The Great Debate: Subject Line Length

The optimal length for email subject lines has been hotly debated over the years, with wildly conflicting conclusions. We looked at emails received by over 2 million email subscribers from over 3,000 retail senders in February 2015, and found that:

  • Most subject lines were between 41 and 50 characters.
  • Subject lines 61 – 70 characters long had the highest read rate (17%).

Overall, our research indicates that there is actually no correlation between the length of a subject line and its read rate. When comparing the number of characters in a subject line to the read rate, the Pearson’s correlation* value was -.03, which shows us that there’s no relationship between the number of characters in a subject line and whether or not the email is opened.

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Just because there’s no specific correlation between length and read rate doesn’t mean you can fully ignore how many characters are in your subject line. Different devices have different display capabilities, so it’s important to keep this in mind as you write your subject lines.

A typical desktop inbox displays about 60 characters of an email’s subject line, while mobile devices show just 25-30 characters. If your audience is primarily reading your emails on smartphones, place the offer or call to action at the beginning of the subject line where it’s more likely to be seen.

Being mindful, too, of how your subject lines may be truncated can also avoid embarrassing brand mistakes, like “license” being truncated to “lice.” Shorter subject lines may see higher performance for a mostly mobile audience, too. Look at your existing data for insights like this, or use subject line length as one of your next elements to test.

Looking for more subject line insights and advice? Download The Art & Science of Effective Subject Lines.

*  The Pearson’s correlation is the most common measure to determine if two sets of data have a relationship. The values for a Pearson correlation can range from -1 to 1, with 0 showing no correlation, a -1 showing a negative correlation, and a +1 showing a positive correlation.

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