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Periods in Abbreviations

Neha Karve

Updated July 2, 2023

Here are the generally accepted guidelines for using periods in acronyms and other abbreviations.

1. In most formal styles, periods are not used in abbreviations with more than one capital letter (URL, DNA, PhD, ATM, PDF, USA, UK, NATO, NASA, CEO).
2. Periods are used in Latin and scholarly abbreviations with lowercase letters (e.g., i.e., etc., p.m., abbr., p.p.) but not in technical abbreviations and those that contain the word per (mph, rpm, ppm, bhp).
3. Periods are often used after initials in a name (L. M. Montgomery, Arthur C. Clarke, R. Daneel Olivaw) but not if the entire name is abbreviated (MLK, JFK, FDR).
4. Abbreviated SI units of measurement never contain periods (cm, kg, km).
5. In American usage, a period is used after a contraction or other shortened form (Dr., Mr., Rev., Mt., Inc., Jr., Jan., Sun., abbr.). In British usage, no period or full stop is used after a contraction (Dr, Mr, Mt, Jr, Ltd) except when a shortened form comprises only the first letters of a word (Rev., Hon., Prof., Jan.).

The use of periods in abbreviations is a matter of style rather than grammar, and style guides differ in their recommendations. In this article, we discuss general guidelines. Many exceptions exist. As a writer or editor, respect the conventions of your field, which take precedence over these “rules.” And remember to stay consistent in usage throughout a document.

Abbreviations with capital letters

Acronyms and other abbreviations with all capital letters generally don’t take periods. Also avoid using periods if an abbreviation contains more than one capital letter.

U.S. or US?

The abbreviation for United States has traditionally been written with internal periods (U.S. rather than US), but now, both are considered correct: U.S. and US.

The APA Publication Manual recommends using periods, while other style manuals, including the Chicago Manual of Style, CSE Manual, and MLA Handbook, now suggest omitting them, as you would for any other abbreviation with all capital letters. The AP Stylebook mandates the use of periods in running text (U.S.) but recommends omitting them in headlines (US). Pick a style, and use it consistently across a document.

Although US may be written with or without periods, style guides agree that USA contains no periods. Periods are also generally omitted in UK and EU, except in AP style, which recommends U.K. but EU (and UK in headlines).

Abbreviations with lowercase letters

In American usage, Latin and other scholarly abbreviations with lowercase letters generally contain periods.

Periods (or full stops) are often omitted in British usage (5 am instead of 5 a.m.).

Acronyms with all lowercase letters contain no periods. (Acronyms are abbreviations pronounced as words. With usage, some become words in their own right.)

Abbreviations and acronyms seen in Internet slang and casual chat generally contain no periods, whether comprising capital or lowercase letters.

Scientific and technical abbreviations

Don’t use periods in abbreviations containing the preposition per and in technical and scientific abbreviations with lowercase letters.

Contractions and shortened forms

Use a period after a contraction or other shortened form.

Some contracted forms become words in their own right. The period is then omitted. (Check whether the abbreviated form is listed as a word in a standard dictionary like Merriam-Webster or Oxford.)

Exceptions in British English

In British usage, no period is used after a contraction.

But if the abbreviation consists of only the first part of a word, a period is still used at the end in British style.

Here are some usage examples.

Name initials

Periods are generally used after initials before a surname.

Periods are often omitted after initials in news copy (e.g., JRR Tolkien rather than J. R. R. Tolkien). The use of spaces between initials (J. R. R. or J.R.R.) is also a matter of style. The Chicago Manual of Style, APA Publication Manual, and MLA Handbook suggest inserting spaces; the AP Stylebook recommends omitting them.

Initials are not separated by periods when used in place of an entire name (including the surname).

Abbreviated units of measurement

Don’t use a period after an abbreviated unit of measurement or a symbol for an SI unit.

However, some abbreviated units of time, considered acceptable in nontechnical, nonacademic writing, can take periods.

Abbreviations of non-metric units, common in American usage, may also be followed by a period.