When to Insert a Comma Before “So”
Including two tricks to help you identify what this simple conjunction is up to
While sometimes it’s easy to know when to use a comma before “so,” at other times, it’s not. Turns out there’s a reason for this — and a trick to help you know which way to go.
“So” seems so simple
The conjunction “so” sometimes connects two independent clauses and at other times doesn’t. When it does, a comma is usually needed because the two clauses it connects can stand on their own as individual sentences. Neither depends on the other, and both are equally important.
For example, a comma should be used in this sentence: “She found herself wide awake hours before dawn, so she got out of bed to pack for her trip.”
It’s easy to see that “She found herself wide awake hours before dawn” and “she got out of bed to pack for her trip” are complete sentences that stand on their own and are equally important. And it’s easy to remember that a comma is needed before “so” in such a sentence.
But wait, there’s more. An even easier way to make sure that’s correct is to replace “so” with “therefore.”
In the above example, that option works: “She found herself wide awake hours before dawn, therefore she got out of bed to pack for her trip.” This simple switch emphasizes that the first part of the sentence is the reason for the second part of the sentence—and confirms that both parts can stand on their own, so a comma is needed before “so.”
Exception to the rule
When “so” connects two short independent clauses, it’s often okay to skip the comma. Consider this example: “He was tall so he could reach it.”
While “He was tall” and “he could reach it” are both independent clauses, both are also short and easily understood as equally important, so there’s no need to include a comma. But if you do, that’s fine, too. (My preference? Add comma: “He was tall, so he could reach it.”)
“Therefore” vs. “so that”
But what about when the word “so” doesn’t connect two independent clauses? It’s easy to identify when this is the case by trying to replace “so” with “so that.” If “so that” fits, chances are good “so” is connecting a subordinate clause (which can’t stand on its own) to an independent clause (which can), and no comma is needed before it.
Consider this example: “They arrived early so they could help set up.”
Replacing “so” with “therefore” results in: “They arrived early, therefore they could help set up.”
While technically this works, the switch has changed the meaning of the sentence. If the reason “they” arrived early was in order to help, “so that” is a better alternative than “therefore,” which means a comma should not be added before “so” in the original sentence.
Key takeaways
Add a comma before “so” if “so” can be replaced by “therefore,” which indicates “so” is connecting two independent clauses.
Exception: When “so” connects two short independent clauses, it’s okay to skip the comma.
Don’t add a comma before “so” if “so” can be replaced by “so that,” which indicates “so” is connecting a subordinate clause to an independent clause.
An earlier version of this post appeared in The Writing Cooperative.
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I write fiction, poetry, and nonfiction when I’m not working as a copy editor. Author of the novel One Sister’s Song and the e-book Not Nearly Everything You Need to Know About Writing.