Your Step-by-Step Guide to Testing Email Subject Lines


You’ve been invited to dinner with the executive management team of your company. This is your chance to hobnob with the big wigs, and you want to look your best. That’s why the floor of your bedroom is covered in discarded clothes and accessories as you assemble the perfect outfit for the event. It’s also why you recruited your partner to give you honest feedback. In other words, without even realizing it, you are already a master split tester.

We all naturally understand the benefits of split testing outfit choices or food at restaurants. Why, then, wouldn’t you just as naturally use split testing as effectively as possible for your business’s email subject lines?

Split testing, also called A/B testing, is your ticket to more email opens, more clicks, and higher conversion rates. After you find the perfect outfit for your big dinner and impress your bosses, impress them even more with your plan to invigorate your email responses with heavily tested, measured, proven email subject lines.


A Quick Reminder About Why Email Subject Lines Are Important

You are a marketing professional, so you already know how critical it is to develop strong, catchy email subject lines to woo your audiences. You know, for example, that estimates say the average person receives a whopping 92 emails a day, and at least 33% of people make the decision to open an email solely based on the subject line.

In other words, a lot is riding on your email subject lines, which means you can’t leave them to gut instinct. Instead, you need data. To get that data, you test what types of subject lines inspire your audience to read your messages.

Here are the steps for testing email subject lines.

Step One: Choose Your Audience Segment

You really can’t please everyone, nor should you want to. The point is not to discover the greatest email subject line the world has ever known (though if you do, kudos). Instead, you want to understand what subject lines resonate with your unique audiences. The best way to do that is to target a specific audience segment.

For example, a politician trying to solicit campaign donations is better served writing one email to her list of college students who are worried about student loans, and a different one to her list of new parents concerned about childcare costs. That politician likely wouldn’t use the same email subject line for both messages.

Think about what aspect of the marketing campaign, email content, or action item speaks most to your particular audience segment, and then brainstorm subject lines. It’s generally easier to A/B test email subject lines if you’ve already segmented your email list. Use a marketing automation solution or customer relationship management (CRM) system that easily manages email segmentation.

Step Two: Decide What Aspect to Test

It’s easy to get split-testing fever and feel tempted to test just about everything you can think of. Resist that impulse. You should only test one aspect of the subject line at a time so that you can get accurate results.

Here are some of the most commonly tested attributes:


Even adding or taking away a single word can make a difference in how an audience responds to your email. For example, Optimizely tested two nearly identical subject lines:

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  2. [Ebook] Why You’re Crazy to Spend on SEM But Not A/B Testing

Even though option B includes just one additional word, it netted the company a 30% increase in clicks. If your email list includes 30,000 people, a 30% increase means 9,000 more people clicking through to your website or online store.


Common wisdom suggests that the best email subject lines are shorter, rather than longer. Many marketers are probably aware of the single highest grossing campaign email the Obama team sent, which featured the subject line, “Hey.”

However, what works for some audiences won’t necessarily work for yours. Until you A/B test longer versus shorter email subject lines, you won’t really know what your audience wants.


Developing personalized subject lines that either include the recipient’s name or company have become a popular marketing tactic, especially with the help of sophisticated CRM systems. You can also test this factor by using “you” in subject lines:

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For some companies, using a subject line like A, personalized with “You Need to Know,” can earn your email a higher open rate. Again, don’t make assumptions. Your audiences are getting savvier every day, and while personalized subject lines may have been a hit in the past, don’t assume they’ll have the same impact today. As they say, “Always be (A/B) testing.”


Don’t forget to test how your subject line looks. Even small design changes can mean thousands of additional email opens. All caps, emojis, punctuation, and spacing can all impact your metrics. Design carefully with these options, as the wrong combination can land you in the spam folder.

Step Three: Test a Portion of Your List

If your list is big enough to create credible sample sizes, consider A/B testing only a portion of the list. This way, you can determine which subject line will likely give you the best results, and then you can send the strongest contender to your full list. Some marketers like to A/B test their entire list, and this makes sense when you have a smaller list. However, by testing the full list, you send your weaker-performing email subject to half the list’s recipients.

Almost all major email clients offer A/B testing features, and this makes it easy for you to split test email subject lines. If you aren’t using an email client or marketing automation, you can manually A/B test by splitting your email list into two segments and sending a different subject line to each list. Try to email both lists as close together as possible (same day and time) to eliminate other factors in your comparison of the results.

Step Four: Time to Analyze

Speaking of results: If you are a proud data geek, then welcome to your happy place. Once the split testing results are in, analyze the data. During this process, keep your eye on your goals. Yes, open rates and click-through rates are important, but the most important metric is almost always going to be conversions. Even if one subject line leads to a greater number of opens, that doesn’t mean it will also necessarily lead to more click-throughs and more conversions.
Don’t be afraid to dig deep. Pull lots of exciting, important data. For example, you might discover that even though subject line A accumulated more overall opens, subject line B was opened by more recipients in the coveted 25 to 35 age range. It may be worth segmenting your list differently ahead of your next email campaign.

Step Five: Keep Up the Testing

A/B testing is not a “one and done” step, nor should it be considered a chore. The foundation of strong marketing rests upon constant testing and iteration. Maybe your first A/B test taught you that your audience prefers shorter subject lines. Great: Next time, use shorter subject lines and test personalization or a different design feature.

Keep testing, because tastes change. Audiences can quickly get used to seeing a certain type of subject line that is popular at the moment, such as those using emojis, and may start ignoring them.

Every time you communicate with your audience you have an opportunity to test, tweak, and improve your messaging. The more you improve your communication, the more your message will resonate, and the more your customers will reward you with their clicks and their loyalty.

Great emails don’t just happen. They are built upon experience and data, not gut instinct. A/B testing your subject lines is one of the best tools in the marketer’s playbook, because it will help you gain a better understanding of your audience, one email campaign at a time. Don’t forget, too, that while this article is only focused on email subject line split testing, you should split test everything you can, including your email teaser, header, images, content, design, call to action, and more. Have fun geeking out on all the data and analytics that will help your conversion rate and bottom line.

About the Author:

Jessica Bennett is a writer, editor, and novelist. Her clients span a number of industries, and she's written blog posts, product descriptions, articles, white papers, and press releases— all in the name of inbound marketing. She's proud to be Inbound Certified.

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