Apply the title capitalization rules of your style guide.
Style guides disagree about which small words stay lowercase. AP lowercases articles, short conjunctions and prepositions of three letters or fewer, and capitalizes words of four letters or more. APA lowercases the same family of small words, with a list that includes as, if, off and via. Chicago and MLA lowercase all articles, coordinating conjunctions and prepositions of any length, so Through and Between become through and between in the middle of a title.
In every style the first word, the last word and the first word after a colon are capitalized, and parts of hyphenated words are capitalized. Initialisms such as U.S. are kept in capitals. A keep brands option restores common spellings such as iPhone, YouTube and NASA, and leaves any word that already has inner capitals as you typed it. An all capitals input is lowercased first so that it can be converted.
Use the one your publication or institution requires. If none is required, pick one and apply it consistently. AP is common in news, APA in sciences, Chicago and MLA in the humanities.
AP capitalizes any word of four letters or more. Chicago keeps all prepositions lowercase regardless of length unless they are the first or last word.
The Title Case Converter capitalizes every word. This tool applies the real style guide rules for minor words.