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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. ALL CAPS text, title-cased headings, and randomly mixed-case content all need sentence case correction, and doing it manually in a large document is error-prone. Sentence Case Fixer intelligently converts text to sentence case while respecting acronyms (preserving 'API', 'HTML', 'NASA'), the pronoun 'I', and optionally a user-defined list of proper nouns that should stay capitalized. It processes every sentence independently, detecting boundaries at periods, exclamation marks, and question marks, so each sentence gets its own initial capital while the rest of the sentence is lowercased.

Why use it

  • Sentence Boundary Detection: Each sentence is independently processed — capitalization resets at every period, exclamation mark, and question mark.

  • Acronym Preservation: ALL-CAPS sequences that look like acronyms are preserved intact so 'API' does not become 'api'.

  • Pronoun 'I' Always Capitalized: The first-person singular pronoun 'I' is always kept uppercase regardless of its position in the sentence.

  • Custom Proper Noun Exceptions: Add brand names, people's names, and technical terms to an exception list so their capitalization is preserved.

  • Handles ALL CAPS and Mixed Case Input: Works correctly whether your input is all uppercase, all lowercase, or a random mixture.

  • Instant Correction: Output updates as you type — no submit button, immediate feedback on every change.

How to use

  1. Paste your text into the input area.
  2. The sentence case output appears instantly with each sentence's first letter capitalized.
  3. Toggle 'Preserve acronyms' to keep sequences like 'API', 'NASA', and 'HTML' in ALL CAPS.
  4. Toggle 'Capitalize I' to ensure the first-person pronoun 'I' is always capitalized.
  5. Add custom proper nouns (comma-separated) to the exceptions list to preserve their capitalization.
  6. Click Copy to copy the corrected text to your clipboard.

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

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 API AND HTML FOR WEB DEVELOPMENT.

Expected result

We use API and HTML for web development.

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

  • Build your custom exceptions list with product names, brand names, and technical terms before pasting large documents to minimize manual corrections.
  • After applying sentence case, run the output through Text Diff against your original to verify only capitalization changed and no words were modified.
  • For legal or formal documents, review acronym preservation carefully — legal abbreviations like 'LLC', 'GDPR', and 'IP' should be in the exceptions list.
  • The pronoun 'I' is always capitalized, but 'i' in chemical formulas or programming variable names may be incorrectly uppercased — add context-specific exceptions as needed.
  • 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?
An acronym is detected as two or more consecutive uppercase letters. Sequences like 'API', 'HTML', 'NASA', 'USA' are preserved. A single uppercase letter (like the start of a proper noun) is not treated as an acronym.
Does it handle the pronoun 'I' correctly?
Yes. The first-person singular pronoun 'I' is always capitalized regardless of where it appears in a sentence. This is a fundamental English grammar rule applied independently of sentence case logic.
Can I add my own proper nouns to preserve?
Yes. The custom exceptions field accepts comma-separated words or phrases. Any token matching an exception (case-insensitively) will have its capitalization preserved as entered in the exceptions list.
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.
Will it lowercase words that are intentionally capitalized mid-sentence?
Yes — any word not identified as an acronym, the pronoun 'I', or a custom exception will be lowercased. Add important proper nouns to the exceptions list to prevent them from being lowercased.

Explore the category

Case Converter Convert text between letter cases Strip HTML Tags 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.