Best Practices for Subject lines that get emails opened

man sitting at a desk on a computer

The single most important sentence in any email that you write is the subject line. 

If your subject line doesn’t immediately grab your reader’s attention—your email will never get opened. And with the average office worker receiving more than 121 emails a day1, your subject line needs to stand out from the competition. 

Sending out a series of A/B tests can help determine what kind of subject lines work for your audience—but those tests have their limits. Since there are so many factors involved, it’s almost impossible to isolate the cause of an email that performs well. 

A well-crafted subject line may result in your audience having a higher engagement with your brand, better conversions, and ultimately—more revenue for your business.

At Klaviyo, our experts have analyzed data from our customers over the past year, and a number of trends have emerged. Using this information to make small tweaks to your subject line can be what decides if someone will open your email—or clicks the delete button.


When writing a subject line, the goal is to get your reader’s attention. In real life, shouting may get someone to notice you. But in an email, writing in all caps translates to being loud—and possibly ignored. 

Klaviyo’s research shows that campaigns that did not fully capitalize their subject lines performed ~12% better than campaigns that did, with approximately a 2% higher absolute median open rate. It’s important to note that this only proves correlation, not causation.

Subject line in all caps Open rate (median) Percentage of campaigns
Yes 15.36% 9.15%
No 17.36% 90.85%

But this doesn’t tell the whole story. There are a number of other factors that can impact your marketing strategy:

  • The difference in the performance of emails between companies that frequently use subject lines written in all capital letters versus companies that use few to none.
  • The difference in performance between emails with subject lines written in all capital letters versus those that don’t within the same company.

For the first point, we can divide companies into three bins by what frequency they use subject lines written in all capital letters:

Percentage of subject lines written in all capital letters Median of account open rate Percentage of companies
0-20 20.77% 86.3%
20-80 19.57% 11.2%
80-100 20.51% 2.5%

Companies that write most of their emails with subject lines in all capital letters perform almost equally well to companies that rarely do so. While this may be counterintuitive, the “loud” effect of subject lines written in all caps is actually part of some brands—and they have discovered that it works very well for their audience. 

Klaviyo also surveyed companies that sent a mix of emails, with some subject lines written in all caps and some written in either title or sentence case. For these companies, the median open rate difference was only 0.15% between emails sent exclusively with subject lines written in title or sentence case compared with fully capitalized emails. The former performed slightly better.

Clearly, simply making the change from writing subject lines in all caps to either title or sentence case will not automatically increase the open rate of your emails. Klaviyo’s research proves that open rates are more of a function of your ongoing engagement with your customers, among other factors.

What about punctuation marks???

Some people like to emphasize the urgency of their emails by using an exclamation point in their subject line. But getting carried away with punctuation marks can cause your open rates to nosedive. 

Klaviyo’s research shows that campaigns that use a series of consecutive exclamation points or question marks performed ~19% worse than campaigns that did not, with an approximately 3.3% lower open rate.

Consecutive punctuation marks Median open rate Percentage of campaigns
Yes 13.93% 2.44%
No 17.26% 97.56%

The long and short of subject lines

Most people’s attention spans are very short—so generally it’s a good idea to keep your subject lines brief and to the point. And with many people reading emails on their mobile devices, brevity is especially important, as most screens only display 30-40 characters in a subject line. And Klayvio’s data proves that emails with longer subject lines tend to have lower open rates.

subject_line_length graph

There is no magic formula for getting more people to open and read your emails, but following best practices could lead to improvements. 

Along with avoiding long subject lines and repeated punctuation marks, there are a few more things that you can do to increase your open rate. First, try to avoid writing spammy subject lines. Experiment with Klaviyo’s A/B test functionality for campaigns to see what works with your audience. And finally, have someone proofread your subject line and email before you hit the send button.

Ready to experiment with your own email subject lines? Check out these 7 tips to help you get started.  



1“How Many Emails Does The Average Person Receive Per Day?”

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