How should you treat references to states? The form depends on which style guide you adhere to and why the state is being referenced. Details about how to refer to states follow.
The Chicago Manual of Style and The Associated Press Stylebook agree on one thing: When referring to a state on its own, spell the state name out (“California became a state in 1850”). However, when referring to a city and the state in which it is located, although Chicago recommends retaining the spelled-out version of the state name (“San Diego, California”), AP style calls for abbreviating the state name (“San Diego, Calif.”) if it consists of more than six letters. (Chicago also has abbreviations if you insist, but they don’t always match AP’s style.)
The AP style abbreviations arbitrarily range in length from two to six letters, and all two-word names are abbreviated with the initials, such as N.Y. for “New York” — with the exception of West Virginia’s abbreviation, which for some reason is rendered W.Va. (Note that AP style omits state names for a specified list of cities considered familiar enough that the state in which they are located need not be mentioned.) In headlines, the periods are omitted.
However, when giving an address, or in tables or other uses in which space is limited, use the US Postal Service’s symbol system, which consists of a two-letter abbreviation in which both letters are always capitalized and no periods are used (for example, NY for “New York”).
Other style handbooks have their own guidelines, so, if you are writing or editing for a particularly company as a staff member or a freelancer, determine which resource is considered the authority on state abbreviation.
Note, too, that abbreviation of country names is rare and not recommended. US and UK are frequently used as nouns in informal contexts, but the names should be spelled out except as adjectives — “the US response,” for example, or “the UK’s role” — and Chicago recommends omitting periods in these cases, as is advised for all capitalized abbreviations.