{Email} Marketing Campaign Subject Lines: Length

I mentioned in my intro to Subject Lines, {Email}’s Oldest Topic That No One Agrees On: Subject Lines, that there are a ton of references, tests, best practices, and professional/expert opinions on what’s best and most effective, and I also mentioned that most experts and tests don’t agree with each other. Optimal subject line length, while in theory a simple topic, is one which has email marketing experts running in circles.

I believe that optimal subject line length is a topic that isn’t necessarily something you can test for – at least not practically, because of the wide range of variants that can alter test validity and therefore produce skewed results. As an example, if a sale offer is sent this week to a target to run an A/B test splitting the target in equal parts (50%/50%), testing two different subject line lengths, the results might be dependent on the nature of the offer, the products the offer applies to, the target it was sent to, the day/time it was set, etc.

Ok, let’s run a second round of this test, and attempt to minimize variables. Let’s send the same test to the same target using the same offer and content, but we’ll send it two weeks later, using the same two subject lines, but swapping the subject line used per segment, so those that received A previously will receive B this round, and visa versa. Smart. But, is it? If you’re sending the same offer, can you expect conversions to be on par to the offer sent two weeks prior? Considering it’s the same target, you’ll likely see a drop in conversion count and rate, as those likely to convert may have converted the first time around. There is literally no way to run a second test without some skew.

Ok, we’ll run a hundred tests instead. That’ll work, right? Nope. While you will reduce your variance and minimize impact of outliers by increasing test count, you simply cannot derive finite results. I’m sorry.

So, then what? Don’t worry, and don’t overthink it.

Some reports have “determined” that the optimal subject line length is 6 words, and others may say the perfect number is 7. While I don’t doubt the validity of the tests and results, I do question the applicability and relevance of their findings because setting a word-count target is fairly arbitrary and can produce some truly garbage subject lines from people believing their emails simply won’t be opened, or won’t be effective, if they use 6- or 8-word subject lines. You can become so focused on meeting the word count that you compromise the quality of your work.

Other reports may tell you 30-character subject lines work best, while some may say 60’s the way to go. And, I’m sure they’re all 100% correct. Yes, every one of those conflicting reports could be 100% correct. I believe there is literally no way to determine finite, concrete best practice, and I believe that any “perfect” recommendation is only as accurate and effective as the target, sender, and subject matter it references.

Instead, my recommendation is really quite simple: write your subject lines in the length that allows you to best inform your reader of what’s inside the email.

If you want to press, if you want to force a number out of me, or if you want a baseline to work from, I’d say that subject lines should be roughly 35 characters in length. No, this is not an arbitrary number. And, no, I have not exhaustively tested this character count. And, no, I do not think my lack of exhaustively testing (which is completely impossible) flaws either of recommendations.

Why this recommendation, then? Common analysis reports indicate that at least 50% of emails are opened on mobile devices. And, Apple iPhones are the most popular, or, at least, are the most common. And, most mobile devices display roughly 35-50 characters in the subject line of both native and third-party mobile email applications, dependent on settings (zoom, etc.), view mode (portrait/landscape), etc. So, if you shoot for 35 characters, and hit it, your entire subject line will be seen, and hopefully read, by most of your target. Go over 40, and you start to lose some of your message, and with it, some of the impact.

“Ok, whatever. I write really long subject lines, and I get more people to open my emails because people need to open them to see what the subject line says.” Actual response to my recommendation in the past. I believe that person was previously a used car salesman who listed all their cars priced at “Call for price” or “TOO LOW TO LI$T”. No need to waste the time of your subscribers, as that long subject line might belong to the last email you are able to send them before they unsubscribe. Tell them what’s inside, don’t make them take an action to find out your car prices are really just 15% over blue book.


If you have any insights on subject lines, please let me know in the comments below – would love to incorporate more perspectives.

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