📝 Run-on Sentence Checker: Find Long Sentences and Comma Splices

What is the Run-on Sentence Checker?

A run-on sentence joins two or more complete thoughts without the punctuation or the joining word that should hold them apart. The two classic forms are the fused sentence, where clauses simply crash together with no punctuation at all ("I finished the draft I sent it to my editor"), and the comma splice, where a lone comma tries to do the job that a period, a semicolon, or a comma-plus-conjunction should be doing ("I finished the draft, I sent it to my editor"). Length alone does not make a sentence a run-on: a long sentence can be perfectly correct if its clauses are joined properly. This tool therefore looks for the two most common warning signs and asks you to review them, rather than claiming a definitive verdict.

The first rule is length. This checker flags any sentence over 40 words. That number is a practical threshold, not a grammar law. Plain-language guidance from sources such as the US federal plainlanguage.gov project recommends keeping most sentences short, often around 15 to 20 words, because comprehension drops as sentences grow. A 40-word cutoff is deliberately generous: it lets ordinary complex sentences through and only surfaces the genuinely sprawling ones, which are the sentences most likely to hide a run-on or to exhaust a reader. When a sentence trips this rule, the fix is usually to find the natural break, often at an "and" or a comma, and split it into two.

The second rule targets the comma splice, which is far harder to spot by eye than a long sentence. The checker scans for a comma that is followed immediately by one of ten common clause starters (it, he, she, they, we, you, I, this, that, there) and then a verb, with no coordinating conjunction (and, but, or, nor, for, so, yet) sitting between the comma and the new clause. That pattern (", it was", ", they went", ", this means") is a strong signal that a second independent clause has been welded on with only a comma. Because the rule is pattern based, it is transparent and predictable, but it is also narrow on purpose: it will miss splices that start with a noun ("the manager agreed, the deal closed") and it can occasionally flag a correct sentence, so every hit is offered as a prompt to look, not a ruling.

Once two independent clauses have been identified, English gives you four standard repairs, and picking between them is a style choice rather than a right-or-wrong one. You can split the clauses into two sentences with a period; join them with a semicolon when the ideas are closely linked; add a comma plus a coordinating conjunction (the FANBOYS: for, and, nor, but, or, yet, so); or subordinate one clause with a word like because, although, or while so it can no longer stand alone. This tool does not rewrite your text for you, and that is intentional: automatic rewrites often change your meaning, whereas seeing the flagged sentence and choosing the repair yourself keeps your voice intact while still fixing the error.

When to use it

• Proofreading an essay, blog post, or article before publishing to catch sentences that ramble on past the point of clarity.
• Checking student writing for comma splices and fused sentences, a mistake that costs marks in almost every rubric.
• Tightening business emails, reports, and proposals so each sentence carries one clear idea instead of three.
• Improving readability scores by finding and shortening the handful of very long sentences that drag an otherwise clear draft down.
• Reviewing content translated or drafted by AI, which often produces grammatically valid but overly long, comma-heavy sentences.
• Editing a cover letter or personal statement where one tangled sentence can undermine an otherwise strong impression.

How to use the Run-on Sentence Checker

1. Paste or type your text into the input box. A sample paragraph is loaded so you can see how flagging works right away.
2. Press Check sentences, or just keep typing, and the tool re-analyses the text automatically.
3. Read the four stat boxes: sentence count, average words per sentence, longest sentence, and how many sentences were flagged.
4. Scroll the flagged list and open each highlighted sentence: a badge tells you whether it is too long, a possible comma splice, or both.
5. Fix each flagged sentence in your own document (split it, add a conjunction, use a semicolon, or subordinate a clause), then use Copy report to save the findings.

Formula & method

Worked examples

Sentence length and reader comprehension (plain-language guidance)

Four standard ways to repair a run-on or comma splice

Common mistakes to avoid

• Assuming every long sentence is a run-on. Length and grammar are different things. A 45-word sentence can be flawless if its clauses are joined with proper punctuation and conjunctions. This tool flags long sentences as candidates to review, not as confirmed errors, so read each one before you cut it.
• Treating a comma splice fix as adding more commas. Piling on commas does not fix a splice. The repair is to change the join between two independent clauses: use a period, a semicolon, a comma plus a conjunction like but or so, or subordinate one clause. Extra commas inside a clause do not resolve the two-clauses-one-comma problem.
• Expecting the tool to catch every run-on. The comma-splice rule only looks for a comma followed by one of ten pronouns and a verb. It will miss fused sentences with no comma at all, and splices that begin with a noun such as "the team agreed, the plan shipped." Use the flags as a starting point and still proofread by ear.
• Fixing flagged sentences with an automatic rewrite. Letting software rewrite a sentence often shifts the meaning or flattens your voice. Because this checker shows you the problem instead of rewriting it, you stay in control: choose the repair that keeps your intended emphasis and tone.

Glossary

Run-on sentence

Two or more independent clauses joined without the correct punctuation or conjunction, so the sentence keeps going past where it should stop.

Comma splice

A specific run-on where a single comma, with no coordinating conjunction, is used to join two independent clauses, as in "It rained, we stayed inside."

Fused sentence

A run-on with no punctuation at all between two independent clauses, for example "It rained we stayed inside."

Independent clause

A group of words with a subject and a verb that can stand alone as a complete sentence.

Coordinating conjunction

One of the FANBOYS words (for, and, nor, but, or, yet, so) used with a comma to correctly join two independent clauses.

Comma splice pronoun starter

A pronoun such as it, he, she, they, we, you, I, this, that, or there that, when it follows a comma directly, often signals the start of a new independent clause.

Frequently asked questions