Is "One" Capitalized in a Title?

Yes. "One" is a number, pronoun, or determiner ("One Day," "The One Rule"), and every style guide capitalizes it in a title.

"One" by Style Guide

Style guide	"One" mid-title	Why
AP	Capitalize	Determiners and pronouns are major words
APA	Capitalize	Determiners and pronouns are major words
Chicago	Capitalize	Determiners and pronouns are major words
MLA	Capitalize	Determiners and pronouns are major words
AMA	Capitalize	Determiners and pronouns are major words
Bluebook	Capitalize	Determiners and pronouns are major words

Examples in Chicago Style

• One Rule for Better Titles
• The One Tool You Need
• One Day at a Time
• From One Writer to Another

Spelled-out numbers are capitalized in titles like any other major word: "Two," "Ten," "One Hundred."

For the short list of words that do stay lowercase, see the full list of words that stay lowercase.

"One" in Sentence Case

APA reference lists, many news sites, and most UI text use sentence case instead of title case. There, only the first word and proper nouns get capitals, so "one" is lowercase unless it starts the title: "One rule for better titles." Pick one of the two styles for a whole set of headings rather than mixing them.

Quick Answers

Should "one" be capitalized in a title?

Yes. "One" is a determiner or pronoun, and every style guide capitalizes determiners and pronouns in title case.

Is "one" capitalized in APA?

Yes. APA capitalizes major words, including determiners and pronouns, regardless of length.