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Sentence Case Fixer

Fix inconsistent capitalization by converting text to sentence case with acronym and pronoun controls.

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About this tool

Sentence case — capitalizing only the first letter of each sentence and proper nouns, leaving everything else lowercase — sounds simple, but applying it consistently to pasted text is surprisingly difficult. Title-cased headings and randomly mixed-case content need sentence case correction, and doing it manually in a large document is error-prone. Sentence Case Fixer lowercases your text, then capitalizes the first letter after each period, exclamation mark or question mark followed by a space, and after every line break. Two options are on by default: Capitalize pronoun I turns a standalone 'i' into 'I', and Preserve acronyms keeps any word written in all caps in your input (two or more letters or digits, such as API, HTML or NASA) in all caps everywhere it appears. Because every all-caps word counts as an acronym, untick Preserve acronyms when converting text that is entirely in capitals.

Why use it

  • Sentence Boundary Detection: Capitalization resets after every period, exclamation mark and question mark followed by a space, and after each line break.

  • Acronym Preservation: Words typed in all caps, like 'API', stay in all caps instead of becoming 'api'.

  • Pronoun 'I' Capitalized: A standalone lowercase 'i' becomes 'I' wherever it appears in a sentence.

  • Handles Mixed Case Input: Randomly capitalized or title-cased text is lowercased and re-capitalized at each sentence start.

  • Quote-Aware Starts: An opening quote or bracket before the first word is skipped, so the word inside still gets its capital.

How to use

  1. Paste your text into the Input text box.
  2. Keep 'Preserve acronyms (NASA, API)' ticked to keep all-caps words such as API in capitals; untick it when the whole input is ALL CAPS.
  3. Keep 'Capitalize pronoun I' ticked to turn a standalone 'i' into 'I'.
  4. Click Fix sentence case to fill the Output box.
  5. Click Copy to copy the corrected text, or Clear to start over.

When it helps

  • When converting a block of ALL CAPS text (e.g., from an old document or legal file) to properly cased prose.
  • When cleaning up text pasted from a title-case source that should be in sentence case for body copy.
  • When correcting mixed-case input submitted by users in a form field before displaying it.
  • When normalizing the capitalization of product descriptions, review text, or user comments before publishing.
  • When post-processing text extracted from PDFs or OCR output that has inconsistent capitalization.
  • When preparing text for a style guide that mandates sentence case for all UI labels and headings.

Examples

ALL CAPS to sentence case (Preserve acronyms unticked)

Input

THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG. THE END.

Expected result

The quick brown fox jumps over the lazy dog. The end.

With acronym preservation

Input

we use the API and HTML for web pages. i think NASA likes it too.

Expected result

We use the API and HTML for web pages. I think NASA likes it too.

Mixed-case cleanup

Input

this Is A Randomly Capitalized Sentence. and so Is This!

Expected result

This is a randomly capitalized sentence. And so is this!

Tips

  • Untick Preserve acronyms before converting ALL CAPS text; otherwise every word is kept in capitals.
  • After applying sentence case, run the output through Text Diff against your original to verify only capitalization changed and no words were modified.
  • Proper nouns such as people's and place names are lowercased mid-sentence, so re-capitalize them in the output.
  • Abbreviations that end in a period, like 'Dr.' or 'e.g.', make the next word start with a capital; review those spots.
  • For UI label normalization, sentence case is increasingly preferred over Title Case in modern design systems — use this tool to batch-convert labels.

Frequently Asked Questions

What is sentence case?
Sentence case capitalizes only the first letter of the first word in each sentence (and proper nouns). All other words are lowercase. This is the standard capitalization for most body text in English, contrasted with Title Case which capitalizes each major word.
How does the tool detect sentence boundaries?
Sentence boundaries are detected at terminal punctuation: periods, exclamation marks, and question marks. The first letter of the word after each boundary is capitalized. Abbreviations ending in periods (like 'Dr.') may cause occasional over-capitalization.
What counts as an acronym for preservation?
Any word in your input written entirely in capitals with at least two characters (letters or digits, optionally joined by - or /), such as API, HTML or COVID-19. Every occurrence of that word is then restored to capitals, in any case, so if 'THE' appears in caps once, every 'the' stays 'THE'.
Why does ALL CAPS text stay in capitals?
With Preserve acronyms ticked, every all-caps word of two or more letters is treated as an acronym, so an ALL CAPS input comes back unchanged. Untick Preserve acronyms and click Fix sentence case again to convert it.
Does it handle the pronoun 'I' correctly?
With Capitalize pronoun I ticked, every standalone 'i' becomes 'I'. That includes the 'i' in abbreviations like 'i.e.', which becomes 'I.e.', so check those by hand.
Can I add my own proper nouns to preserve?
No. There is no exceptions list. Names and brands typed with a single capital (like 'London') are lowercased unless they start a sentence; only all-caps words are kept when Preserve acronyms is on.
Does it work on non-English text?
The sentence boundary detection and capitalization logic is designed for English. It may work reasonably on other Latin-script languages but will not respect language-specific capitalization rules for proper nouns or German nouns.
What happens with a question that ends with a question mark followed by a quoted answer?
The tool capitalizes the first letter after the question mark. In dialogue, this may produce incorrect capitalization for continuation sentences. Manual correction after auto-fixing is recommended for complex dialogue punctuation.

Explore the category

Case Converter Convert text between letter cases HTML to Text Converter Remove HTML tags and extract clean plain text with optional formatting controls. Diacritic & Accent Remover Strip accents, umlauts, and diacritical marks from text, converting characters like é, ü, ñ to plain ASCII. Title Case Converter Convert headlines to AP, APA, Chicago, MLA, NYT, or Wikipedia style Speed Read Flashcard (RSVP) Paste any text and read it word-by-word at a fixed WPM. RSVP-style speed reader with adjustable speed, pivot highlighting, and keyboard controls. Text Diff Compare two texts line-by-line

Glossary

Sentence case
A capitalization style where only the first letter of the first word in a sentence and proper nouns are capitalized. All other letters are lowercase.
Title case
A capitalization style where the first letter of each major word is capitalized. Common in headings, book titles, and article titles.
Acronym
An abbreviation formed from the initial letters of a phrase, typically written in all capitals: API, HTML, NASA, USA.
Proper noun
The name of a specific person, place, organization, or thing, which is always capitalized in English regardless of sentence position.
Terminal punctuation
Punctuation marks that end a sentence: period (.), exclamation mark (!), and question mark (?).
First-person pronoun
The pronoun 'I' used to refer to oneself as the speaker or writer. Always capitalized in English regardless of position.
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