A run-on sentence isn’t just a long sentence — it’s two or more complete thoughts crammed together without the punctuation that should separate them. They’re one of the most common writing mistakes, and they’re sneaky: because the ideas are related, the sentence often sounds fine in your head even when it’s broken on the page. This guide shows you how to recognize a run-on, how to tell it apart from a sentence that’s merely long, and four dependable ways to fix one.
What is a run-on sentence?
A run-on sentence happens when two independent clauses — each of which could stand alone as its own sentence — are joined without proper punctuation or a connecting word.
There are two main types:
- Fused sentence: two clauses jammed together with no punctuation at all.
“The deadline is Friday I haven’t started writing.”
- Comma splice: two clauses joined by only a comma, which isn’t strong enough to do the job.
“The deadline is Friday, I haven’t started writing.”
Both are run-ons. In each case, “The deadline is Friday” and “I haven’t started writing” are complete sentences, and a comma — or nothing — can’t legally hold them together.
Long is not the same as run-on
This is the most important distinction, and it trips people up constantly. A sentence can be long and perfectly correct, or short and still a run-on.
“Because the supplier missed its shipping window in March, the launch slipped by three weeks, which forced the marketing team to redo the entire campaign calendar.” ✅
That’s a 28-word sentence and it’s grammatically fine — it has one independent clause properly supported by dependent ones. Meanwhile:
“It’s late, I’m tired.” ❌
That’s a four-word run-on (a comma splice). Length has nothing to do with it. What matters is whether you’ve stuck two independent clauses together without the right glue.
That said, run-ons and overlong sentences often travel together, because the longer a sentence gets the more tempting it is to keep tacking on clauses with commas. Keeping an eye on ideal sentence length reduces how often you create run-ons in the first place.
How to spot a run-on
Three quick tests will catch almost every run-on in your draft:
- The split test. Find the comma (or the gap) where the sentence seems to turn. Can both halves stand alone as complete sentences? If yes, and they’re joined by only a comma or nothing, it’s a run-on.
- The “and” test. Read the sentence and notice where you mentally insert an and or a but. That’s usually the seam where two independent clauses meet without a connector.
- The read-aloud test. Run-ons make you run out of breath or stumble. If you instinctively pause hard in the middle of a sentence, that pause often marks a missing period or semicolon.
Four ways to fix a run-on
Once you’ve found a run-on, you have four reliable repairs. They’re not interchangeable — each creates a slightly different rhythm, so pick the one that fits.
1. Use a period (make two sentences)
The simplest fix. If the two ideas are distinct, just separate them.
- Before: “The deadline is Friday, I haven’t started writing.”
- After: “The deadline is Friday. I haven’t started writing.”
2. Use a comma + coordinating conjunction
Keep the comma, but add one of the FANBOYS words — for, and, nor, but, or, yet, so — to give it the strength it needs.
- Before: “The deadline is Friday, I haven’t started writing.”
- After: “The deadline is Friday, but I haven’t started writing.”
3. Use a semicolon
When two independent clauses are closely related and you want to keep them in one sentence, a semicolon is the correct connector.
- Before: “The data looks good we should ship.”
- After: “The data looks good; we should ship.”
4. Subordinate one clause
Make one clause depend on the other using a word like because, although, since, or while. This shows the relationship between the ideas.
- Before: “The deadline is Friday, I haven’t started writing.”
- After: “Although the deadline is Friday, I haven’t started writing.”
| Fix | When to use it | Result |
|---|---|---|
| Period | Ideas are distinct and equal | Two crisp sentences |
| Comma + conjunction | Ideas are related and balanced | One flowing sentence |
| Semicolon | Ideas are tightly linked, formal tone | One connected sentence |
| Subordination | One idea depends on the other | One sentence with clear cause/effect |
A worked example
Here’s a paragraph riddled with run-ons:
The product shipped late, customers were frustrated, we issued refunds it cost us a fortune, the lesson was clear we needed a better process.
Untangled with a mix of fixes:
The product shipped late, and customers were frustrated. We issued refunds, which cost us a fortune. The lesson was clear: we needed a better process.
Notice the variety — a comma-and-conjunction, a subordinate clause, and a colon. Using the same fix every time produces monotony, so vary your repairs the way you’d vary sentence length.
How to catch run-ons in your own writing
Run-ons hide best in long paragraphs, where one bloated sentence blends into the next. The fastest way to expose them is to break your text into individual sentences and look at each one in isolation. Paste your draft into the sentence counter — it splits the text on sentence boundaries, so a “sentence” that’s secretly two or three thoughts stands out immediately, and the count itself flags paragraphs that have fewer sentences than they should for their length.
A second signal comes from your readability score: run-ons inflate your average words-per-sentence, which drags the score toward a higher, harder grade level. If the readability score checker reports a grade level higher than you expect, run-ons are a likely culprit, and the how to improve your readability score guide walks through the sentence-level cleanup.
While you’re tightening punctuation, it’s worth checking your lists too — the Oxford comma is another small mark that decides whether a sentence reads cleanly on the first pass.
The bottom line
A run-on sentence joins two complete thoughts without the right punctuation — either with nothing (a fused sentence) or with just a comma (a comma splice). Length is a red herring; what matters is whether two independent clauses are stuck together incorrectly. Fix run-ons with a period, a comma-plus-conjunction, a semicolon, or by subordinating one clause, and vary your fixes so the rhythm stays natural. To find the ones hiding in your draft, break it into sentences with the sentence counter and watch your readability score for the tell-tale spike in words per sentence.
Frequently Asked Questions
What is a run-on sentence?
A run-on sentence is two or more independent clauses (complete thoughts) joined without proper punctuation or a connecting word. The two types are fused sentences (no punctuation between the clauses) and comma splices (only a comma between them).
Is a long sentence automatically a run-on?
No. A sentence can be long and grammatically correct if it has one main clause properly supported by dependent clauses. Conversely, a very short sentence like “It’s late, I’m tired” is a run-on because it splices two independent clauses with only a comma. Length and run-on status are unrelated.
What is a comma splice?
A comma splice is a specific kind of run-on where two independent clauses are joined by only a comma — for example, “I came early, the doors were locked.” Fix it with a period, a semicolon, or a comma plus a conjunction like but.
How do I fix a run-on sentence?
There are four standard fixes: split it into two sentences with a period; add a coordinating conjunction (and, but, so) after the comma; use a semicolon; or subordinate one clause with a word like because or although. Choose the fix that best reflects the relationship between the two ideas.
How can I find run-on sentences in my writing?
Paste your text into the sentence counter to break it into individual sentences — a “sentence” that contains two or three complete thoughts becomes obvious. You can also watch your average words-per-sentence in the readability score checker; run-ons push it up.
Are semicolons a good way to fix run-ons?
Yes, when the two clauses are closely related and you want to keep them in a single sentence. A semicolon correctly joins two independent clauses, as in “The data looks good; we should ship.” Just don’t overuse them — a period is often clearer.