Case converter, with the style guides built in.

Twelve conversions, one specimen line

The fastest way to see what each button does is to run one sentence through all of them. Every cell below is real output from the tool on this page — the table is generated at build time by calling the same functions the buttons call, so it cannot drift out of step with the converter.

Two of those deserve a note. Sentence case keeps API because the word was already deliberately capitalised, and it re-capitalises the pronoun “I” and its contractions — the one English word that always takes a capital wherever it lands. iNVERSE is not the opposite of UPPERCASE; it swaps each letter individually, which is why a normally-typed sentence comes back looking like a stuck Caps Lock.

Title Case is four different things

“Title Case” sounds like one rule. It is four, and they disagree in public. Every guide agrees on the uncontroversial part — capitalise nouns, verbs, adjectives, adverbs and pronouns, lowercase the articles a, an and the in the middle of a title. The fight is about prepositions, and it comes down to a single question:

Does a word get a capital because of how long it is, or because of what it does?

AP and APA answer “how long”. Four letters or more gets a capital, whatever the word class. That is why AP writes Gone With the Wind — “with” is four letters, so up it goes, while the three-letter “the” stays down.

Chicago and MLA answer “what it does”. A preposition is a minor word and stays lowercase. MLA applies that with no exceptions at all, at any length. Chicago used to as well — and then, in its 18th edition, it moved halfway to the other camp and began capitalising prepositions of five letters or more, which is the single most consequential change on this page.

The other difference is quieter and catches people out constantly: APA has no “capitalise the last word” rule. AP, Chicago and MLA all say the final word of a title takes a capital no matter what it is. APA’s published rule list simply does not contain that clause, so a title that ends on a short preposition ends on a lowercase letter.

The disagreement, measured

Eight headlines, chosen so that each one isolates a specific point of divergence. 5 of the eight produce more than one answer; 1 produces three different answers on its own. Every cell was produced by running the input through this site’s converter — nothing was typed by hand.

Read the fourth line carefully. What Are You Waiting For under AP, Chicago and MLA; What Are You Waiting for under APA. Nothing is broken — that lowercase “for” is what APA’s rule list produces, because the rule that would have rescued it does not exist in APA.

What Chicago’s 18th edition changed

The 18th edition, published in 2024, kept the word-class approach but added a length floor: prepositions of five letters or more now take a capital. Most title case tools still ship the 17th edition behaviour. Run the same eight headlines through both and only 2 change — but they change in the direction most editors will notice first.

If you are working to a house style that has not been rewritten since 2024, tick Use the superseded CMOS 17 preposition rule under the Chicago button and the converter reverts to lowercasing every preposition regardless of length. It is scoped to that one rule, which is why the label names the rule and not the edition.

The four rule sets, stated plainly

What no converter can decide for you

Some of these rules turn on grammar, and grammar needs a parser that spelling alone cannot stand in for. Guessing here would be quietly wrong, so CaseConv applies the conservative reading and tells you which words it was unsure about — the amber “Judgment calls” panel appears under the style-guide buttons whenever the output contains one. Here is the complete list of what it will not decide:

• Is "up" in “Backing Up Your Laptop” a preposition or the adverbial particle of a phrasal verb? All four guides capitalise an adverbial particle. Deciding which one it is needs a parser, so CaseConv applies the preposition reading (lowercase) and flags the word under “Judgment calls” for you to fix.
• Does AP capitalise “to” when it marks an infinitive — “How To Win” or “How to Win”? It does, and this one is not in dispute: AP’s composition-titles entry explicitly capitalises “to” in infinitives — “What I Want To Be When I Grow Up.” Telling an infinitive marker from a preposition still needs a parser, so CaseConv lowercases mid-title “to” under every guide and flags it for you to fix. Chicago and MLA lowercase it either way.
• Does Chicago lowercase “so” and “yet”? Chicago’s rule names exactly five coordinating conjunctions — and, but, for, or, nor — and “so” and “yet” are not among them, so CaseConv capitalises them under Chicago. MLA names all seven FANBOYS, so MLA lowercases them. Secondary summaries of Chicago disagree with each other here; we follow the wording the manual’s publisher and the most consistent secondary reproductions give, and flag both words whichever guide you pick.
• Does AP capitalise the first word of a subtitle after a colon? AP publishes no rule for subtitles inside a composition title, so there is no AP text to follow. APA, Chicago and MLA all capitalise it, and AP’s own published copy does the same, so CaseConv extends that shared behaviour to AP and invents no AP-specific exception.

The clearest case is the last headline in the comparison table. “Backing up your laptop” contains a phrasal verb, and in every one of the four guides the particle of a phrasal verb is capitalised — the correct output is Backing Up Your Laptop. But “up” is also an ordinary preposition, and the two are spelled identically. The converter prints the preposition reading, flags the word, and leaves the last five per cent to you. Any tool that silently picks one is guessing on your behalf.

The six programmer cases

The bottom row of buttons rewrites text as an identifier. All six start the same way: the input is broken into words — on spaces, on punctuation, and on the internal capitals of an existing identifier, so getUserName and get user name both yield the same three words — and then rejoined with a different separator and capitalisation.

Folding ID to Id is deliberate and it follows the Google Java Style Guide, which spells the procedure out: divide the phrase into words, “lowercase everything (including acronyms), then uppercase only the first character” of each word. The guide’s own examples are XmlHttpRequest and newCustomerId; the capital runs of XMLHTTPRequest and newCustomerID are exactly what it drops. The payoff is that the boundary between words never disappears into a run of capitals.

One known limit follows from the same rule: a word that legitimately contains an internal capital — IPv6, iOS — is split at that capital, because nothing distinguishes it from a real word boundary without a dictionary. Type them lowercase (ipv6) and they survive intact.

Which convention belongs where

Picking the right case is not taste; in most ecosystems it is written down, and linters enforce it. The last column says how much weight each row carries: spec means a first-party specification, style guide or documentation page states the rule, and convention means near-universal practice that no first-party document actually mandates. 3 of the 17 rows are convention, and the last column says so on every one of them.

Where case conversion changes the character count

Uppercasing is usually a one-for-one swap, so the character count holds steady. Usually. A handful of lowercase characters have no single-character capital, and uppercasing them makes the string longer — which matters the moment you are working to a field limit, a meta description budget or a database column width.

The German sharp s is the one you will actually hit. Uppercasing ß produces SS: two characters where there was one. Here is the measurement, taken by running the string through this page’s own counter: