Does Subject Line Length Matter?

Last updated: 
July 28, 2022

The issue: When it comes to Subject Lines does length matter?

What is interesting about the issue of Subject Line length, is the question is very rarely framed in that way. It usually starts with “evidence” that shorter Subject Lines are better than long ones.

Back in 2007 almost everybody in the industry seemed to have convinced themselves that shorter (defined by the number of characters in the Subject Line) is better. This was driven by concern that long Subject Lines would not be seen in full because longer ones got truncated by the email client. The emergence of smartphones as the primary device for email arbitrage only reinforced this belief. However, it just didn’t make sense to us. If they were right, then this Subject Line:


would always beat this Subject Line:

90% off all New Season Nike Apparel, iPads, and Rolex watches available online today.

We decided to take an in-depth look at the data, first in 2008 and again in 2015, and found the “shorter is better” argument to be false.  In fact, an article by Econsultancy, Four Factors That Can Impact the Success of Email Subject Lines, agreed with one of our key findings that Subject Lines over 60 characters generated higher open rates than those between 21 to 60 characters even though the end might have been cut off when viewed on a smart phone.

Over the years, we’ve continued to hear this question, so we decided to revisit the issue using the huge amount of data about Subject Lines and their performance held within Subject Line Pro, a virtual testing tool that delivers real, accurate results in seconds.

Length Does Matter – and longer is (almost) always better

While it is true that this is a complicated question with no simple answers, what we see in the data leads us to the conclusion that longer tends to be better than shorter if you take meaning into account and there IS a causal relationship between Subject Line length and performance.

The only way to conclude that length has no impact on Subject Lines is to approach it like a machine or someone looking at a language using an alphabet they had never seen would and assign no meaning to the characters and by definition spaces in the Subject Line like these below:

• You need to get a life :)

• [If you are still looking for hidden meaning in these Subject Lines then you really do need to get a life and probably deserve beer and brisket

• Voted best steak restaurant in New York City steak, lobster, cocktail & $50 win chip, $20

• A drink for everyone

However, if you focus on how 2 things – how MUCH information the Subject Line is attempting to convey and ambiguity (the proposition unclear or requires the email to be opened to know what exactly is on offer) you come up with a very different understanding. We call every piece of information in the Subject Line a proposition. So, a single proposition unambiguous Subject Line would look like this: 

Tennis shoes 50% off today!

An ambiguous single proposition Subject Line would look like: Summer starts here!

Multiple proposition Subject Lines look like this one from GILT the masters of the multiple proposition: 

Last Chance for Extra 30% Off | Jonathan Simkhai, Christian Louboutin Shoes, Jetsetter Must-Haves and More Start Today at 8am ET

A few things to note:

• Multiple proposition Subject Lines contain more info and are naturally longer

• Single propositions are naturally short

• Multiple propositions are less likely to be ambiguous or misleading to the recipient

• Making a single proposition long for the sake of it is not worth the effort

• Making multiple propositions short is only possible if the recipient is very knowledgeable or the concepts are widely understood

The fact that single propositions are usually short and multiple propositions usually longer may explain the dead zone identified in the Econsultancy article and our research, but broadly speaking VERY short is good, but longer is generally better especially for clicks if you use the extra characters to pour more meaning into the Subject Lines.

The exception is when they are ambiguous. So very short Subject Lines tend to give you very high open rates because the only way the recipient can work out what to do with the email is open it.  This explains why Subject Lines like WOW! or OOPS work so well for opens unless you do it all the time. When it comes to clicks though short ambiguous Subject Lines usually perform poorly and may be perceived as click bait.

Right-size your Subject Lines (and email team).

Size matters when it comes to Subject Lines. It can also make a difference when creating and managing omnichannel campaigns. Making our digital marketing experts part of your team can significantly improve your results while enabling you to run a lean, mean internal team.

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What does the Subject Line Pro data tell us?

Subject Line Pro contains data from billions of emails sent and billions of Subject Lines under analysis from lots of different clients and sectors, so it’s a great source of insight about subject lines.  While writing the algorithms that take the amount of information within the Subject Line into account AS WELL as tone/emotion/length etc. was a real challenge, it was worth it because it makes the tests results much more accurate. When we normalize the data according to each sender’s average engagement rates, here’s what we see:

This chart shows the relationship between length and open rate for all Subject Line types and does not take how much information into account. The gray bars are the percentage of Subject Lines of that length within the database. So, as you can see the vast majority of Subject Lines sent are short. Our data matches fairly well with Econsultancy: short and long subject lines are better, and ‘medium’ subject lines perform worse.

• The average Subject Line is much longer and there are very few Subject Lines of 30 characters or less containing 3+ pieces of information

• Long Subject Lines are much better than short

• There are so few "less than" 30-character Subject Lines, the margin of error on "less than" 30-character Subject Lines is large so that big spike at the left of the chart isn’t to be trusted.

Here is what the data tells us about single proposition Subject Lines:

Somewhat surprisingly for single propositions very short is good - after that (over 25characters) longer is still slightly better.

Here are the 2 charts overlaid:

Whichever way you look at it one thing is certain, YOU HAVE TO TEST. Decision making without testing is just guesswork. Contact us today to learn how Subject Line Pro can help you improve your email testing program.

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