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The study, from Email Services Provider Epsilon International, analysed the impact of more than one billion emails over nearly 20,000 separate campaigns sent out by five US retail clients and five US consumer goods clients, in what is understood to be the biggest ever study of the impact of subject line length.

The research found that while shorter subject lines are generally associated with better performing messages, word choice and word order, especially in the first 50 characters of a subject line, trump the issue of overall subject line length.

Subject Line Length (SLL) is certainly a factor, the study concludes. This is not just because time-poor consumers want subject lines that match their attention spans. AOL, Yahoo! and Hotmail, which collectively control around 57% of the US email market, display only the first 38 to 47 characters of a subject line in a recipient’s in-box. Mobile devices display even less of the subject line.

As a result, most consumers are basing the decision to open an email on the first 45 or so characters of its subject line.

But the relationship between SLL and open rates turned out to be relatively weak.

Ian Hitt, Epsilon International managing director, says: “Given how often the mantra of short subject lines is repeated in the press, the reality – that while there is an effect, it’s nowhere near as important as many experts have claimed – may be a surprise for many marketers.”

Ian Hitt adds: “Subject Line Length is definitely important. But content is king. What marketers really need to focus on is the words they use and the order they use them in. They need to spend more time writing, re-writing and testing the content of their subject lines, as it is the strongest driver of successful email performance.”

Key details of the research:

Five retail clients and five consumer services clients were selected at random from Epsilon’s email client set.

Messages sent by these clients between June 2007 and June 2008 to U.S. consumers were included in the analysis.

Messages that were sent to fewer than 100 recipients or that had a personalised subject line (variable length) were excluded.

After suppressions, the analysis included 568.7 million retail emails from 2,365 campaigns, and 507.3 million consumer services emails from 17,516 campaigns.

Unique opens and total clicks were calculated for each campaign and then correlated with subject line length.

It is thought that this is the largest analysis on the subject of subject line length.

Key findings:

Overall, shorter subject lines perform better on average.

However, the relationship is relatively weak, certainly weaker than would have been expected by how often the mantra of short subject lines is repeated.

The research also shows that exceptions exist — some clients get better performance from longer subject lines.

Marketers don’t necessarily need to limit themselves to subject lines between 38 and 47 characters.  However, they do need to ensure the crux of their message is easily understood within that number of characters.

Word choice and word order, especially in the first 50 characters of a subject line, trump the overall subject line length.

Exceptions to the “shorter is better” rule of thumb are plentiful enough that each marketer should test this factor for themselves. 

This is especially true for marketers who send very targeted emails or those who have a niche audience.

Finally, more time should be dedicated to writing, re-writing and testing subject lines than most email marketers currently spend, since a good subject line is the strongest determinant of email performance.