Simple Rules for Plural Acronyms and Abbreviations

Using acronyms and abbreviations is convenient, but it can also be a bit confusing figuring out how to properly convert these shortened versions of words or phrases to plural form. Discover basic rules for forming plural acronyms and plural abbreviations. 

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Basic Rules for Plural Acronyms

An acronym is a group of letters where each letter stands for a specific word in a phrase. True acronyms are pronounced as if they are words (for example: laser). Initialisms are a bit different, but are treated the same as acronyms. They are formed the same and become plural the same way, but are pronounced as individual letters rather than sounding like a word (For example: RBI). 

  • Most acronyms are easy to convert to the plural form simply by adding the letter s to the end. 

  • This can become more complex when the last letter in the acronym is already an s

Last Letter Is Not S

For acronyms that end in a letter other than an s, simply add a lowercase s after the last letter.  In other words, simply follow the ordinary procedure for most nouns

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light amplification by stimulated emission of radiation

LASER or laser (acronym)

LASERs or lasers

runs batted in

RBI (initialism) 


Common Mistakes to Avoid

Be sure to avoid making common mistakes when converting acronyms that end in a letter other than s to plural form. 

  • using a capital s - Avoid using a capital S at the end of an acronym written in all capital letters. A capital S at the end of an acronym means that the first letter of the last word of the phrase the acronym stands for is an s. (For example: LASERS would mean that there is another word after radiation that starts with an s in the string of words used to create the acronym.)

  • using apostrophe s - An apostrophe followed by an s indicates possession in most circumstances. If you add an apostrophe and an s to the end of an acronym, that will mean that you are referring to something owned by whatever the acronym stands for. (For example: FDA’s policy would mean a policy of the Food and Drug Administration.)

Last Letter Is S

When the last letter of an acronym or an initialism is an s, things get more complicated. Unless you are writing in a specific style (such as Chicago or American Psychological Association) that specifies how to handle these situations, you can add a lowercase s or an es, choosing the one that looks least likely to confuse the reader. If both ways look confusing, alter the wording of the sentence so you don’t need to use the acronym in a plural form. Consider spelling out each word rather than using the acronym or rephrasing the sentence. 

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Plural Options

time and relative dimensions in Space

TARDIS (acronym)


material safety data sheet

MSDS (initialism)


The Apostrophe Exception

While most style manuals advise against using an apostrophe for anything other than to indicate possession, some print publications follow a different guideline. Since print media outlets often use all capital letters for their headlines, they will use an apostrophe before an s when using the plural form of an acronym or initialism. This is to prevent confusion about what the letters in the acronym or initialism actually stand for. 

  • In an all-capital letter headline, the meaning of MSDSS would be unclear. Readers would wonder what the second S stands for. 

  • Using MSDS’s leaves less room for confusion as to what the actual acronym is. The context should clarify whether it is plural or singular.

Common Mistakes to Avoid

Be careful to avoid common mistakes when pluralizing acronyms or intiallisms that end with an s

  • placing an apostrophe after the last letter - Avoid placing an apostrophe on its own at the end of the acronym, as that punctuation would be used only for indicating possessive. (For example: MSDS’ requirements would indicate a requirement specified in a single MSDS, not that there are multiple MSDS forms). 

  • adding a period - Do not add a period to the acronym prior to adding an s or es. This would just end the sentence, and then you’d have an errant letter or two after the sentence.

Basic Rules for Plural Abbreviations

Abbreviations are not the same as acronyms. Abbreviations are shorter ways of expressing individual words. The basic rule for abbreviations is based on the number of periods, if any, in the abbreviation.

Let the Period Count Be Your Guide

Some abbreviations include a period (.), while some do not. Some abbreviations include shortened forms of multiple words, so they have more than one period. 

  • For abbreviations that do not have a period, add an s to the end of the shortened form of the word.

  • To make an abbreviation with one period plural, add an s immediately before the period.

  • For an abbreviation that has more than one period, add ‘s immediately after the last period. 

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ml (no period)



oz. (one period)


Doctor of Philosophy

Ph.D. (two periods)


Be Alert for Exceptions

While looking at the number of periods in an abbreviation works for many commonly used abbreviations, there are some exceptions. Since there are irregular plural nouns, it shouldn't surprise you to learn that there are also irregular plural abbreviations. For example, the plural of Mrs. is not Mrss. It is Mmes., which goes way beyond basic rules. Review this quick guide to Messrs., Mmes. and Mses for insights on irregular plural forms of courtesy titles that are used often. 

Applying the Rules for Plural Acronyms and Abbreviations

Be sure that you know the difference between an acronym and an abbreviation so you’re clear on which rules apply to the word you need to pluralize. Applying that knowledge, along with basic apostrophe rules, will help you properly express acronyms and abbreviations in plural form. 

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