Marketing | 5 min read

What everyone ought to know about subject lines


In most things in life, roughly 80% of effects come from only 20% of the causes.

80% of Warren Buffet’s wealth comes from just ten investments. 80% of software problems are caused by 20% of bugs.

It’s called the Pareto Principle, and it means that a small number of things will have a disproportionate impact. If you’re communicating with customers on a regular basis (if you’re not, read this) 80% of your success will come from those important-but-overlooked tasks – a clear focus, a personal greeting and, of course, a compelling subject line.

A quick scan of your inbox should remind you that startups tend to criminally underestimate the power of subject lines. If yours is anything like mine you’ll see that “Just checking in”, “Just a thought” and the mildly terrifying “I need to talk to you” are the status quo. With email like this, it really doesn’t really matter how good your product is, or even what’s inside the email. Your recipients will never get that far.

In the same way the best editors can spend half their time on an article just writing the headline (a blog post for another day), a significant amount of time should go into a subject line that drives opens, clicks and replies.

Ok, you get it, subject lines are important, but what ones perform best? Let’s find out.

Short is best…but exactly how short?

This isn’t the time for purple prose. This is email. You have a limited amount of words to play with so be clear and direct. When viewed on a desktop the average inbox will show about 60 characters of the subject line. But increasingly email is read on mobile devices – over 50% according to some estimates. In that scenario you can’t be sure that any more than 30-40 characters will be visible, which means you have less than 6-8 words to get to the point.

Based on analysis of emails sent in Intercom, our data bears this out. Emails with subject lines of 5 words and under have the greatest reply rates (which implies a message has been opened and likely read), while longer subject lines were correlated with lower reply rates.

subject line characters

Of course, it can’t just be any 5 words; you have to make sure they’re interesting too. Hillary Clinton’s campaign team achieved excellent results with simple and personal subject lines such as “I’d love to meet you”, “You and me, {name}?”, and my personal favorite, “Dinner?”. Which brings us nicely onto our next point.

clinton subject line

The power of a personal address

We’ve all become experts at ignoring messages that aren’t meant specifically for us. In the same way that banner blindness has trained us to ignore animated ads and over-designed pop-ups, generic, impersonal emails don’t get much of our attention either. When we know a message wasn’t written specifically for us we afford it less importance.

It’s said that a person’s favorite word is their own name, and it’s proven that adding the recipient’s first name to your subject line will increase engagement rates for most users (unless you’re a lawyer, that is), especially when used for targeted onboarding and retention emails. (In Intercom you can do this by using the “first name” attribute in your subject line)

Notion subject line

Unfortunately, greedy, lazy organizations have embraced first name personalization as a way to personalize as many emails as they can as cheaply as they can, relying on the law of large numbers. That’s why some of the best performing subject lines are tailored to behavior, not just to easily gleaned information such as your name.

For example, Airbnb saw great results with a series of behavioral subject lines, where each subject line was personalized based on the properties you had viewed. As you have absolutely zero leverage for negotiation inside someone else’s inbox, it makes a lot more sense to use subject lines that you know people will be interested in.

Airbnb subject line

Ask a question, get an answer

Adding questions to a subject line is one of those sacred cows of email marketing. It’s also why you see so many bad ones. Too many subject lines – “Were Jesus and his disciples the most successful startup ever?” – follow Betteridge’s Law of Headlines. If the answer is quite obviously no, your email is going straight to trash.

Good questions are ones your recipients can empathise with or would like to see answered. Marketing legend Drayton Bird is the the master of this, using subject lines that pique his reader’s curiosity: “How much are your customers worth?”, “Run your own business?” and my personal favorite, “What if your mother was blind?”

Closed questions like these can be extremely powerful but don’t miss out on using open questions too. Open questions are impossible for the reader to answer without opening your email. Here’s a good example from Groove’s CEO trying to re-engage customers who have slipped away.

Groove subject line

The golden rule…

There’s always the temptation to game the system with subject lines and copy that generate clicks rather than communicate clearly.

Credit: XKCD’s “Headlines

But there’s a law of diminishing returns with those kind of growth-hacking tricks and the effectiveness of your emails will ultimately decline as customers start to see through them. For example, add the words “Win an iPhone 8” to your subject lines and watch your open rates double, but there’ll be no significant change in behavior aside from recipient disappointment, which you likely don’t have a metric for.

So here’s the golden rule for subject lines – your subject line should describe the subject of your email. Can the reader tell from the subject line what you’re writing about without going further? If not, why are you insisting that they guess? You’re not Agatha Christie, and you’re not paid to surprise your audience, so whatever it is you’re trying to communicate, say it in your subject line.

The Messaging Starter Kit for Customer Engagement

Engage new users and convert them to active customers with our series of message templates.