How to A/B Test Subject Lines for Higher Email Open Rates

Why A/B Testing Subject Lines Is Essential

Email marketing success heavily relies on one factor: getting recipients to open your emails. No matter how valuable your content or offer is, if your subject line doesn’t grab attention, your email gets ignored. This is where A/B testing subject lines comes in.

Creating an A/B test, also known as split testing, allows you to compare two variations of a subject line to determine which performs better. By consistently testing and refining your approach, you can significantly boost open rates, engagement, and conversions.

In this guide, we’ll walk through how to A/B test subject lines effectively, analyze data, and use insights to refine future campaigns.

Step 1: Define Your Goal

Before running an A/B test, you need to determine what success looks like. Some common goals for subject line testing include:

• Increasing email open rates – The most common goal, measuring the percentage of recipients who open your email.
• Boosting click-through rates (CTR) – If the subject line leads to more clicks on links inside the email.
• Improving conversion rates – The final goal if you want email recipients to take a specific action (e.g., making a purchase or signing up for a webinar).

Once you define the goal, you can create subject line variations designed to impact that specific metric.

Step 2: Choose a Single Variable to Test

To get meaningful results, focus on changing one element of the subject line at a time. When you are writing your subject lines, you’ll want to pick your theme first then write variations of that subject line, testing your variable. You can also of course text completely different subject lines to determine what content your audience is most interested in. Here are key subject line components you can test:

• Length: Short vs. long subject lines.
• Tone: Formal vs. casual.
• Personalization: Including the recipient’s name vs. not using it.
• Urgency: Adding time-sensitive words like “limited time” vs. a neutral approach.
• Question vs. statement: Asking a question vs. making a bold statement.
• Use of numbers: Including statistics or list-style formats (e.g., “3 Ways to Improve Your Open Rate”).

For example, if you want to test urgency, you could compare:
A: “Last Chance! 50% Off Ends Tonight”
B: “50% Off – Shop Now Before It’s Gone”

By testing one element at a time, you can pinpoint which factor drives better results.

Step 3: Segment Your Audience Properly

A/B testing works best when your sample size is large enough to provide reliable data. If your email list is small, running a test on a subset of your audience can yield misleading results.

A good practice is to:

1. Split your audience evenly – Randomly divide your list into two equal, representative groups. Many email marketing platforms will do this for you, and there are a few different popular ways you can test. You could completely split your audience 50/50 and implement the data you receive into future tests. This works better for smaller contact lists, since your test is happening over a larger number of contacts. You can also A/B test subject lines to a random percentage of your audience and send the winning subject line to the remainder of your contacts. This works a lot better for larger audiences that can still test to a significant number of contacts that only make up 10-20% of their population.
2. Ensure statistical significance – The more recipients in the test, the more reliable your results will be. The exact amount of contacts that demonstrates statistical significance does of course change depending on your current email metrics. For example, if you have a very engaged audience as a whole, you don’t need as many contacts to receive actionable data in your A/B test as opposed to a contact list that is significantly less engaged.
3. Keep testing conditions the same – Send both subject lines at the same time and on the same day of the week to avoid skewed results. You will of course want to test your sending times to determine the most optimal time for your audience, but isolating your variable is important, so 

Step 4: Analyze Your A/B Test Results

Once your email has been sent, allow 24–48 hours to gather sufficient data before drawing conclusions. Key metrics to track include:

• Open Rate (%) – The percentage of recipients who opened the email.
• Click-Through Rate (CTR) – The percentage of recipients who clicked on links inside the email.
• Conversion Rate (%) – The percentage of recipients who completed a desired action.

If one subject line significantly outperforms the other, that variation should be used moving forward. However, if results are too close, it may indicate that the subject line change had little impact.

