Using the converter
Paste on the left and pick a Case. Sentence case lowers everything first, then capitalises the first letter after each full stop, question mark, exclamation mark and line break.
The other five are Title Case, Capitalise Each Word, UPPERCASE, lowercase and aLtErNaTiNg. Swap is switched off for all of them, because lowering a capital throws away the fact that it was one.
Sentence case and title case are different jobs
Sentence case capitalises the first word of a sentence and nothing else. Title case capitalises most words in a heading. Picking the wrong one on a heading is the commonest reason a page looks slightly off without anyone being able to say why.
Neither can be done perfectly by machine. Sentence case can't know that "paris" is a proper noun, so it won't restore the capital on a name that arrived lowercase. The one exception here is a lone "i", which is always a capital in English and is always restored.
Run the result past your own eyes. Proper nouns and acronyms are exactly what an automatic pass gets wrong, and all-caps text is the worst case, because the capitals that should have stayed were destroyed before you pasted it.
Nobody agrees what title case is
There's no single rule, and the tools disagree because the style guides disagree. Chicago capitalises every word except articles, coordinating conjunctions and prepositions, whatever their length, so "The Lord of the Rings" keeps a lowercase "of".
AP capitalises prepositions of four letters or more, so AP writes "Gone With the Wind" where Chicago writes "Gone with the Wind". The New York Times uses five. MLA capitalises no prepositions at all. All four are correct within their own house.
This tool follows the shorter Chicago-style list. A small word still takes a capital when it lands first or last in the line, which every one of those guides agrees on.
Related conversions
Text pasted out of a PDF usually needs line breaks removed before the case is worth fixing, because a break in the middle of a sentence makes the next word look like a new sentence.
For decoration rather than correction, bubble text and upside-down text change the characters instead of their case.
Questions
What is sentence case?
Capitals on the first word of each sentence and on proper nouns, with everything else lowercase. It's the style used for ordinary prose.
How do I fix text that is in all caps?
Paste it above and choose Sentence case. Everything is lowered first, then the start of each sentence is capitalised.
Why didn't it capitalise a name?
Nothing in the text says which words are proper nouns. All-caps input destroys that information before it reaches the tool, so names have to be restored by hand.
What's the difference between title case and capitalising each word?
Title case leaves short words like "of" and "the" lowercase in the middle of a heading. Capitalise Each Word capitalises every word without exception.