What makes a great subject line? Focus on these 8 principles to increase open rates

Profile photo of author Tiff Regaudie
Tiff Regaudie
13min read
Email marketing
April 28, 2023
On the left side of the image, the text "subject lines" is stacked three times in all caps, in a shade of dark lavender on a background of lighter lavender. The gradient of the image fades into a darker lavender on the right side of the image, where text in white font reads, "email subject line principles to increase open rates". In the bottom right corner, the background turns to lemon, with the Klaviyo flag in lavender.

We’re obsessed with subject lines. And you should be, too.

Without a persuasive subject line that piques curiosity, your email body copy can’t do the work it was destined to do. No opens means no clicks—and no clicks means no revenue.

We’ve scoured through some of the best email subject line examples from our customers to reverse-engineer best practices and come up with some first principles to help you craft and test subject lines—and drive higher open rates.

Before you start: open rates post-Apple iOS 15

We can’t talk about subject lines without first addressing how they’re measured: open rates.

Launched in June 2021, Apple’s iOS 15 privacy features allow Apple Mail users to block third parties from tracking email opens. Considering Apple owns 58% of email client market share, that means more than half of your brand’s open rate metrics don’t reflect how many people actually opened your email.

Ananda Farge, email marketing associate director at The Stable, says she still values open rate—but only relative to itself and other metrics that inform her about the overall health of an email program.

If we had a 60% open rate on Monday, and on Tuesday we see a 40% open rate, we can compare campaigns and make decisions from there. Maybe deliverability dropped off, so we’ll look at that.
Ananda Farge
Email marketing associate director, The Stable

“We know that a 60% open rate doesn’t mean 60% of people actually opened your email,” Farge says. “But if we had a 60% open rate on Monday, and on Tuesday we see a 40% open rate, we can compare campaigns and make decisions from there. Maybe deliverability dropped off, so we’ll look at that.”

More practically, the best way to manage this reality is to use your digital marketing platform to differentiate Apple’s Mail Privacy Protection (MPP) from true recipient engagement. The purpose of this is to avoid seeing inflated email open rates that may lead you to believe your subject lines are more effective than they really are.

This will be a smaller segment than what you’re used to.
Dayna Scandone
Principal customer success manager, Klaviyo

Klaviyo handles this with a property called “Apple Privacy Open,” which is either true or false for all open events. If Apple Privacy Open is true, it means the message was opened on a device with MPP turned on. That means the open may be attributable to Apple’s MPP and doesn’t necessarily reflect a true email open.

The next step is to separate MPP opens from true opens by adding a filter where Apple Privacy Open equals false. This is the segment to test subject lines with, because you know an open from this segment is actually an open.

Gif shows how to create a Klaviyo segment of true Apple Mail opens. The parameters are “What someone has done (or not done),” followed by “Opened email at least once over all time,” where “Apple Privacy Open” equals “False.”
Source: Klaviyo Help Center

“This will be a smaller segment than what you’re used to,” points out Dayna Scandone, principal customer success manager at Klaviyo. “If the email marketing campaign is important, you can always follow up with a second campaign to everyone else and use Smart Sending to ensure no one gets the campaign twice.”

With that in mind, here are 8 email subject line first principles and best practices to test for your own brand and audience.

1. Personalization makes subject lines stand out in the inbox

Generally speaking, the larger the company, the more likely they are to use personalization in email subject lines. Klaviyo research finds that mid-market companies, for example, tend to personalize 3.35% of their email subject lines with the recipient’s first name, while small to midsize businesses only do so in 2.45% of their email subject lines.

Image shows a vertical bar graph depicting the percentage of email campaigns in which the subject line is personalized using the recipient’s name. Mid-market companies use personalization slightly more than SMBs (3.35% vs. 2.45%, respectively).

Most of the time, “personalization” in a subject line means using the reader’s first name—but our research suggests first name alone won’t do much to increase your open rates.

Brands that are squeezing every ounce of value from audience segmentation are driving better results with deeper personalization tactics. This is because audience segmentation, when paired with personalization, increases the relevance of your email content—and that’s the single most important ingredient for driving opens, clicks, and conversions.

My favorite way to personalize a subject line is to reference the name of the product for an abandoned browse or abandoned cart automation.
Dayna Scandone
Principal customer success manager, Klaviyo

Klaviyo customers can segment in an endless number of ways based on customer data and cohorts. One of the most popular and effective is using predictive analytics segmentation, powered by AI.

