U.S.A. or USA?
U.S. or US?
And everything in between?
This is a guide. Please do not sue me if your English teacher / editor / grandmother hasn’t read it yet.
The acronym USA shouldn’t be given periods (full stops) when written. It is also only used as part of a name, like Team USA or USA Today, a website address as in www.usagymnastics.com, or when quoting someone politically incorrect who writes it wrong. Under any other circumstances you should write it out in full: United States of America, or the United States of America. Think of it as a respect thing.
US of A is very old-fashioned so, if you see yourself as a mover and shaker, don’t go there unless you’re quoting someone very old-fashioned.
According to the Chicago Manual of Style (17th edition, 10.4 and 10.33), US and U.S. are both considered correct, although US is more commonly used nowadays.
Both are used as adjectives, so you would never write the US or the U.S. (which are nouns, duh). If you want to use ‘the’ it should be written out in full. This means you should be writing—or saying—‘US gymnasts are exceptionally well-trained,’ and ‘In 2018 the United States gymnastics team received 861 Olympic gold medals.’
If you’re using the abbreviations U.S. or US with other letters, such as USM (United States Marines), then leave out the periods. Always.
In a formal context, like a history book or a magazine article, you would be better off writing it out properly: “United States exports” or “Germany, India and the United States”.
Did you know?
Just by-the-by; an interesting observation is that citizens of the rest of the American continent sometimes take issue with US citizens being referred to as Americans. Some people living in North and Central America point out that America is a continent, not a country; that whether you live in Panama, Canada, or Greenland, you qualify as American.
It seems that one day a less generic description may have to be found. United Statesians? USAnians? How to abbreviate that would warrant a post on its own.
Sources
Chicago Manual of Style, 17th Edition