Add a comma before "so" when meaning "therefore". Omit the comma to explain why something happened.

Add a comma before "so" when meaning "therefore".

John has won the lottery, so he's going to pay off his debt.

John has won the lottery so he's going to pay off his debt.

Omit the comma before "so" (or "so that") to explain why something happens.

Olivia hurried so she wouldn't be late.

Olivia hurried, so she wouldn't be late.

1. When to Put a Comma Before 'So'

As a conjunction (used to connect clauses or phrases), "so" can have two different meanings:

1. "Therefore", "and for this reason", or "consequently"
2. "In order that" or "with the result that" to introduce a reason or explanation

Place a comma in front of "so" when using it as a synonym of "therefore" (coordinating conjunction).

My daughter is smart, so she often identifies the best solutions.

My daughter is smart so she often identifies the best solutions.

You can easily replace "so" by "therefore" in this sentence without changing its meaning.

My daughter is smart; therefore, she can identify the best solutions.

Note that in the example above, "so" can also be replaced by "and for this reason" or "consequently".

Do not use a comma before "so" or "so that" when meaning "in order that" or "with the result that" (to introduce a reason or explanation).

Peter wants to learn Spanish so he can study in Madrid next year.

Peter wants to learn Spanish, so he can study in Madrid next year.

Notice that replacing "so" by "therefore" makes no sense in this sentence.

Peter wants to learn Spanish; therefore, he can study in Madrid next year.

"So" is often followed by "that" in sentences like these:

Leave early so that you don't miss the train to Seattle.

Follow the same comma rules when using other subordinating conjunctions.

2. More Examples ('So' as a Coordinating Conjunction—Comma)

• She overslept, so she missed the flight to London.
• Oliver is biased, so he is an unreliable witness.
• Sophia is talented and energetic, so she got the promotion.
• James was in Chicago, so he couldn't have committed the crime at that place and time.
• Charlotte missed the flight because she was late to the airport, so she doesn't have any real rights.

3. More Examples ('So' as a Subordinating Conjunction—No Comma)

• Since my parents are getting older, I moved to Chicago (so/so that) I can visit them more often.
• I'm thinking about planning my meals in advance (so/so that) I can take control of my weight.
• Lift heavier weights (so/so that) you can build muscle.
• Avoid sweets and cakes (so/so that) you can reduce your sugar intake and prevent diabetes.
• I spoke slowly (so/so that) my students would understand me.

4. Conclusion

To determine if a comma is needed before "so", follow these simple steps:

1. Mentally replace the word "so" by "therefore".
2. If the result makes sense, add a comma before "so".