This tool predicts customer lifetime value, repeat purchase rate, and churn—and it lets a marketer create segments based on those predictive metrics to increase customer lifetime value and loyalty or even mitigate a lost customer.

During BFCM 2023 alone, Klaviyo customers sent 357M messages from segments created with predictive analytics.

How to personalize email subject lines in Klaviyo

Scandone, who works with Klaviyo customers day in and day out, says her favorite way to personalize a subject line “is to reference the name of the product for an abandoned browse or abandoned cart automation.” Review requests or replenishment reminders are also great places to use this syntax, she adds.

So how do you actually accomplish this in Klaviyo?

The answer is dynamic variables—which sounds advanced, but is actually just a tagging system that pulls information from your customer data platform into your subject line. For a first name, it looks something like this:

{{ first_name|default:’there’ }}

Here, “first name” will be replaced in the subject line with the recipient’s actual first name. But if a brand doesn’t have access to someone’s first name, the “default:’there’” instructs the email platform to replace the name value with “there” (for “Hey there”).

For a product title, the tag may look something like this:

{{ event.extra.line_items.0.product.title }}

Once you have a handle on product titles in subject lines, you can go even further by collecting more nuanced data through quizzes and other forms and using that information in subject lines.

For example, if you’re a skincare brand and your quiz asks the customer what type of skin they have, you might craft a subject line like this: [First Name], shop [moisturizers] for [sensitive] skin.

Visit our help center for more ways to use dynamic variables.

2. Subject lines perform better when they’re short

“Most email opens are happening on mobile devices, so it’s critical to think mobile-first when you’re writing subject lines,” Scandone points out. “You should always be testing your campaigns on desktop and mobile to ensure they’re showing up exactly how you want them to, and that includes the subject line.”

You should always be testing your campaigns on desktop and mobile to ensure they’re showing up exactly how you want them to, and that includes the subject line.
Dayna Scandone
Principal customer success manager, Klaviyo

According to our research, as subject lines get wordier, subscribers on your email list are less likely to open your emails.

Image shows a line graph depicting word count on the x axis and median open rate on the y axis. The line starts in the 30-40% range for subject lines under 5 words, dips below 30% for subject lines 5-10 words, and ends up hovering around 25% for subject lines of 15-20 words.

So, how long should your subject lines be?

On average, subject lines across all business sizes are about 7 words long (including emojis). According to our data, subject lines of this length drive open rates of about 30%.

Image shows a vertical bar graph depicting average subject line word count by business segment. Both mid-market businesses and SMBs tend to send emails with subject lines around 6-8 words long.

If you want to challenge yourself to shorten your subject lines even more—and potentially boost open rates—you can lean on another important tool for piquing curiosity: preview text.

3. Thoughtful preview text can promise a solution to a problem

We can’t talk about subject lines without mentioning the importance of preview text.

Image shows a screenshot of a Gmail inbox with the preview text of an email highlighted. The email is from sender name “SupportCentral,” the subject line reads “Things your boss wants to know,” and the preview text reads “Give ‘em the numbers.”

“Solving a pain point in your subject line can work well to encourage email opens,” says Toccara Karizma, CEO of Karizma Marketing. One of the best ways to do this is to present it in your subject line and hint at a solution in your preview text.

Solving a pain point in your subject line can work well to encourage email opens.
Toccara Karizma
CEO, Karizma Marketing

4. Urgency and scarcity prompt opens—but proceed with caution

Limited-time offers, exclusive offers, “running out of stock” emails, and clearance sales are actually about creating urgency and scarcity—two things that inspire people to act.

The most obvious way to create a sense of urgency is using phrases like “limited time” and “ending soon.”

A word of caution: While these kinds of words don’t have the same detrimental effect on email deliverability as they used to, we still recommend careful and appropriate use based on your audience to make sure your email doesn’t end up in the spam folder.

Whereas an audience segment with low purchase intent might find your use of urgent language, exclamation points, and all caps annoying, a segment with high buying intent or purchase history may welcome it.

Your complete guide to email deliverability: avoid the spam folder for maximum engagement
Learn more about what really affects email deliverability, including spam trigger terms to avoid.

