advance, but, instead, provides specified benefits to everyone who meets the eligibility criteria. For example, once Congress sets the rules for Medicare—the health insurance program for the elderly—the exact amount of spending varies from year to year based on the size of the elderly population and how much or how little they use heath care services.