From the At a Glance: "To increase understanding of the choices that the nation faces when considering the defense budget, the Congressional Budget Office has updated its 2016 primer on the structure of the armed forces. This updated version is based on the spending plans and personnel numbers outlined in the Department of Defense's (DoD's) Future Years Defense Program for fiscal years 2021 to 2025. The primer contains entries that describe the size, costs, and functions of every major element of the military's force structure. Those elements include the major combat units that form the traditional backbone of the armed forces, such as armored brigades, aircraft carrier strike groups, and tactical aircraft squadrons. They also include specialized organizations that provide specific capabilities to DoD, such as special-operations forces and missile defense. CBO [Congressional Budget Office]'s analysis of the military's force structure focuses on the day-to-day operating costs covered by DoD's operation and support (O&S) budget. That budget, which has typically totaled more than $400 billion a year in recent years, excludes spending to buy weapon systems, build military facilities, and conduct ongoing military operations."
Congressional Budget Office: http://www.cbo.gov/