5. Context rules all good content performance

“Consider the context [of your email],” says Nikki Elbaz, an email copywriter. “By context, I mean the inbox. What kinds of emails does your subscriber usually receive? Will emojis/questions/whatever style subject lines you’re writing stand out or blend in?”

Consider the context. What kinds of emails does your subscriber usually receive? Will emojis/questions/whatever style subject lines you’re writing stand out or blend in?
Nikki Elbaz
Email copywriter

Most email marketers understand context, in theory—they know their emails are showing up in flooded inboxes, received by overwhelmed, busy people. But if you want an example that demonstrates application rather than just comprehension, check out this one from shaving brand Supply.

With this cheeky subject line for BFCM, Supply makes damn good use of an emoji to show they understand the context of the average inbox. With solid reinforcement from their preview text, Supply crafts one of the most creative yet simple subject lines we’ve ever seen.

Image shows a screenbox of an inbox promotions tab, with an email from shaving brand Supply at the top followed by emails from Beardbrand, ChatBot, and Wisepops. Supply’s subject line reads, “This email is better than” with an arrow pointing down emoji. The preview text reads, “Prove us wrong. Don’t open it.”
Image source: Twitter

6. Most great subject lines create a curiosity gap

The curiosity gap is the “space between the information we’re given (what we know) and the information that’s initially withheld (what we don’t know),” according to Enchanting Marketing. This is what you want to create with your email subject line‚ because opening your email is how your audience closes the gap.

Curiosity is especially important if you’re hyping a new product, collection, or campaign that hasn’t launched yet. But keep in mind that what you spark curiosity about will depend on the purchase intent of your audience segment.

The curiosity gap is the space between the information we’re given (what we know) and the information that’s initially withheld (what we don’t know).
Enchanting Marketing

In the early stages of an email subscriber relationship, you may want to spark curiosity about the more salient points of your brand story or the problems you solve.

But after you’ve established a relationship and subscribers are indicating higher purchase intent—through website visits, abandoned carts, and more—that’s when you might want to spark curiosity about your actual product releases.

A word of caution: Just as you wouldn’t enjoy reading a novel without a resolution, email recipients don’t appreciate a curiosity gap that stays open. If you’re going to pique curiosity in a subject line, make sure your email copy keeps its promise by satisfying that curiosity.

7. Emojis aren’t bad in subject lines, but use them sparingly

If you’re trying to stand out in a crowded inbox, you might think emojis are a good way to get noticed.

But you may be blending in more than you realize: according to Klaviyo research, 42% of mid-market businesses and 43.5% of SMBs use emojis in their subject lines.

Here are some quick tips for using emojis in subject lines:

  • Don’t use more than one.
  • Use emojis to amplify the message, not replace words.
  • Remember that the goal of the subject line is to deliver a clear message. If an emoji helps you do that, go for it.

A word of caution: Emojis show up differently depending on the inbox provider, so make sure to preview your subject lines across inbox providers and devices.

8. Test AI to craft subject lines at scale based on past performance

Artificial intelligence may not be a great writer (yet). But it’s an effective assistant when:

  • You don’t know where to start with crafting subject lines.
  • You need to create a lot of subject lines at once.
  • You have enough past performance data to inform current subject lines.

More on that last point: Klaviyo’s Subject Line AI learns over time what’s working for your brand and offers ways to learn from and improve your future subject lines. If you like one of the suggestions, it’s easy to copy and paste it into your subject line field. Click refresh under any suggestion for more examples.

And if you’re struggling to remember some of the best practices from this article, you can find them within the subject line assistant itself. Recommended length, ways to evoke curiosity, and other tips bespoke to your brand are there to help on the spot.

Next steps: A/B test these elements for your subject line

We’ve handed over the knowledge, but to discover what works best for your brand, you’ll need to A/B test each element to find out what works for your brand. Based on these best practices, test these one at a time:

  • Product name mention vs. first name only
  • Subject line word count
  • Preview text wording
  • Urgency language vs. softer language
  • Email context scenarios
  • One emoji vs. none
  • AI-generated subject lines vs. human-generated subject lines
Power smarter digital relationships.
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Tiff Regaudie
Tiff Regaudie
Tiff (she/they) is a writer and content consultant who specializes in marketing, health, and the attention economy. Before devoting herself to freelance writing full-time, they led content teams at various startups and nonprofits in Toronto, Canada.