Run-On Sentence Checker: Find and Fix Comma Splices

How to use the run-on sentence checker

Paste or type your text in the box (up to 20,000 characters). The run-on sentence checker splits it into sentences and gives each one a verdict as you type. Nothing is uploaded: the checking happens in your browser, so essays, emails and private drafts stay on your device.

1. Read the summary. The four boxes show the number of sentences, how many run-ons were found, your longest sentence and your average sentence length in words.
2. Go through the sentences. Each sentence is marked Comma splice, Possible comma splice, Possible fused sentence, Style note or OK. In a flagged sentence, the words around the join are highlighted, and a note explains why it was flagged: which words form the first clause, and which subject and verb start the second.
3. Pick a fix. The buttons under a finding rewrite that sentence in place: split it into two sentences, use a semicolon, add a comma with and, but or so, or use a semicolon with however or therefore. Choose the one that matches your meaning. Undo takes back the last change.
4. Copy the corrected text. Copy text puts the whole text, with your fixes, on your clipboard.

One setting changes what counts as a long sentence: 30, 40, 50 words (40 is the default). Tap Try a sample to see a short text with correct sentences and one of each kind of problem. In the sample, the checker finds 3 run-ons in 8 sentences and adds 2 style notes.

Please read the results with one thing in mind: this is a rule-based tool, not an AI. It checks word patterns, such as a comma followed by a subject and a verb. It does not understand meaning. It is built to stay quiet when it is unsure, so it will miss some run-ons, and it can be wrong about an unusual sentence. That is why every finding shows its reason, and why the less certain ones are labeled “possible”. Every line break ends a sentence, so if your text was copied from a PDF and has line breaks in the middle of sentences, join the lines first with remove line breaks.

What is a run-on sentence?

A run-on sentence is a sentence in which two or more independent clauses are joined without the right punctuation or linking word. An independent clause has a subject and a verb and can stand alone as a sentence: It was late is one, and we went home is another. Put them together with nothing in between, or with only a comma, and you have a run-on.

Run-ons make readers work harder. Without a clear break, the reader has to figure out where one thought stops and the next one starts, and may have to read the sentence twice.

How long is a run-on sentence?

Any length. This surprises many people, because “run-on” sounds like “a sentence that runs on and on”. But a run-on is a joining mistake, not a length. It was late we went home. has only 6 words and is a run-on. A sentence of 41 words can be perfectly correct if its clauses are joined properly. So there is no number of words that makes a sentence a run-on, and counting words will not tell you whether you have one.

The two types of run-on sentences

There are two kinds, and the only difference is what sits between the clauses. Some teachers use “run-on” for the fused sentence only and treat the comma splice as a separate error; many handbooks use “run-on” for both. The fixes are the same either way.

Comma splice

A comma splice joins two independent clauses with only a comma. A comma is too weak for that job. Examples:

• The alarm went off, I slept through it.
• My phone died, I could not call you.
• We wanted to hike, the trail was closed.

In each one, the words before the comma could be a sentence, and so could the words after it. A second kind of comma splice uses a comma with a word like however, therefore or then. These words feel like conjunctions, but they are adverbs (more on that below).

Fused sentence

A fused sentence joins two independent clauses with nothing at all: no punctuation and no linking word. Examples:

• I was hungry I made a sandwich.
• She called me I was asleep.
• It was raining we stayed inside.

Fused sentences are harder for a program to find than comma splices, because there is no comma to mark the spot. In This is the house we bought, a subject and verb also follow a noun directly, and that sentence is correct. The tool therefore words these findings as “Possible fused sentence” and flags only clear patterns, such as an adjective or an object pronoun followed by a new subject and verb (I was tired I went to bed).

How to fix a run-on sentence: four ways

Every run-on can be fixed in four standard ways. Here is one comma splice, The store was closed, we went home., fixed with each of them:

The first three are the buttons in the tool. A close relative of the semicolon fix adds a linking adverb: The store was closed; therefore, we went home. The fourth fix needs a word that fits the meaning (because, although, when), so the tool leaves it to you. It is often the best choice, because it tells the reader how the two ideas are related. When the dependent clause comes first, put a comma after it.

Which fix should you choose?

• A period when the two ideas are separate, or when the sentence is already long.
• A semicolon when the ideas are closely linked and about equal in weight. See our guide to semicolons.
• A comma and a conjunction when you want to name the link: and adds, but contrasts, so gives a result.
• A subordinating word when one idea explains the other or depends on it.

Do not fix every run-on with a period. A page of very short sentences reads as choppy, and it hides the connections between your ideas.

