Should You Use a Headline or Sentence as Your Email Subject Line?

Experiment No. 014

Should You Capitalize Your Email Subject Line?

Don’t underestimate the importance of an email subject line. According to a study, 33% of people open an email based on the subject line. And the majority of people (69%) look at the subject line to determine if it is spam.

You need to follow some subject line best practices, but there is also some room to get creative.

When it comes to the style of a subject line you can follow one of three types

Three Types of Subject Line Styles

Headline or Title Capitalization: The first letter of every word is capitalized, except for words like “the” and “an.” Follow style manuals such as Chicago or APA.
Sentence: Capitalize only the first letter of the subject line.
All lowercase: Every letter is lowercase, including the first letter.

The question, then, is which one is better? The only way to find out is to test. We always use a headline-style subject line so the next question is—Would a sentence-style subject line increase the open rate?

Before we dive into the test, I asked this question on LinkedIn to get other professionals' thoughts or experiences.

Poll: Subject Line Style

This is the question I posted on LinkedIn.

“Should you write a subject line in headline style or sentence style?

“Some nonprofits use headline style, while others use sentences. So which is better?

”I intend to run a test to find out. Before I do, I would love to know what you think.”

The results show people weren’t too sure which type of subject line would perform better. A majority (41%) of people said a sentence-style subject line would increase the open rate. The same amount of people (30%) said it would decrease the number of open or make no difference.

A few people who commented on the post felt the context of the subject line matters. One individual commented:

“I think it depends on the content of your subject line more than the type. You can use a sentence like "Please fulfill your pledge to XYZ." OR you can try a sentence like "Your pledge to XYZ means the world to us." I think there are a lot of variables and this could fluctuate based on your audience...for instance, where I work, the first example would work well for our post-50th crowd. They prefer traditional, straightforward language. But our "younger" audiences are more interested in impact and emotion, so the second example would resonate better with them.”

Test: Subject Line Style

With most people saying a sentence-style subject line would increase the click and conversion rate, it was time to run the test.

Our subject line style is to capitalize every important word. We use the Chicago Manual.

Research Question: Will a sentence-style subject line where only the first word is capitalized, increase the open rate?

Hypothesis: Because a sentence-style subject line looks more personal it will encourage more people to open the email.

Test Element: Subject line

Control - Headline-style subject line with every important word capitalized

Test - Sentence-style subject line with only the first word capitalized

The subject lines and email were the exact same. The only difference was capitalization.

Key Metric: Open rate

Other Metrics: Click rate, conversions

Results & Application

Results: The sentence-style email performed a little better. However, the results were not statistically significant.

Open rate - 1% higher (32% for the test & 31% for the control)
Conversion rate - same amount.

Application: While the sentence-style subject line increased the number of opens it didn’t perform well enough to say it was reliable.

Future Tests: A good test will often lead to more questions. This test raised a few. Test ideas include:

Test it again, in the same way, to see if we get the same results.
What context will a sentence-style subject line work best?
Does a sentence-style subject line work better with a different audience? For instance, would a younger generation respond to a sentence-style subject line

Findings From Other Studies About Subject Lines

AWeber, an email marketing platform, looked at how experts capitalized subject lines. The majority of experts (60%) use sentence-case subject lines. While, 34% use title case and 6% use all lowercase.

One A/B test found that an all-lowercase subject line outperformed the headline-style subject line and got 35% more opens.

HIPB2B looked at their inbox and found that most brands (62%) favored sentence-style subject lines.

What This Means For You

According to this test, a sentence-style subject line performs better than a headline-style subject line. If you use a headline-style subject line, it is fine to switch to a sentence-style one without causing too much confusion for your audience. You might even see a slight increase in the performance. If you use a sentence-stye subject line don’t switch now.

Not sure? That’s why it s always best to test.