Plurals of Acronyms and Initialisms

The plural of an acronym or initialism (an abbreviation formed by using the initial letters of multiword terms and sounded out letter by letter, such as FBI or CEO) is usually formed by adding -s. No apostrophe is needed. Some style guides permit plurals to be formed by adding -s, but there are good reasons not to use an apostrophe when forming the plural of an acronym or initialism. First, an acronym or initialism may have a possessive form (CEO’s office, FBI’s manpower), so it’s important to distinguish between the forms. Second, the mistake of forming plurals of nouns by adding -s is rampant (the Smith’s when it should be the Smiths), and there is no good reason to reinforce this mistake.

There are still two cases when adding -s to an acronym or initialism is preferred: if periods are used with capital letters (M.B.A.’s) or if lowercase letters are used (jpeg’s). The reason -s is still used in these cases is to avoid possible confusion of the -s being a part of the acronym or initialism.
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