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What is the optimum email subject line length?

Take a moment to think about everything you think you know about email subject line length.


Now, chuck it all out the window.

Why? Because everything you think you know about the ‘best’ length for an email subject line just might be wrong.

We here at Phrasee have been studying subject line science for quite a while. We’ve analysed literally millions of email subject lines and tracked their performance over time, we’ve even put our awesome AI technology to the task.

What have we learned?

Some things can’t be quantified quite so easily. Sometimes things just aren’t as simple as “best length”. When it comes to something as complex as human communication and language, it just… well, it depends. 


Conventional wisdom says subject lines should be short.

Image credit: NBC


“Brevity is the soul of wit” and whatnot.

It’s not the worst advice in the world, but it’s important to remember that human language is complicated, and that what you’re selling is complicated too.

Some email marketing blogs and advice go beyond conventional wisdom and gut instinct and show you numbers. Charts, even. Ooh! Numbers that come with percentage signs and hyperlinks – shiny! They must be true!

You’d think that would be getting us closer to the elusive truth about email subject line length. Sadly, this isn’t the case. Enter: bad statistics. When tested scientifically, such numbers rarely hold up. Why do we think that? Well, if you read enough of those blogs, you’ll start to see that not only do they contradict each other, but sometimes they even contradict themselves.

As we at Phrasee developed and implemented sound scientific methodology to solve the mystery of the optimal subject line length once and for all, confidence abounded. Surely we would crack the code and be lauded across the email marketing sphere by those who had waited ages for the answer. We would become legends!

But it wasn’t to be.

For one thing, Some cheeky monkeys already beat us to the punch all the way back in 2012. For another, science wasn’t prepared to give us the definitive answers we were looking for.

Alas, such is the nature of science.

But that doesn’t mean we didn’t learn anything


What does science say about email subject line length?


The science on email subject line length is…  inconclusive.

The mathematical correlation between subject line length and subject line performance doesn’t exist – statistically speaking.

Does this mean that subject line length doesn’t matter at all?


It just means that when it comes to email subject line length, things can get complicated. The only honest answer to the question “what is the optimum email subject line is: “It depends”.

Now we know what you’re probably thinking. “If the answer to everything is “it depends” how can any of us ever hope to get it right? If they’re so clever and they’ve gone through the trouble of debunking sham statistics and they still don’t have the answer, what hope is there for any of us?”

Fear not.

There are better things to focus your attention on.


Which variables matter more than length when shooting for better email subject line performance?

Image credit: Nickelodeon


There are six major variables that affect email subject line performance that every email marketer should bear in mind when writing a subject line. They are:

  • Sentiments
  • Individual word choice
  • Language sophistication
  • Emojis
  • Diversity (which we like to call “Phreshness”)
  • Word count.

You’ll also need to consider your brand’s unique tone of voice, which should match with your overarching brand voice and style. There are also several things to avoid when crafting your stellar subject line, like using spam words, copying your competitors or making false promises.

Like we said, it’s complicated.

But for us here at Phrasee, “complicated” isn’t sanctimony, it’s the solution! We work with massive data sets and intelligent AI to parse through all of those variables and find, not the mythical optimum subject line standard for all, but the best options for you and your brand.

All things considered, there are better things to worry about than subject line length.

Because, at the end of the day, what works or doesn’t work will all depend on your goals and your audience.

Instead of betting the whole farm on one of many variables, we think there’s a better way to the ideal subject line – subject line science!

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Trevor Beers, Senior Language Technician, Phrasee

Phrasee pheatures: Trevor Beers

Our very own Senior Language Technician, Trevor Beers, shares advice for those looking to join the tech industry, and insights into life at Phrasee.

Discover how applying actual AI to language can deliver big results!