There is much advice about subject lines out there – the key is to know what the trends are… and to see how they match up to your email marketing intuition and email monetization strategies.

A study by Return Path found that open rates were 12.5% higher where subject lines contained less than 49 characters, compared to those that were 50 characters or more, while click-through rates were 75% higher for the shorter subject lines. Pinpointe and Net Imperative also found this to be the case, with shorter subject lines having slightly higher opening rates and much higher click-through rates.

Other studies have also found that open rates of 35 characters or less have higher open and click-through rates. However, a caveat to all of this – the majority of this research is cross-sector and focused on large pure play retail senders, and therefore may not be transferable to your business.

However, Alchemy Worx found that although emails with subject lines less than 60 characters long were the most opened, click to open rates were optimised by subject lines with over 70 characters. Click to open refers to the percentage of people who click on an email once they have opened it. As a result, subject lines 60-70 characters in length fell into a “˜dead zone’, where neither open nor click to open rates were optimised. Mail Chimp again found that subject lines of less than 50 characters were more effective, unless they were being sent to targeted audiences.

It is clear that prevailing research indicates that either very short or very long subject lines are optimal, but there are certainly some variables around this trend. Does our 2012 research follow suit, and how are different industries affected?

What is the optimal number of characters for your subject line?

The majority of the research that has been conducted into subject line length has been focused on high street retailers – so, massive campaigns (lists of 500k+) with similar purposes. However, we were curious if the same trends held for all of our clients, no matter what size their list is. The results are fascinating, to say the least.

When only considering open rates, it appears that either very short or very long subject lines deliver better results. However, when also considering the click-through and click to open rates, there is strong evidence that longer subject lines are more effective. They drive opens and importantly also lead to action being taken once the email has been opened.

Following our research, our friends at Mailchimp also posted some of their data in an excellent blogpost. They found (interestingly) that for a lot of their clients (something over 6 billion of the 12 billion, a bit more than 50% of their data) the subject line length shows no correlation with open and click results. What about the other 50%?

What have you found to work best for you? Let us know in the comments below.