What are the relative sizes of the named 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:
There doesn’t seem to be any mnemonic to this. You just have to memorize them, I guess.
Why I Looked It Up
I was reading a history of World War 1, and the unit sizes are used constantly. I couldn’t figure out what was big and what was small, and what was bigger or smaller than anything else.