What “sentence case” actually means
Sentence case is the plainest of the capitalisation conventions: capitalise the first word of a sentence, then carry on in lowercase, letting only words that deserve a capital in their own right keep one. It is how the paragraph you are reading is written, and there is no style guide fight about it, because there is almost nothing to disagree over.
That absence of argument is why it has been quietly winning. Product interfaces switched to sentence case for buttons and headings years ago, and academic publishers have moved the same way for article titles and headings — a title in sentence case is easier to read, easier to search for, and impossible to get subtly wrong the way a title case heading can be under four competing manuals.
The eight inputs that separate a good implementation from a bad one
Sentence case looks trivial until you write it. The hard part is not the capital — it is knowing where a sentence ends, and knowing which words to leave alone. Every “after” column below is real output from the tool on this page, generated when the page was built.
| Input | Output | What it shows |
|---|---|---|
| the meeting is on FRIDAY. bring the NDA. | The meeting is on FRIDAY. Bring the NDA. | Sentence boundaries at a full stop, and acronyms left as typed. |
| WE NEED THIS BY NOON — NO EXCEPTIONS. | We need this by noon — no exceptions. | A line with no lowercase at all is folded down first, then re-capitalised. |
| i told them i'd call. i didn't. | I told them I'd call. I didn't. | The pronoun “I” and its contractions are re-capitalised anywhere in the sentence. |
| what now? nothing. really? ok. | What now? Nothing. Really? Ok. | Question marks and exclamation marks end a sentence as reliably as a full stop. |
| the API returned 404; the CDN cache was stale. | The API returned 404; the CDN cache was stale. | A semicolon does not start a new sentence; the acronyms survive. |
| ready... set... go! | Ready... Set... Go! | An ellipsis is treated as sentence-ending punctuation. |
| dr. patel will chair it. | Dr. Patel will chair it. | A known limit: an abbreviation’s full stop looks exactly like a sentence end. |
| the iPhone shipped. eBay listed it. | The iPhone shipped. eBay listed it. | Deliberate internal capitals survive; nothing normalises them away. |
Sentence ends
A new sentence starts after a full stop, a question mark, an exclamation mark or an ellipsis — and at the start of every line. A semicolon does not start a new sentence, which is correct: a semicolon joins two clauses instead of separating two sentences, and capitalising after one is a real copy-editing error — style has no say in it.
The pronoun “I”
English has exactly one word that takes a capital wherever it appears, and it is one letter long. The converter re-capitalises a standalone i and its contractions — I'm, I've, I'd, I'll — with either kind of apostrophe, straight or curly. This matters most when rescuing text from a source that lost its capitals: the “i” is the mistake a reader spots first.
Words that were already deliberately capitalised
A word carrying a capital anywhere after its first letter was cased on purpose: NDA, API, iPhone, eBay, McDonald. Those are left exactly as typed. It is a heuristic rather than knowledge, and it is deliberately one-directional — the converter will never destroy a capital you put there, and will never invent one you did not. If you want everything reconsidered from scratch, untick Keep NASA, iPhone as typed.
A line in full capitals
A line containing no lowercase letters at all is shouting, not styling. If the “already capitalised” rule applied there, a full-caps paragraph would survive sentence case completely untouched — the exact opposite of what someone pasting one wants. So a line with no lowercase is folded down first, then re-capitalised sentence by sentence. That single rule is why this page fixes an all-caps email in one click.
The limitation, stated plainly
There is one thing this cannot get right, and no sentence-case tool can: an abbreviation’s full stop is indistinguishable from a sentence-ending one. “dr. patel” becomes “Dr. Patel”, which happens to be correct. “approx. ten metres” becomes “Approx. Ten metres”, which is not. The text simply does not contain the information needed to tell the two apart.
The practical answer is to convert first and then scan for abbreviations — there are usually few, they cluster in technical writing, and they are quick to spot once you know to look. Everything else the converter does is deterministic, so the only thing you ever need to re-read is the word after a full stop that was not really a full stop.
A three-step recipe for wrecked text
The most common job on this page is a paragraph that arrived in the wrong case entirely — a form submission in caps, a transcript with no capitals at all, a paste out of a spreadsheet. The reliable sequence is:
- Lowercase everything if the text is a mix of shouting and normal, so you are starting from a known state.
- Apply sentence case to put one capital back per sentence, along with the pronoun “I”.
- Re-capitalise the proper nouns by hand. No tool can recover a name that was folded down, and there are usually fewer of them than you expect.
Because the tool converts the textarea in place, step two runs on the result of step one without any copying between boxes — and Undo steps back through the whole sequence one conversion at a time if you take a wrong turn.