According to: http://grammar.ccc.commnet.edu/grammar/abbreviations.htm
Abbreviations of units of measure are written without periods (with the exception of "in" when it could be confused with the preposition). We use periods for most lower-case abbreviations such as e.g. and i.e. and c.o.d. For very common abbreviations, leave out the periods, as in rpm and mph. When an abbreviation with a period ends a sentence, that period will suffice to end the sentence: He lives in Washington, D.C. Suffixes for people's names require periods: Joe Smith Jr. lives in Erie. In formal text it is not a good idea to abbreviate military titles — Lieutenant Colonel Chester Piascyk — but in informal text Lt. Col. Chester Piascyk would be acceptable. (Note the space after "Lt.") Academic degrees can be written with periods or not, but don't insert spaces — Ph.D. or PhD, M.B.A. or MBA .
According to Grammar Girl: http://grammar.quickanddirtytips.com/acronym-grammar.aspx
Any shortened form of a word is an abbreviation, for example, "etc." for "etcetera" and "Oct." for "October;" but acronyms are special kinds of abbreviations that can be pronounced as words, such as "NASA" (National Aeronautics and Space Administration) and "OPEC" (Organization of the Petroleum Exporting Countries). This makes acronyms a subset of abbreviations. All acronyms are abbreviations, but not all abbreviations are acronyms.
You may be wondering whether you need to put periods after each letter in an acronym or initialism. There's no strict rule. Some publications put periods after each letter, arguing that because each letter is essentially an abbreviation for a word, periods are necessary. Other publications don't put periods after each letter, arguing that the copy looks cleaner without them, and that because they are made up of all capital letters, the fact that they are abbreviations is implied.
Finally, when you're using any kind of abbreviation in a formal document, it's important to spell out the entire phrase the first time you use it and put the abbreviation in parentheses after the words so people know what your abbreviation means.
According to Oxford Dictionary: http://oxforddictionaries.com/us/words/punctuation-in-abbreviations-american
People are often uncertain about whether or not to use periods in abbreviations. Here are some guidelines:
If the abbreviation consists only of the first part of a word, then you should put a period at the end
Wed. [= Wednesday] Dec. [= December]
If an abbreviation consists of the first and last letters of a word, the American rule is to include a period at the end:
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