• EM |
• Articles |
• Style |
• Abbreviations

Capital Letters in Abbreviations

Neha Karve

Updated September 20, 2023

Whether an abbreviation contains capital or lowercase letters is based on convention and established usage rather than a fixed set of rules. Many abbreviations are capitalized (MBA, VP, DNA, ATM, URL), while others are lowercased (e.g., i.e., etc., laser). Some abbreviations may be either capitalized or lowercased (NATO/Nato, COVID/Covid). Still others contain a mix of capital and lowercase letters (PhD, mRNA, kW).

Capitalization can depend on whether an abbreviation is an initialism, of which the letters are individually pronounced (USA, UK), or an acronym, pronounced as a single word (NATO, radar). Abbreviated Latin terms are usually lowercased (i.e., a.m.), as are many scientific and technical abbreviations (bhp, kg). We discuss below which abbreviations are generally capitalized and which are lowercased.

Initialisms

Initialisms, which comprise the first letters of the words of a term, generally contain capital letters to make them easily identifiable as abbreviations rather than words. In an initialism, the individual letters are separately pronounced.

Many scientific and technical abbreviations and those containing the word per are lowercased.

Initialisms seen in informal usage (such as in text messages and on social media) are also often lowercased: ttyl, rofl, brb.

Acronyms

Acronyms usually contain all capital letters to indicate that they are abbreviations rather than words. (An acronym is an abbreviation pronounced as a single word.)

With frequent usage, some acronyms become words in their own right and are no longer capitalized.

Some acronyms may be either capitalized or lowercased. Dictionaries often list both options.

With usage, some acronyms may take only an initial capital letter. In British usage, acronyms are often written with only the first letter capitalized.

In some styles, only the initial letter is capitalized in acronyms longer than five letters. The AP Stylebook, for example, recommends capitalizing only the first letter of acronyms of six letters or more.

The following examples show how acronyms, which are pronounced as words, are sometimes lowercased with use. The BBC capitalizes the first letter of many acronyms but lowercases the rest of the term (Nato, Unicef). Other publications, like the New York Times, still capitalize the entire term (NATO, UNICEF) but treat UNESCO as a name, with only the first letter capitalized (Unesco).

Mix of capital and lowercase letters

Some abbreviations may contain a mix of capital and lowercase letters. Prepositions like of, for example, may be represented by a lowercase letter in an acronym that is otherwise capitalized.

Contractions and other shortened forms

Contractions of social titles that appear before a name are capitalized.

Contracted names of months and days of the week also start with a capital letter. (These names are considered proper nouns and are therefore capitalized.)

A contraction that is part of a proper noun (for example, the name of a person, organization, or geographical feature, or the title of a book) is also capitalized.

Contractions that are not proper nouns and don’t appear as titles before a name need not be capitalized.

Latin abbreviations

Latin abbreviations are generally not capitalized but are lowercased instead, with periods to separate the letters.

Contracted forms of Latin words are also not capitalized.

Occasionally, Latin abbreviations may be capitalized.

The abbreviations a.m. and p.m. are also sometimes capitalized, although they are usually lowercased in formal writing.

Scientific and technical abbreviations

Many scientific and technical abbreviations are lowercased. They may also contain a mix of capital and lowercase letters.

Units of measurement

In general, don’t capitalize abbreviated forms of SI and other units of measurement.

But do capitalize abbreviated units of measurement named after people. (Note that such units are still lowercased when spelled out.)

The abbreviated form of liter may be capitalized (L instead of l), even though it is not named after a person, to avoid confusion with the numeral 1. This is often preferred in North American and Australian writing.

Also capitalize degree Celsius.

Prefixes for quantities smaller than the standard unit are lowercased (like milli- and deci-). Prefixes for quantities larger than the standard unit are capitalized (like mega- and giga-), except kilo-, hecto-, and deka-.

Units of computer information are capitalized differently. The abbreviation for bit is lowercased, while byte is capitalized. Also note that with these units, the abbreviation for kilo- is capitalized.

Full forms of abbreviations

In general, avoid capitalizing the full form of an abbreviation, unless it contains a proper noun or proper adjective.

Do capitalize the full form of an abbreviation of a proper noun. Note that minor words (prepositions like of, conjunctions like and, and articles like the) are still lowercased.