Defined. The correct length for a subject line.

Is there really a best length for an email subject line?

Yes, there is an answer to the question of what is the right length for a subject line. In the words of Deep Thought, "you're probably not  going to like it". Nonetheless, I am going to tell you.

In case you don't know, Deep Thought is the computer in Hitch Hikers Guide to the Galaxy, built to answer the ultimate question of "what is the meaning of life, the universe and everything".

Deep Thought was the most powerful computer ever made and took some millennium to work out the answer, it is after all a difficult question. Needless to say great expectation and excitement built up around the day the answer would be given.

On that day Deep Thought announced there was indeed an answer, but warned that his makers would probably not like it. Deep Thought pronounced the answer as the number 42. Naturally this was a confusing and disappointing answer. Deep Thought proposed the problem was one of not actually understanding the question to which the answer is 42. The search for the ultimate question to which the answer is 42 began.

No, I'm not going to say the perfect subject line is 42 characters. It is with some trepidation I am happy to give the answer to the perfect length for a subject line. The length is the length needed to create the most powerful message possible that results in the desired marketing objective. Typically that objective is sales conversion (not email opens).

Trying to define the perfect length as a fixed number of characters is nonsense. The subject line length is simply the result of the best set of words. The length is the output of the words selected, not the input to selecting the words.

These subject lines and others like them have all done well, although they different in length considerably.

• 50% off sale
• Tight, taut and tuned!
• We've missed you, so here's an extra special discount from us
• Get 15% off in our summer range, new Nike and Reebok favourites

There has been a lot of published research into subject line length. This research shows that both short lines and long lines can work. The often heard myth that subject lines must be short does not stack up, long lines can work better. But that is not the same as saying that long is always better, or indeed that short is always better.

The way our brains read and react to subject lines goes deeper than evaluation purely on length. Whilst getting a message across quickly is good, the words are more important than the length. If an extra three words give more power and clarity then it’s not going to be a problem to add them.

Whilst not so often talked about as length, word order is important. The first words get more focus. Say a subject line mentions two products, the order in which they are mentioned can change what gets clicked in the email.

Can we define rules around the words to use? There are certainly some words that are often powerful such as; free, new, save, % off and exclusive.

But how about evaluation of a complete subject line? I tried using Google Prediction tools to build a model that could predict a good subject line based on the words. Testing using historical data found it was not reliable. You may be aware of the subjectline.com tool which will score a subject line, however, before you rush over and try it out read on.

The webpage for that tool says "results from this tool are based on 1 billion+ email messages and over 400 unique rules are tested". I tried "The weather today is great", it scores a perfect 100. I tried 15 sets of subject lines that had been previously split tested to see if the tool could predict the winner. It got 8 of 15 correct. That's 53%, or not much better than a random guess.

One way to evaluate a subject line is to consider how well it addresses each of these attributes, from the perspective of the reader:

• Clarity
• Appeal
• Benefit
• Exclusivity
• Credibility
• Relevance

As a mental exercise take a subject line and score it for weak, OK, strong in each of these attributes.

Considering the previous examples, "50% off sale" scores well for benefit. It's not strong on clarity and relevance since it says nothing about what has 50% off. Whereas "Tight, taut and tuned!" has no clarity but strong appeal in the form of intrigue. It was also a very on brand message.

A subject line doesn't need to have all of the above attributes, but scoring well on 3+ of these will help success.

And of course, don't forget get the ultimate way to know what's best is to A/B test your subject lines...