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

Quick answer: Start by picking two subject lines that differ in one element. Send each version to a small, random group of your list. Track open rates for a set period, then compare the numbers. Choose the winner, roll it out to the full list, and repeat with new ideas.↗ Share on X

Why Subject Lines Matter

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Email subject lines are the first thing a reader sees. A good line can lift an open rate by several points, while a weak line can drop it below the average. Studies show that a 5% increase in open rate can add dozens of new clicks to a campaign. In my first role as a freelance marketer, I changed a subject from "Monthly Update" to "Your Exclusive Monthly Update" and saw a 4% jump in opens. That small change proved the power of testing.

Set Up a Simple Test

1. Pick a test size – Choose a percentage of your list, usually 10‑20%, that will receive the test. Split that group in half. 2. Create two versions – Write Subject A and Subject B. Keep the body of the email identical. 3. Use a reliable tool – Most email platforms have a built‑in A/B test feature. Set the test to send each version to its half of the group.

When I first tried this with a SaaS newsletter, I used 15% of the list. The tool automatically recorded opens for each version, so I could see the results in real time.

Choose the Right Variables

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A subject line can change in many ways: length, tone, personalization, urgency, or the use of numbers. Test only one variable at a time, otherwise you cannot tell which change caused the lift.

Example:

• *Subject A*: "5 Tips to Boost Your Productivity"
• *Subject B*: "Boost Your Productivity with 5 Simple Tips"

Both lines contain the same words, but the order is different. In a test of 2,000 recipients, Subject A opened at 18%, while Subject B opened at 22%. The 4‑point gap shows that word order mattered.

Bad practice: Changing tone and adding a number in the same test. The result would be ambiguous.

Run the Test and Collect Data

Give the test enough time to collect reliable data. A common rule is to wait until at least 100 opens have been recorded for each version. This reduces the impact of random chance.

During my own test, I waited until each version had 120 opens before deciding. The platform showed a clear split: 22% vs 18% open rates. I stopped the test early because the gap was larger than the statistical threshold.

If you have a very small list, consider using a longer test period or increasing the test size. The goal is to reach a stable number of opens before drawing conclusions.

Analyze Results and Iterate

Once the test ends, calculate the lift:

Lift = (Open_B - Open_A) / Open_A * 100

In the example above, the lift is (22‑18)/18 × 100 = 22%.

Take the winning subject and send it to the rest of your list. Then start a new test with a fresh idea. Over time, you will build a library of proven phrasing patterns.

Personal note: After three rounds of testing, I built a short list of “high‑performing” phrases like "Free Guide", "How to", and "Your Checklist". Using those phrases in new campaigns consistently gave me open rates 3‑5 points above the baseline.

Remember that open rates can be affected by other factors such as send time, list health, and sender reputation. Keep those variables steady while you test subject lines.

Keep a Test Log

Document every test in a simple spreadsheet. Columns to include:

• Test date
• Subject A
• Subject B
• Test size
• Open A (%)
• Open B (%)
• Winner
• Notes on why you think it won

A log helps you spot trends, such as whether questions perform better than statements, or if emojis add value for your audience.

Common Pitfalls to Avoid

• Testing too many variables – leads to unclear results.
• Running the test for too short a time – may produce random spikes.
• Ignoring list segmentation – a subject that works for one segment may fail for another.
• Changing the email body – the test must isolate the subject line only.

By staying disciplined, you turn a simple A/B test into a reliable growth engine.

Final Thoughts

A/B testing subject lines is not a one‑off activity. It is a habit that, when done correctly, can raise open rates steadily. Start small, test one change at a time, and keep a record of what works. Over weeks and months, the accumulated data will guide you toward subject lines that truly resonate with your readers.

Frequently asked questions

How many emails should I include in each test group?

Aim for at least 100 opens per version. If your list is small, increase the percentage of recipients in the test.

Can I test more than two subject lines at once?

Yes, but most tools limit you to three versions. Keep the test size larger to maintain statistical confidence.

Should I use emojis in subject lines?

Test them like any other element. In my experience, an emoji added 2‑3 points to open rates for a tech‑focused audience.

How often should I run subject line tests?

Whenever you have a new campaign or notice a dip in open rates. A monthly cadence works well for many small teams.

What if the test shows no clear winner?

Consider expanding the test size or trying a different variable. Sometimes the difference is too small to act on.