• by George Bilbrey , Op-Ed Contributor, June 8, 2015

The optimum length of a subject line has been debated as long as companies have been sending email. Marketers, consultants, and agencies have labored over distilling pitches to a handful of words or characters, aiming for the Goldilocks “just right” length. The subject line needs to have enough detail to set expectations and earn subscriber attention, but not so much that readers stop following and ignore the message.

We recently sifted through more than eighteen million subject lines sent to more than two million subscribers to find out whether subscribers’ actual read rates pointed to an optimal length. We found none: the sweet spot remains elusive. In fact, at least in this sample, it didn’t exist — not mathematically, anyway.

It was easier to find the most popular subject line lengths. Three groupings (21-30 characters, 31-40 characters, and 41-50 characters) each represented 20% of the sample. And all of these groupings shared an unexpected common trait: some of the lowest-read rates in the sample, between 12% and 13%. Slightly longer (51-60 characters) and slightly shorter (11-20 characters) coincided with higher read rates, both around 14%. Even longer subject lines (61-70 characters) coincided with an even higher read rate: 15%. Much longer ones (81-90 characters) were also read more often than the popular groupings at 14%.

Here’s the good news: A quick chi-squared test for correlation confirmed that these results were driven by coincidence. There was no meaningful correlation between subject line length and read rates, at least in this (extremely large) sample.

That's really good news for marketers. It means  you can eliminate a variable from your testing program and concentrate on more important ones. You can focus on communication instead of an arbitrary element of message structure.

It makes sense that your efforts to optimize context, offers, urgency, and other elements that convey meaning are more likely to pay off by getting people to read your messages -- which is the ultimate goal of your subject line. Whatever your campaign is designed to accomplish, from clicks to purchases, it cannot succeed unless your messages are read.

One cautionary note before you decide to completely ignore subject line length and start writing especially long ones: Know where your email is being read. If it’s mostly on phones or anywhere that might truncate longer subject lines, use common sense and distill your message to something that reads well in those environments. Also, it’s worth citing one more statistic from our quest for the perfect subject line length: the really long subject lines, containing more than 100 characters, had -- by far -- the worst read rates: less than 10%. Coincidence?