What is the Sentence Checker?
This sentence checker looks at the shape of every sentence in your text — not spelling, not word choice — and flags the ones that are too long, too short, or strung together with too many commas and conjunctions to follow easily. Paste in a paragraph and it splits the text into sentences, counts the words in each one, and applies four structural rules: sentences over 30 words are flagged as hard to follow, sentences of two words or fewer (without an exclamation or question mark) are flagged as possible fragments, sentences chaining three or more conjunctions get flagged as run-ons, and sentences with five or more commas are flagged as likely comma splices. It also catches missing capital letters at the start of a sentence.
Long, tangled sentences are one of the most common reasons a piece of writing feels exhausting to read even when every word in it is spelled correctly and the grammar is technically fine. A 45-word sentence stitched together with "and… and… but… so…" is legal English — it just asks a lot of a reader's working memory. This tool exists to point at exactly those sentences so you can split them, rather than leaving you to spot the problem by ear.
Why use a free sentence checker?
Finds run-ons and comma splices specifically
Sentences chaining three or more conjunctions, or carrying five or more commas, are flagged as likely run-ons — the pattern behind most "this sentence is exhausting" feedback.
Catches accidental fragments
A sentence of one or two words that is not an exclamation or a deliberate stylistic beat ("Fine.") gets flagged, so you can check it was not a broken edit.
Flags sentences over 30 words
Long sentences are not automatically wrong, but past 30 words they are hard for most readers to hold in their head — this is the length where a split is usually worth considering.
Word count per sentence, shown in context
Every finding shows the sentence in context with its exact word count, so you can judge for yourself whether it needs splitting.
Nothing uploaded
Checking runs entirely in your browser — your draft is never sent to a server.
How do you use the Sentence Checker?
- 1
Paste your text
Sentence structure checks work on anything from a single paragraph to a full article. The tool splits it into sentences automatically.
- 2
Review the flagged sentences
Each finding shows the sentence in context and explains which rule it tripped — length, comma count, conjunction count, or a missing capital letter.
- 3
Fix or split manually
Capitalisation issues have a one-click fix. Long or run-on sentences need a manual split — the tool tells you what is wrong, not how to rewrite it, since that is a judgment call about meaning.
- 4
Re-check after editing
Split a long sentence and the checker will re-scan it as two shorter ones, confirming the fix actually resolved the flag.
What does it look like in practice?
A run-on chaining conjunctions
Input
The project kicked off in March and the team was excited and we set an ambitious timeline and everyone agreed to it even though the scope was never actually written down.
Result
Flagged: 4 conjunctions chained in one sentence — consider splitting into two or three sentences.
Four uses of "and" in a row is the pattern this rule targets. Splitting at the natural breaks ("...in March. The team was excited...") usually reads better without losing any information.
A likely comma splice
Input
The scope, the budget, the staffing, and the deadline were never actually written down anywhere official, which is a problem now.
Result
Not flagged — 4 commas, under the 5-comma threshold.
This shows the threshold matters: four commas across a list and a clause reads fine, so it is correctly left alone rather than over-flagged.
A likely accidental fragment
Input
Fine. it slipped.
Result
Flagged: "it slipped" is 2 words — check it isn't an accidental fragment. Also flagged: missing capital letter on "it".
"Fine." is a deliberate one-word sentence and is left alone. "it slipped" is flagged both for its short length and for starting with a lowercase letter — together, a strong sign of an unintentional fragment rather than a stylistic choice.
How accurate is it?
Every flag is a direct count against your text — words per sentence, commas per sentence, conjunctions per sentence — measured against fixed thresholds (30 words, 5 commas, 3 conjunctions). There is no scoring model behind it; the same sentence always triggers the same flags.
- Sentence splitting is done on ".", "!" and "?" boundaries, so a stray missing space after a period can occasionally merge or split a sentence incorrectly — check any oddly-shaped result against the original.
- The 30-word, 5-comma and 3-conjunction thresholds are reasonable editorial rules of thumb, not scientifically validated cutoffs — a well-constructed 32-word sentence can still read cleanly, and this tool will still flag it for you to judge.
- This checker does not evaluate grammar or meaning — it only measures shape. A short sentence can still be grammatically wrong, and a long sentence can still be perfectly clear.
What should you know before using it?
What this tool is, where it runs and what it costs — the facts people usually check before trusting a free tool with their work.
| Detail | Sentence Checker |
|---|---|
| Category | Writing ToolsOne of 12 tools in this category. |
| Where it runs | Entirely in your browserYour input is never transmitted. Verify it yourself in the Network tab of your browser DevTools. |
| Price | FreeNo account, no trial, no usage cap, no paid tier. |
| Works offline | Yes, once the page has loaded |
| Also known as | free sentence checker, sentence checker online, online sentence checkerOther names people search for this by. |
Which tools relate to the sentence checker?
The sentence checker is one of 12 writing tools on this site. These are the ones most often used alongside it — either because they handle the next step in the same job, or because they answer a question this tool raises.
Readability Score Checker
Sentence length is a direct input to Flesch, Fog and SMOG — the sentences flagged here are usually the same ones dragging down a readability score.
Spell Checker
Covers individual word spelling rather than sentence shape — run it alongside this tool for typos this checker does not look at.
Online Proofreader
Runs this same structural check together with spelling, punctuation and commonly-confused-word checks in a single combined pass.
Paragraph Rewriter
Once a sentence is flagged as a run-on, this tool offers a mechanical clause-reordering rewrite you can accept or reject, rather than leaving the fix entirely manual.
When should you use a sentence checker?
Because the sentence checker runs entirely in your browser, it suits work you would not want to hand to a third-party server — client files, unpublished drafts, anything under an NDA. These are the situations people reach for it in most often.
Editing long-form articles
Run a finished draft through before publishing to catch the sentences that grew too long across multiple editing passes — a common way run-ons sneak in.
Academic and report writing
Dense subordinate-clause sentences are common in formal writing; this checker flags the specific ones worth splitting without asking you to rewrite the whole document.
Email and business writing
Short, choppy fragments and overlong run-on sentences both undermine clarity in a business email — this checker flags both extremes in the same pass.
Editing dictated or transcribed text
Speech-to-text drafts often run several spoken clauses together with "and" — a quick structural check finds the natural sentence breaks that transcription missed.
Frequently asked questions
What does a sentence checker actually check?
How long is too long for a sentence?
What counts as a run-on sentence here?
Will it flag a short sentence I used on purpose for effect?
Does this tool fix sentences automatically?
Is this the same as a grammar checker?
Is my text sent to a server when I use this tool?
Is the sentence checker safe to use?
This tool runs entirely inside your browser. Your text, files and settings are processed on your own device and are never uploaded to our servers — there is nothing for us to store, log or leak. You can confirm it yourself: open your browser DevTools, switch to the Network tab, and use the tool. You will not see an upload request. It also means the tool keeps working if your connection drops mid-task.
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