Step 5: Implement Insights and Repeat Testing

A single A/B test provides valuable insights, but the real magic happens when you continuously optimize your subject lines based on repeated testing. Here’s how:

1. Track and document results – Keep a spreadsheet of past tests, including what was tested and the outcome.
2. Identify patterns – Over time, you may notice that certain words, structures, or tones consistently perform better. Continuing to use those is how you dial in your A/B testing and identify audience trends. You may find these trends change over time, but what’s important is that you are identifying and taking note of those trends.
3. Refine future subject lines – Use winning subject lines as a baseline, but continue experimenting with new variations.

By treating A/B testing as an ongoing process rather than a one-time experiment, you can continually refine your subject line strategy and maximize email engagement. Remember, A/B testing is about generating helpful and actionable data to continue to improve your desired metric. If you are not seeing improvements, it’s a sign to change or refine your strategy.

Best Practices for A/B Testing Subject Lines

To run consistently successful A/B tests, follow these best practices:

• Test one variable at a time – Avoid testing multiple elements in the same subject line to ensure clear results.
• Use a large enough sample size – Small lists can lead to misleading data. With smaller audiences, testing with your entire audience as a 50/50 send can still give you actionable data over time. Larger audiences should test a smaller percentage of their audience followed up by a full send to their remaining audience with the winning subject line.
• Send emails at the same time – Testing under consistent conditions eliminates bias. You will want to test different dates and times, but for A/B testing, controlling time is important to isolate the subject line.
• Keep variations subtle – Drastic changes may produce unreliable conclusions. While you do want to test for content type over time, small variations can lead to significant results.
• Monitor long-term trends – Track results over multiple tests to spot patterns. Depending on your content, knowing trends throughout the holiday season or entirely different behavior should inform your future subject tests.
• Apply insights to future campaigns – Don’t just use the winning subject line; understand why it performed better.

Common A/B Testing Mistakes to Avoid

Even experienced marketers can make errors in A/B testing. Here are some pitfalls to avoid:

• Testing with an insufficient sample size – Small tests can lead to false positives. This does of course change depending on the engagement of your audience, but an audience size of at least 1,000 allows you to effectively A/B test subject lines through a 50/50 test.
• Running tests for too short a time – Give recipients enough time to open emails before analyzing results. If you are testing a portion of your audience, allowing for at least a four hour window will often give you enough data to work with, depending on your send time.
• Ignoring statistical significance – Just because one variation had a higher open rate doesn’t mean it will always win.
• Testing too many variables at once – Changing multiple elements makes it hard to determine what caused the improvement.

By avoiding these mistakes, you’ll ensure your A/B tests provide actionable and reliable insights that can be used to optimize your email marketing strategy.

A/B Testing Subject Lines FAQs

How long should an A/B test run for subject lines?
A/B tests should run for at least 24–48 hours to allow enough recipients to engage before analyzing results.

What’s the best way to choose subject lines for A/B testing?
Start with two variations that change one key element, such as length, tone, or urgency, while keeping other factors the same.

Can I A/B test more than two subject lines?
Yes, but whether or not you should will depend on what you are testing. For testing subject line structure with the same sentiment, two subject lines will isolate your variable. If you are highlighting different types of content within your email and testing which your audience is most engaged with, you can test three subject lines.

What is the ideal subject line length for high open rates?
Subject lines between 6–10 words (or 30–50 characters) tend to perform best, but this may differ with your audience. Subject line length is a great variable to A/B test over time.

What happens if my A/B test results are inconclusive?
If results are too close, test another variation or analyze a larger sample size for better accuracy. If more than one test comes up inconclusive, that variable may not impact your results. You may still want to test that variable again in the future.

How often should I A/B test subject lines?
Regularly! The best email marketers continuously test new subject lines to stay ahead of trends and audience preferences.

Final Thoughts: Why A/B Testing Subject Lines is a Must-Do

A/B testing subject lines is not just a marketing tactic—it’s a data-driven approach to maximizing engagement and conversions. Every well-crafted test can give you important insight into your specific audience. By consistently testing, analyzing, and iterating, you can discover what resonates most with your audience.

If you’re serious about your email marketing, start running A/B tests to gather invaluable data on your audience. If you’re interested in working with naturally creative and data-driven optimists on your next email marketing campaign, get in touch today.