The Plurals of Abbreviations and Acronyms

The Quick Answer

The plural of an abbreviation is formed by adding "s." For example:
  • RTA > RTAs correct tick
  • (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 correct tick
  • (COS = Chief of Staff)
  • COS > COS'S correct tick
  • (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 correct tick
  • ATM > ATMs correct tick
  • Once I have finished this PC, I will only have 3 PCs to configure. correct tick
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. correct tick
  • (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. wrong cross
  • (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 (small American flag) / Full Stops (small British flag)

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. correct tick
  • Not one of the S.O.S.'s was received by the coastguard. correct tick
Do not put a period after the "s" when forming the plural of an abbreviation with full stops. For example:
wrong MOT sign

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. wrong cross
  • (CCJ - County Court Judgment)
  • We have only done two M.O.T.s since January. correct tick

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. correct tick
  • (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.
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This page was written by Craig Shrives.