Some people reason that since “RBI” stands for “runs batted in,” there is no need for an additional “S” to indicate a plural, and speak of “120 RBI.” However, though somewhat illogical, it is standard to treat the initialism as a word and say “RBIs.” In writing, one can add an optional apostrophe: “RBI’s.” Definitely nonstandard is the logical but weird “RsBI.”

The same pattern applies to other such plural initialisms as “WMDs” (“weapons of mass destruction”), “POWs” (“prisoners of war”), and “MREs” (“meals ready to eat”); but “RPMs” (“revolutions per minute”) is less widely accepted.

List of errors