Infantry Units

It varies from service to service, country to country, and era to era, but I found this listing of U.S. Army unit sizes (PDF) in the late 20th century:

• Army: 100,000
• Corps: 30,000+
• Division: 15,000+
• Brigade: 4,500
• Regiment: 1,500
• Battalion: 700
• Company: 175
• Platoon: 40
• Squad: 10

Note that “an Army” (the top-most division) is not “The Army.” When we speak of the Army, we mean every single person in the Army. But an Army is a subdivision of that. The Army actually consists of multiple armies.

The U.S. Army itself has a multimedia tool that lets you click through descriptions of the sizes (but it’s hard to link into).

The differences between the list above are:

• The Army adds a “Team” to the bottom, consisting of four people. (My personal experience confirms this: I served as a medic in a Marine Corps platoon, and the smallest unit was a four-person “Fire Team.”)
• The Army subdivides the topmost unit into:
  • Field Army (90,000)
  • Army Group (400,000)
  • Army Region (1,000,000+; it notes that this only happens during a time of war)
• The Army doesn’t include “Regiment.” There is the concept of a Regiment in the U.S. Army, but it seems to be more of an administrative construct, not a deployable unit.

For the UK, I cobbled together the following list from multiple sources. It’s very close to the U.S. structure:

• Division
• Brigade
• Regiment
• Battalion
• Company
• Platoon
• Section

There doesn’t seem to be any mnemonic to this. You just have to memorize them, I guess.