Squad - 9 to 10 soldiers. Typically commanded by a sergeant or staff sergeant, a squad or section is the smallest element in the Army structure, and its size is dependent on its function.
Platoon - 16 to 44 soldiers. A platoon is led by a lieutenant with an NCO as second in command, and consists of two to four squads or sections.
Company - 62 to 190 soldiers. Three to five platoons form a company, which is commanded by a captain with a first sergeant as the commander's principle NCO assistant. An artillery unit of equivalent size is called a battery, and a comparable armored or air cavalry unit is called a troop.
Brigade - 3,000 to 5,000 solders. A brigade headquarters commands the tactical operation of two to five organic or attached combat battalions. Normally commanded by a colonel with a command sergeant major as senior NCO, brigades are employed on independent or semi-independent operations. Armored cavalry, ranger and special forces units this size are categorized as regiments or groups.
Division - 10,000 to 15,000 soldiers. Usually consisting of three brigade-sized elements and commanded by a major general, divisions are numbered and assigned missions based on their structures. The division performs major tactical operations for the corps and can conduct sustained battles and engagements.
Corps - 20,000 to 45,000 soldiers. Two to five divisions constitute a corps, which is typically commanded by a lieutenant general. As the deployable level of command required to synchronize and sustain combat operations, the corps provides the framework for multi-national operations.
Army - 50,000 + soliders. Typically commanded by a lieutenant general or higher, an army combines two or more corps. A theater army is the ranking Army component in a unified command, and it has operational and support responsibilities that are assigned by the theater commander in chief. The commander in chief and theater army commander may order formation of a field army to direct operations of assigned corps and divisions. An army group plans and directs campaigns in a theater, and is composed of two or more field armies under a designated commander. Army groups have not been employed by the Army since World War II.