Sentence Case Converter
Convert text to sentence case: lower-case everything, then capitalise the first letter of each sentence. Paste a paragraph and read the result instantly.
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How It Works
Sentence case is the ordinary capitalisation of running prose: the first word of a sentence is capitalised, everything else is lower-case except proper nouns. This converter implements the mechanical half of that rule — a lowercase pass followed by a capitalisation pass — and opens the case converter already in sentence mode.
Step one: lower-case everything. The whole string is lower-cased first. That makes a headline typed in capitals readable again, but it also flattens proper nouns and acronyms: NASA becomes Nasa, and iPhone becomes iphone. There is no dictionary of names in the tool, so those fixes are always manual.
Step two: capitalise sentence starts. A regular expression then looks for two positions — the start of the text, and any spot that follows a period, exclamation mark or question mark plus at least one whitespace character — and upper-cases the next lower-case letter it finds there. Three sentences separated by ". ", "! " and "? " therefore each get a capital, and so does the first word of the whole paste.
Why the space matters. The rule requires whitespace after the terminator, which is what separates "hello. world" from "hello.world": the first gains a capital on World, the second does not, because a period with no space after it is usually part of a domain name, a file name or a decimal rather than a sentence break. That single condition is why version strings and URLs survive a conversion looking intact.
Known blind spots. Abbreviations are not recognised, so the word after an abbreviation point gets a capital it should not have: "Dr. smith arrived" becomes "Dr. Smith arrived", and "e.g. this example" becomes "E.g. This example". A terminator followed by a digit is left alone, which is why "3.14" survives a conversion intact. Whitespace-only input is passed through unchanged, and empty input returns an empty string rather than an error.
How to use the converter
- Paste the paragraph or headline into the input box; the conversion happens as you type.
- Check that the mode selector reads Sentence case, since the tool also offers lower, upper, title and camel styles.
- Restore proper nouns and acronyms the lower-case pass flattened.
- Fix any sentence that starts after an abbreviation, where the rule cannot tell a real sentence break from a title.
Input and what the rule does to it
| Input | Output | Why |
|---|---|---|
| hello world. second sentence. | Hello world. Second sentence. | A period plus a space marks a new sentence |
| HELLO WORLD. THIS IS A TEST. | Hello world. This is a test. | Everything is lower-cased before the capitalisation pass |
| hello.world | Hello.world | No whitespace after the period, so it is not a sentence break |
| NASA is here. | Nasa is here. | Acronyms have no exception in the rule |
Examples
Input
hello world. second sentence.Output
Hello world. Second sentence.Each sentence start is capitalised; everything else is lower-cased.
Input
HELLO WORLD. THIS IS A TEST.Output
Hello world. This is a test.Boundary case: a fully upper-case headline is normalised, not just re-capitalised.
Input
hello.worldOutput
Hello.worldBoundary case: no space after the period, so the domain-like fragment is not treated as a new sentence.
Input
what? yes! right.Output
What? Yes! Right.Question marks and exclamation marks start sentences just as periods do.
Frequently Asked Questions
- What counts as a new sentence here?
- A period, exclamation mark or question mark followed by at least one space, or the very start of the paste. A terminator with no space after it does not start a sentence, which is deliberate: it keeps domain names such as example.com and file names such as report.final intact instead of capitalising them mid-word.
- Why are proper nouns and acronyms lower-cased?
- The first pass lower-cases the entire string so that text typed in capitals becomes readable, and the tool has no dictionary of names to protect. NASA becomes Nasa and London becomes london. Restore those few words by hand; for prose with many names, title case or no conversion at all is usually less work.
- Does it capitalise after an abbreviation like Dr.?
- Yes, and that is wrong. The rule cannot distinguish an abbreviation point from a sentence break, so "Dr. smith arrived" gains a capital S in the middle of the sentence. Fix the affected words after converting, or paste sentences separately so the abbreviation never sits before a space.
- Can it convert a whole document?
- It converts everything you paste, in one pass, and returns the full result. There is no partial selection, so for a long document paste it in sections and check each one. Conversion runs in your browser, so nothing is uploaded and the tool keeps working offline.
Limitations & Accuracy
Every letter is lower-cased before the capitalisation pass, so proper nouns and acronyms lose their capitals and must be fixed by hand. A sentence break needs whitespace after the terminator, which keeps domain names safe but also means a line ending in a period with no following space will not start a new sentence. Abbreviations are not recognised, and characters without case are left unchanged.