The seven styles
UPPERCASE turns every letter into a capital. lowercase does the reverse. That second one is the fastest fix for text that arrived shouting.
Title Case capitalises the first letter of each word and lowercases everything after it.
Capitalize Each Word looks almost identical but leaves the rest of each word alone. That difference shows up the second your text contains a brand name, because Title Case will happily turn iPhone intoIphone while Capitalize Each Word keeps it intact.
Sentence case capitalises one letter per sentence, the way normal prose works. Then there are two joke styles. aLtErNaTiNg flips case letter by letter, and iNVERSE swaps whatever case each letter already had. Both exist for social posts and neither has a serious use.
Title Case is not one rule
There is no single correct title case.
Two major style guides disagree about it, and every automatic converter has to quietly pick a side.
AP style runs on length. Words of 4 letters or more get a capital. Shorter ones do not. That gives you "A Guide to Writing Well", with "to" left small at 2 letters.
Chicago style ignores length. It caps the important words and leaves the joining words small, however long they run. Words like "a", "the", "and", "but" and "between". So "between" stays small in Chicago, while AP caps it for running to 7 letters.
They agree on one point. The first and last word of a title are always capitalised, whatever they happen to be.
This converter uses the simple rule: capitalise the first letter of every word. Writing to a house style? Treat the output as a first pass and fix the small words by hand. No converter can read your style guide.
The bug that broke a compiler
Changing letter case looks like the most trivial operation in computing. There is a famous counterexample.
Turkish has four i's, not two. Dotted i at U+0069 and İ at U+0130. Dotless ı at U+0131 and I at U+0049. Four separate letters, four separate meanings. Not styling choices.
So on a machine set to Turkish, uppercasing i correctly gives you İ, not I. Code that assumed otherwise fails in ways that read as impossible. Compare "i".toUpperCase() to "I"and you get false. Only in Turkey.
Not hypothetical. It hit the Kotlin compiler itself. On Turkish machines "IntArray".decapitalize() came back as "ıntArray", dotless i and all. The fix meant combing the whole compiler for case conversions. Kotlin 1.5 shipped safe replacements. By Kotlin 2.1 the old ones threw a compile error.
One rule falls out of this. Never convert case without naming a locale. Working on an identifier or a filename rather than human prose? Ask for the invariant locale by name. In Java that is Locale.ROOT, and in JavaScripttoLocaleUpperCase takes an explicit tag for exactly this reason.
Where each style earns its keep
Sentence case took over app design about a decade ago. Google, Apple and Microsoft all switched their buttons and menus to it. The argument was that it reads faster and shouts less than Title Case On Every Control.
Title Case still owns headlines, article titles, and citations.
UPPERCASE works in short bursts. A label. A warning. Four or five words at most.
It stops working the moment it runs to a full sentence, and there is a reason. Look at the word "typing" in lower case. The t and the i poke up, the y and the p hang down. Your eye uses that ragged outline to spot the whole word at a glance. Capitals flatten every word into the same rectangle, so you are back to reading letter by letter.
lowercase is mostly a repair job. Caps lock was on, or a headline got pasted into a paragraph. Flatten it, then apply sentence case.
Frequently asked questions
What is the difference between Title Case and Capitalize Each Word?
Title Case lowercases everything after the first letter of each word. Capitalize Each Word leaves it untouched. Pick the second if your text contains iPhone, eBay, or an acronym you need intact.
Can I paste a long document?
Yes. No length cap, and it converts in one click.
Why did my brand name get mangled?
Title Case rewrote the letters after the first one. Switch to Capitalize Each Word.
Does this do camelCase and snake_case?
Not here. Those are programming identifier styles and they live on the case style converter.
Which title case rule does this use?
The simple one, first letter of every word. AP and Chicago disagree about small words, so adjust those by hand if you are writing to a style guide.
Sentence case or title case for buttons?
Sentence case, by current convention. Every major platform design system moved that way, and it reads faster.
Related: camelCase and snake_case · count words and characters · clean up messy text · find and replace