From the Federation of American Scientist, Military Analysis network:
http://www.fas.org/programs/ssp/man/usm ... t_org.html
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Squad/Section: 4 to 10 people [usually 10 People]
Platoon/Detachment: Between 2 or more Squads (Usually 3) [Around 40 People]
Company/Battery/Troop: Between 2 or more Platoons (Usually 3) [Around 130 People]
Battalion/Squadron: Between 2 or more Companies (Usually 3) [Around 450 People]
Regiment/Group: Between 2 or more Battalions (Usually 3) [Around 1,500 People]
Brigade: This used to contain 3 Regiments and support, but is not the case in Modern War. Most Brigade units consist of at least 2 Battalions, but usually between 5 and 7 Battalions. [Varies between 2,000 and 6,000 People]
Division: This used to contain 3 Brigades and support, but is not the case in Modern times. Most Modern Divisions consist of at least 8 Battalions, but usually between 9 and 18 Battalions. [Varies between 8,000 and 12,000 People]
Corps: This used to contain 3 Divisions and support, but is not the case in Modern times. Most Modern Corps consist of at least 9 Brigades and support, but usually between 6 and 10 Brigades. [Varies between 20,000 and 45,000 People]
Army: This used to contain 3 Corps and support, but is not the case in Modern times. Most Modern Armies consist of at least 8 Divisions and support, but usually between 6 and 9 Divisions. [Varies between 120,000 and 145,000 People]
Army Group: Consist of at least 2 Army's and usually up to 3. [Varies between 300,000 and 450,000 People]