Sentence case is how ordinary writing looks: one capital letter at the start of each sentence, lowercase for everything else apart from names. It is the form you need when someone sends you a paragraph typed entirely in capitals, or a block of lowercase text with no capitals at all, and you want it to look like writing again.
Paste the text above. The tool lowercases everything and then puts a capital back after each full stop, question mark, exclamation mark or line break, and at the very start.
What it gets right
“tHIS IS A SENTENCE. and here is another one! is it right? yes.” comes out as “This is a sentence. And here is another one! Is it right? Yes.” Mixed up capitals from a fumbled caps lock, all-caps customer emails, headings copied from a document in small caps, and subtitles pasted from a transcript are all quick to repair this way.
Each new line also starts with a capital. That suits lists and subtitle files, though it means a sentence deliberately broken across two lines will get a capital on the second half, so check for that.
What it cannot know
The tool does not understand names, so you will have to restore these by hand:
The word “I”, and contractions such as “I’m” and “I’ll”. People’s names and place names. Months and days of the week. Brand names, acronyms like NASA, and initialisms like USB. The first word after a colon, if your style wants it capitalised. Abbreviations that end with a full stop: after “e.g. the”, the tool sees a sentence ending and capitalises “The”.
For a long text, it is quickest to convert first, then search for the usual suspects. A find for “ i “ is a good start, and the find and replace tool makes short work of fixing “i” to “I” in one go (turn on the match-case box).
When another tool is better
If the result should be a title or heading, use title case. If you only want everything small, the lowercase converter does that and nothing more. After the case is fixed, text that has come out of a PDF often needs extra spaces removed and line breaks cleaned up, and our guide to cleaning text copied from a PDF walks through the usual order of steps.
The rule of thumb for web headings is that sentence case reads as friendlier and is easier to get right; many editorial style guides have moved to it for exactly that reason.