Periods in US (or U.S.) and USA
The abbreviation for United States may be written with or without periods between the letters—U.S. or US—depending on the style you follow. Although it has conventionally been written with internal periods, style manuals now differ in their recommendations.
- The APA Publication Manual recommends using periods to separate the two letters of the abbreviation: U.S.
- Other style authorities, including the Chicago Manual of Style, the CSE Manual, and the MLA Handbook, now suggest omitting periods in the abbreviation US (as you would for other abbreviations with two or more capital letters).
- The AP Stylebook mandates the use of periods in running text (U.S.) but recommends omitting them in headlines (US).
- Periods are omitted in British style, as recommended by the New Oxford Style Manual, BBC News style guide, and Guardian and Observer style guide.
- Dictionaries such as Merriam-Webster and Oxford list US (without periods) as the preferred abbreviation, and U.S. (with periods) as a variant.
- Finally, as dictated by tradition, the U.S. Government Publishing Office (GPO) Style Manual mandates the use of periods between the letters of the abbreviation (U.S.).
Thus, both styles are acceptable—US and U.S., with and without periods. General advice is to omit the periods in formal styles, except if you specifically follow AP or APA style or are writing for a government publication. If in doubt, respect the style followed by your client, organization, university, or publication. In news copy, omit the periods in headlines.
Abbreviation vs. full form
In academic and other formal writing, prefer to use the full form (United States) in running text, restricting the use of the abbreviation (US or U.S.) to captions, headings, headlines, tables, and charts.
Also prefer the full form, United States, over the abbreviated U.S. or US when you need to use the name as a noun (the economy of the United States), using the abbreviation only adjectivally (the US dollar).
U.S.A. or USA?
Don’t use periods in USA (for United States of America). Style authorities agree that the abbreviation USA is written without internal periods.
Other abbreviated names
Periods are generally omitted in UK and EU, except in AP style, which recommends U.K. and U.N. but EU (and UK and UN in headlines). Acronyms (which are abbreviations pronounced as words) such as NASA, NATO, UNICEF, and UNESCO also don’t contain periods.