Comma before too? Do you or don't you put a comma before too at the end of a sentence?
Yes, You Do
Put a comma before the final too when too means also or as well. For example, which of these two sentences is correct?
The second sentence is correct. We use a comma before too when too is at the end of the sentence. We also put commas around too when too is embedded in the sentence, as in this example.
I know, too, that chocolate is tasty.
Zen Comma Rule N tells us to "Separate conjunctive adverbs with commas." Too, when used to mean also or as well, is a conjunctive adverb, along with therefore, however, additionally, and other such words.
Conjunctive Adverb: An adverb that connects (i.e., joins) two clauses, that shows how the meaning of the second clause relates to the meaning of the first clause.
When we use too as a conjunctive adverb, therefore, we need to separate it from the rest of the sentence. This means when too is the final word in the sentence, we precede it with a comma.
Explanation and Comparison
Let's look at another sentence that uses a conjunctive adverb:
The senator was ashamed. However, he remained in office.
Here, we have two discrete ideas, one per sentence. The second sentence begins with the conjunctive adverb however, which tells us that the meaning and the importance of the second sentence are somehow connected to the idea in the first sentence. That's what a conjunctive adverb does. Let's look at another example.
The principal expelled the student. He fired the teacher, as well.
The phrase as well is a conjunctive adverb. It tells the reader that the meaning being expressed in the second clause (i.e., He fired the teacher) somehow connects to the first clause (i.e., The principal expelled the student).
We see from the last example that the conjunctive adverb as well is separated from the sentence with a comma. When used in this manner, as well is a synonym for too. This tells not only that too is a conjunctive adverb but also that the final too needs to be separated from the sentence with a comma, as follows.
The principal expelled the student. He fired the teacher, too.
This example has the same meaning as the previous example. The only difference is we have swapped synonyms, changing as well for too. The comma remains before the final conjunctive adverb.
Words don't determine punctuation. The function of those words, i.e., what those words do, determines the punctuation. As we see here, the function remains the same, so the punctuation, too, remains the same.
Too in Other Places
When we move too to other places in the sentence, Rule N still applies. Placing too at the beginning of a sentence is uncommon and, frankly, awkward, so we'll skip it. However, too is common in other places. The most common place (other than the end of the sentence) is following the subject of the clause.
I, too, will go to the service.
John is a farmer, and Leroy, too, is a farmer.
Notice the commas around too. If we move too to the end of the sentence, the commas remain:
I will go to the service, too.
John is a farmer, and Leroy is a farmer, too.
Even though we moved too, it has the same meaning and same function in the sentence, so the same punctuation applies.
A Note about AP Style
I often hear people say something like "Hey, the AP style guide says don't use that comma, so it's wrong!" Ok, if you write for a newspaper, don't use it—unless it contributes to clarity (which the AP guide supports). The Associated Press (AP) style guide, in general, encourages writers to remove as much punctuation as possible. This may be an effort to save room for journalists' words in narrow newspaper columns.
If you're not a journalist, you don't need to follow AP style. AP style is not for all writers, which is why we also have the APA, MLA, and other style guides.
Here's one of my general principles for using punctuation, and it applies to the comma before the final too.
What this means: If we're going to separate some conjunctive adverbs with commas, we will separate all conjunctive adverbs with commas—including too.