Writing the Plural of an Abbreviation (Apostrophe or Not?)

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. Generally, this is something to be avoided, but, if the author believes the apostrophe helps readers, it can be added. For example:
  • COS > COS's
  • (COS = Chief of Staff)
  • COS > COS'S
  • (An apostrophe is useful when only capital letters can be used, e.g., in titles.)

Forming Plurals of Abbreviations

The plural of an abbreviation is usually formed by adding s to the end (e.g., MOT becomes MOTs, and ATM becomes ATMs). However, although not common practice, it is acceptable to use 's if it is deemed to assist the reader.

  • Once I have finished this PC, I will only have 3 PCs to configure.
  • We have a dozen SUS's to collect from the station.
  • (SUS - Soldier under Sentence)
    (In this example, the writer considers that SUS's is clearer than SUSs and has opted to use the 's version. This is acceptable. Be aware though that lots of grammarians will not like this, but it is the writer's decision whether to use the apostrophe to aid understanding. There is more on this subject in the lesson of using apostrophes to form plurals.)

  • 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.)

Pluralizing Abbreviations with Full Stops (Periods )

The rule above applies to abbreviations with 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.

Placing the Full Stop (Period )

A full stop is not required after the s when forming the plural of an abbreviation with full stops. For example:

should be M.O.T.s
  • 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.

See Also

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