Run-on sentence vs. long sentence vs. sentence fragment

These three are often mixed up. A run-on has too much in one sentence, a fragment has too little, and a long sentence may be perfectly fine.

The tool keeps these apart. Run-ons are findings. Length is only a style note: a sentence over your word limit is marked “Long sentence”, and a sentence where 3 or more clauses are added on with and, but, so or then is marked “Rambling sentence”. Both are grammatical. They are worth a second look only because long chains tire the reader. For the long example in the table, the checker says: Long sentence. This tool does not look for fragments. To see the length of every sentence at a glance, use the sentence counter.

Coordinating conjunctions: the FANBOYS

English has seven coordinating conjunctions, and the word FANBOYS helps you remember them: for, and, nor, but, or, yet, so. These are the only words that can join two independent clauses with just a comma. The comma goes before the conjunction.

When the two clauses are very short, many writers leave the comma out (I called and she answered), and that is not a run-on, because the conjunction is doing the joining. Read more in our guide to commas with FANBOYS and in the guide to the comma before or after “so”.

Conjunctive adverbs: why “however” cannot join sentences with a comma

Words such as however, therefore, moreover, nevertheless, consequently, otherwise and then are conjunctive adverbs. They show how two ideas relate, but grammatically they are adverbs, not conjunctions. One simple test shows the difference: an adverb can move around inside its clause, and a conjunction cannot. You can write My laptop battery, however, was at two percent. You cannot move but that way.

Because however is not a conjunction, a comma in front of it cannot hold two clauses together. This sentence is a comma splice: I wanted to finish my essay, however, my laptop battery was at two percent. The checker’s verdict: Comma splice. Use a semicolon or a period before the adverb, and (for most of these words) a comma after it: I wanted to finish my essay; however, my laptop battery was at two percent.

When however sits in the middle of a single clause, commas on both sides are correct, and the tool does not flag it (for My laptop battery, however, was at two percent. it says: OK). A note on then: a comma before then between two full clauses is common in casual writing, but many handbooks treat it as a comma splice, so in formal writing use and then or a semicolon. Our guides cover the comma before or after “however” and the comma before “then” in detail.

Run-on sentence examples and corrections

Here are 18 run-on sentences, each with one good correction. The third column shows what this checker says about the sentence. It is a pattern checker, so it does not catch everything: it misses 2 of these 18 (marked “Not flagged”). Commands, and second clauses that start with a noun instead of a pronoun, are its weak spots.

Most of these have more than one good fix. The soup is too salty, it needs more water works with a semicolon, with a period, or as The soup is too salty, so it needs more water. Pick the version that says what you mean.

Sentences that look like run-ons but are correct

A comma followed by a subject and a verb is not always a comma splice. The checker is built to leave these patterns alone:

If you are unsure about a comma that has nothing to do with run-ons, the punctuation checker looks at commas, quotation marks, spacing and more.

When is a comma splice acceptable?

In formal writing (school essays, reports, business letters), fix every comma splice. In other kinds of writing, editors accept a few on purpose:

• Short, matching clauses. A run of brief clauses with the same shape can be joined by commas for rhythm: We ran, we laughed, we fell asleep.
• A “not this, that” contrast. It was not a request, it was an order.
• Fiction and dialogue. Novelists use comma splices to show hurried thoughts or the way people really talk.

The tool does not hide these, because it cannot know whether you wrote them on purpose. It labels them as possible and adds a note (for the two examples above it says: Possible comma splice; Possible comma splice). If the splice is a deliberate choice, leave it.

How to identify a run-on sentence in your own writing

For students

• Find the subject and verb pairs. Underline each subject with its verb. If a sentence has two pairs, check what joins them: a FANBOYS word with a comma, a semicolon, or a word like because or when. If the answer is “only a comma” or “nothing”, it is a run-on.
• Try the period test. Put a period where the comma is. If both halves work as sentences, the comma alone was not enough.
• Watch for pronouns after a comma. It, he, she, they, we and this right after a comma often start a new clause.
• Watch for transition words. However, therefore, then and for example need more than a comma in front of them when a full clause follows.

For editing your own drafts

• Read it aloud. Where your voice drops and you take a full breath, a sentence has probably ended, whatever the punctuation says.
• Vary your fixes. Use a mix of periods, conjunctions, semicolons and subordinate clauses, so the text does not turn choppy.
• Treat the tool as a second reader, not a judge. Look at each finding, read the reason and decide for yourself. Then read the whole text once more, because no pattern checker catches every run-on.