Periods in Abbreviations and Acronyms

Summary

In general, don’t insert periods in acronyms and other abbreviations with more than one capital letter.

Examples
  • PC, URL, DVD, PhD, ATM, PDF, USA, UK, EU, NATO, NASA, CEO, VP, SSN

Periods are generally used in abbreviations with lowercase letters.

Examples
  • e.g., i.e., etc., a.m., p.m.

But in technical abbreviations with lowercase letters and in those with the word per, periods are omitted.

Example
  • mph, rpm, bps, ppm, dpi, bhp, dc, fp

Periods are often used after initials in a name.

Examples
  • L. M. Montgomery, Arthur C. Clarke, R. Daneel Olivaw

But don’t use periods between initials if the entire name is abbreviated.

Examples
  • MLK, JFK, FDR

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

Examples
  • 10 cm, 5 ml, 100 kg, 300 ms, 17 km

Use a period after a contraction or other shortened form.

Examples
  • Dr., Mr., Rev., Sen., Mt., Inc., Ltd., Jr., Sr., Jan., Sun., abbr., adj.

In British usage, periods (or full stops) are usually omitted after contractions.

Examples
  • Dr, Mr, Mt, Jr, Sr, Ltd

But if a shortened form comprises only the first letters of a word, the period is still needed in British usage.

Examples
  • Rev., Hon., Prof., Jan., Sun.

The general rule

In general, don’t use periods in acronyms or other abbreviations with two or more capital letters.

Examples
  • USA
  • UK
  • NASA
  • NATO
  • NAFTA
  • EU
  • UN
  • UNICEF
  • WHO
  • CDC
  • FBI
  • MD
  • CEO
  • GmbH
  • LAN
  • HTML
  • ATM
  • DNA
  • PDF
  • URL
  • DVD
  • CD
  • PC
  • CPU
  • SARS
  • AIDS
  • HIV
  • COVID-19
  • SARS-CoV-2
  • DTaP
  • PhD
  • MBA
  • BSc
  • LLB
  • ROFL
  • IMHO
  • ICYMI
  • TIL
  • FYI

But do use periods in abbreviations with lowercase letters.

Examples
  • e.g.
  • i.e.
  • et al.
  • Inc.
  • Corp.
  • Jr.
  • Ltd.
  • Mr.
  • Mrs.
  • Sen.
  • Dr.
  • Prof.
  • Rev.
  • Asst.
  • Jan.
  • Sun.
  • adv.
  • abbr.
  • Eng.

Note that technical abbreviations (including those with lowercase letters) and abbreviated SI units contain no periods.

Examples
  • km (kilometer)
  • ms (millisecond)
  • mps (meters per second)
  • mph (miles per hour)
  • mW (megawatt)
  • emf (electromotive force)
  • Gb (gigabit)
  • sq (square)

The use of periods (or full stops) in abbreviations can differ between British and American usage. This 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.

In 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.

Examples
  • USA, UK, EU, NASA, NATO, NAFTA, OPEC, SAARC, UN, UNESCO, WHO
  • CD, DVD, URL, PC, DOS, HTML, PDF, XML, DNS, FTP, PC
  • DNA, HIV, AIDS, SARS, COVID-19, SARS-Cov-2
  • PhD, MPhil, BSc, MSc
  • MD, DDS, CFA, BA, MA, LLB, MBA
  • CEO, CFO, CTO, PA, SEO
  • GMT, UTC, CST, EST, PST, IST, CET, AM, PM
  • BC, AD, BCE, CE, BP, NNE
  • PS, QED, NB
  • CDC, FBI, DOJ, USCIS, MOJ, NCA, NSA, CIA, SIS, IB
  • IQ, FAQ, ASAP, FYI, ID, OTP
  • FYI, FWIW, ROFL, AFAIK, TIL, ICYMI, OMG, TBH, IMHO, AFAIK, BTW
  • IgG, HbA1C, AlpA, HopZ, ATPase, IgR
Note

While the Chicago Manual of Style and APA Publication Manual suggest omitting periods in abbreviations with capital letters, the AP Stylebook recommends the use of periods in two-letter abbreviations (B.A., B.C., U.N., U.K.), along with Ph.D. and LL.D., but not in ID and EU.

Tip

An acronym is an abbreviation pronounced as a single word. Thus, NASA, NATO, and UNESCO are acronyms, but USA, UK, EU, and UN are not; they are initialisms.

U.S. or US? Periods in US and USA

The abbreviation U.S. (for United States) has traditionally been written with internal periods: U.S. rather than US.

Both are correct: U.S. and US. Respect the style followed by your organization, university, or publication, and stay consistent in the use or omission of periods.

Tip

In running text, prefer to use the full form (United States), restricting the use of the abbreviation (US or U.S.) to captions, headings, headlines, tables, and charts.

Most style authorities agree that USA is written without periods. Periods are also generally omitted in UK and EU, except in AP style, which recommends U.K. but EU (and UK in headlines).

In abbreviations with lowercase letters

In general, use periods in abbreviations with lowercase letters.

