3 Simple Tips for Better Email Subject Lines. 18 Million Emails Studied.


Email messages are important – but sending an email message with a great subject line is even more important. It’s important for one to one communication or for email newsletters.

SendGrid analyzed 18 million emails and distilled three important aspects of what makes a great email subject line:

Shorten Subject Headers: While 7 words is the most common subject line word length (14.0% of subject lines), 3 word subject lines (1.6% of subject lines) have the highest engagement rates (21.2%, compared to 17.2% overall and 15.8% for 7 word subject lines).

Measuring engagement rates by number of character lines tells a similar story: longer subject lines have lower engagement rates. Past 15 characters, each extra character is correlated with a 0.03% absolute decrease in engagement rate (or about 1% per 33 characters).

Choose Your Buzzwords Carefully: Recipients prefer certain words over others.  Subject lines referring to “yesterday” and“tomorrow” have higher engagement rates (20.5% and 22.3%, respectively)  than “today” (11.8%).

Subject lines with “soon” have higher engagement rates (21.9%) versus subject lines referring to now  “now”

Email subject lines that use the word “free” have significantly lower engagement rates (13.1% versus 17.2% for “free”-free subject lines).

Eliminate Links and Hashtags: Subject lines that contain URLs do poorly, with average engagement rates of 9.9% (compared to 17.2% for subject lines that do not contain a URL). Subject lines with URLs in them are rare (fewer than 0.4% of emails).

Subject lines that contain #hashtags do poorly, with average engagement rates of 10.5% for subject lines with a single hashtag (compared to 17.2% for subject lines that do not contain a #hashtag). Thankfully, emails with hashtags in them are rare (fewer than 0.2% of emails).

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