Unless you are following a style guide that specifies otherwise, observe the following conventions.
Abbreviations, shortened forms of words, are commonly employed in scientific and technical writing. However, avoid unnecessary abbreviations, which can confuse a reader. Some abbreviations are always followed by a period. With other abbreviations, however, the use of the period varies from discipline to discipline. Similarly some abbreviations are capitalized and others are not. Consult a style guide in your professional field for appropriate rules regarding capitalization and periods.
The following list outlines common and appropriate uses of abbreviations.
Mr., Ms., B.A., Ph.D., B.C., B.C.E.
cf. (from the Latin for "compare")
et al. (from the Latin "and others")
i.e. (from the Latin "that is")
e.g. (from the Latin "for example")
|in.||inch||cu. in.||(or in.3)||s||second|