A/B Testing

Put your subject lines to the test.

Maximize your email open rates with subject line A/B testing.

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How to run an email subject line test.

A-B Testing

Create two options

Create an email with two subject lines.

Send email

Send to contacts

Send the test to a portion of your contact list.


Test picks best line

We automatically send the winner to your remaining audience.

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Increase your open rates. Improve your email campaigns.

Email marketing experts agree: Running an A/B test on subject lines maximizes your email open rates. It's a best practice used across the industry, and Constant Contact makes it easy to implement in your email campaigns. Our tool tests two subject lines on a percentage of your target audience, then automatically sends the winner to the remainder of your contact list.

The best subject line. Every time.

With subject line A/B testing, confidence comes easy. Turn any email campaign into an A/B test to make sure you're sending a subject line proven to generate more opens and increase customer engagement.

Use our best-in-class editor to create an email for testing. Simply start a new email from the campaign picker, then select Subject Line A/B test.

Select the percentage of your contact list that will receive each subject line. To achieve statistical significance, we recommend testing each line on at least 1,000 contacts, if possible.

Choose how long to wait—6, 12, 24, or 48 hours—before declaring a winner. The tool selects the best subject line based on the number of opens, and automatically sends it to your remaining audience.

While the test is active, access real-time reporting to view engagement for each audience and which line is in the lead. Then see how the winning subject line performed overall.

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Frequently Asked Questions

A/B testing makes your email marketing even more effective by allowing you to test two versions of an email with a small group of your subscribers to see which version performs better. The “winner” of the test is then sent to your larger email list.

You can run a test on just about any part of an email, but one of the best things to test is your email subject line. Subject line A/B testing determines which email subject line is most likely to have the highest open rate, and therefore be seen by the most people. You can also test the content within an email to see what generates the most clicks.

In email marketing, split testing and a/b testing mean the same thing. Both terms refer to splitting a test group into “group a” and “group b”, and sending a different version of an email to each group to determine which version performs the best.

When you A/B test to determine the best version of your email, you’re able to identify subject lines that garner the highest open rates, figure out which email content gets the most clicks, and generally learn more about your email subscribers. In short, A/B testing makes your marketing smarter and more effective.

While you can run an A/B test with as few as three contacts, the larger your test groups are, the better. We recommend A/B testing subject lines with at least 1,000 contacts to get the most meaningful results.

Resource Center

Resources to help you do more business.

How to Write a Great Subject Line

A subject line is the first impression you’ll make on your readers. Make it count.

11 Surefire Subject Line Secrets

Here’s one: The most effective subject lines contain around 4-7 words or 40 characters.

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