Examples
  • i.e., e.g., n.d., n.b.
  • a.m., p.m.
  • ibid., etc., cf., viz., cet. par., ad int.
  • v.t., v.i., t.p., p.p.

Many exceptions exist, such as units of measurement and technical abbreviations; see below.

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

Examples
  • laser
  • sonar
  • scuba
  • fubar
  • snafu

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

Examples
  • lol
  • brb
  • btw
  • omg

In scientific and technical abbreviations

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

Examples
  • mph (miles per hour)
  • rpm (revolutions per minute)
  • ppm (parts per million)
  • Bps (Bytes per second)
  • dpi (dots per inch)
  • bhp (brake horsepower)
  • dc (direct current)
  • fp (freezing point)
  • mDa (megadaltons)
  • kW (kilowatt)

In contractions and shortened forms

Use a period after a contraction or other shortened form.

Examples
  • Dr., Mr., Mrs., Rev.
  • St., Hon.
  • Sun., Mon., Jan., Feb.
  • Sen., Pres., Gov., Sec. Gen.
  • Gen., Lt. Col., Maj., Sgt., Cdr., Capt.
  • Prof., Asst. Prof.
  • vol., ed., trans., et al., misc.
  • adv., adj., conj., abbr., n., ibid.
  • Eng., Lat., Ger., Gk.
  • Mt., Ave.
  • alt., lat., long.
  • Jr., Sr.
  • Corp., Ltd., Assoc., Co., Inc.
Caution

In et al., don’t place a period after et (which means “and”). Et al. is the abbreviated form of the Latin phrase et alii or et aliae (“and others”).

Example
  • Incorrect: Dash et. al., 1998
    Correct: Dash et al., 1998

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.)

Examples
  • lab (laboratory)
  • vet (veteran/veterinarian)
  • ad (advertisement)
  • gym (gymnasium)
  • Brit (British)

Exceptions in British English

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

Examples
  • Dr, Mr, Mrs
  • St
  • Jr, Sr
  • Ltd
  • Mt (e.g., Mt Everest)
  • Asst
  • ca (circa)

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

Examples
  • Hon., Rev.
  • Sun., Mon.
  • Jan., Feb.
  • Gen., Col., Brig.
  • Corp., Assoc., Co.
  • Prof., Asst Prof.
  • trans., ed., vol.
  • alt., lat., long.

Here are some usage examples.

Examples
  • British: Roland Jones Jr set up Roland Bros Ltd in Manila in 1903.
  • American: Roland Jones Jr. set up Roland Bros. Ltd. in Manila in 1903.
  • British: The Hon. Mr Farley Smith has been appointed chair of the committee.
    The contraction “Mr” doesn’t take a period, but “Hon.,” which is the first syllable of the word “honorable,” does.
  • American: The Hon. Mr. Farley Smith has been appointed chair of the committee.
    Periods are used after both “Hon.” and “Mr.”

In name initials

Periods are generally used after initials before a surname.

Examples
  • L. M. Montgomery
  • A. S. Byatt
  • P. G. Wodehouse
  • T. S. Eliot
  • F. Scott Fitzgerald
  • Pearl S. Buck
  • Arthur C. Clarke
  • R. Daneel Olivaw

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.

Tip

If you insert spaces between initials, use nonbreaking spaces to avoid a line break within a name. To insert a nonbreaking space in Microsoft Word, you can either press Ctrl+Shift+Space on the keyboard or choose Symbol from the Insert menu, click the Special Characters tab, highlight the nonbreaking space, and click Insert.

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

Examples
  • FDR
  • JFK
  • LBJ
  • MLK
  • JKR

In abbreviated units of measurement

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

Examples
  • km (kilometer)
  • s (second)
  • kg (kilogram)
  • g (gram)
  • mg (milligram)
  • l (liter)
  • dl (deciliter)
  • ml (milliliter)
  • A (ampere)

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

Examples
  • yr. (year)
  • mo. (month)
  • hr. (hour)
  • min. (minute)
  • sec. (second)

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

Examples
  • mi., yd., ft., in.
  • lb., oz.
  • qt., fl. oz.

Commas, periods, and other punctuation with abbreviations

If a sentence ends in an abbreviation that contains a period, don’t add another period to end the sentence (i.e., don’t use two consecutive periods).

Example
  • Incorrect: The meeting starts at 9 a.m..
    Correct: The meeting starts at 9 a.m.

But use other terminal punctuation—question marks and exclamation points—as usual after the abbreviation. If an abbreviation with periods appears within parentheses, add another period after the closing parenthesis to mark the end of the sentence.

Examples
  • Can we meet at 9 a.m.?
  • It’s 9 a.m.!
  • I’ll call you tomorrow morning (before 9 a.m.).

Use other punctuation as usual with abbreviations that contain periods.

Examples
  • At 9 a.m., the bell rang.
  • Every day is the same as the one before: he wakes at 6 a.m.; he sleeps at 10 p.m.
  • Maya likes waking up at 5 a.m.: that’s when the day is full of possibility.

Quick Quiz

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Did You Know?

Who is a pronoun, while where is an adverb.
Know more:What Is a Pronoun? Types of Pronouns