The Plurals of Abbreviations and Acronyms

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The Quick Answer
The plural of an abbreviation is formed by adding "s." For example:
  • RTA > RTAs
  • (RTA = Road Traffic Accident)
However, the plural of an awkward abbreviation can be formed by adding "'s" (apostrophe s). Generally, this is something to be avoided, but, if you believe the apostrophe helps your readers, you can use one. For example:
  • COS > COS's
  • (COS = Chief of Staff)
  • COS > COS'S
  • (An apostrophe is useful when only capital letters are permissible, e.g., in titles.)

Forming Plurals of Abbreviations and Acronyms

The plural of an abbreviation or acronym is usually formed by adding "s" to the end. For example:
  • MOT > MOTs
  • ATM > ATMs
  • Once I have finished this PC, I will only have 3 PCs to configure.
However, although not a popular practice, it is permissible to use "'s" (apostrophe s) if it assists the reader. For example:
  • We have a dozen SUS's to collect from the station.
  • (SUS - Soldier under Sentence)
    (In this example, the writer considers SUS's to be clearer than SUSs and has opted to use the "'s" version. This is acceptable. Be aware though that this could irk a fair proportion of your readers. Nevertheless, it is the writer's decision whether to use the apostrophe to aid understanding.)
Only "'s" if it helps. For example:
  • The firefighters could not assist with three of the RTAs because the BA's were still in the stores.
  • (RTA - Road Traffic Accident / BA - Breathing Apparatus)
    (There is no good reason to write BA's - especially after showing how abbreviations are pluralized with RTAs; i.e., it's also inconsistent.)
forming the plural of abbreviations and acronyms

Read more about using apostrophes to form awkward plurals.

Pluralizing Abbreviations with Periods () / Full Stops ()

The rule above applies to abbreviations with periods / full stops too. For example:
  • Once I have finished this P.C., I will only have 3 P.C.s to configure.
  • Not one of the S.O.S.'s was received by the coastguard.
Do not put a period after the "s" when forming the plural of an abbreviation with full stops. For example:


This garage sign in the UK should say M.O.T.s not M.O.Ts.

Here is another example:
  • Anna was refused credit due to three C.C.J.s. and mortgage arrears.
  • (CCJ - County Court Judgment)
  • We have only done two M.O.T.s since January.

Avoid the Version with an Apostrophe

As covered in this lesson on apostrophes, "'s" can be used to show possession. For example:
  • The MP's photo shoot will be held at 7 o'clock.
  • (This is the photo shoot of one MP.)
Therefore, using "'s" to show a plural can lead to ambiguity or, at the least, cause the reader to pause momentarily while a mental check is performed to determine whether the abbreviation is a plural or showing possession.

In summary, you should be aware that the "'s" version is an option you can employ to assist the reader in spotting an unwieldy abbreviation instantly. However, in all other cases, you should opt for the "s" version.
Interactive Exercise
Here are three randomly selected questions from a larger exercise, which can be edited, printed to create an exercise worksheet, or sent via email to friends or students.

See Also

Using AD, BC, BCE and CE Full stops (periods) in contractions Using full stops (periods) in abbreviations