5 Data-Backed Best Practices for Email Subject Lines

Woman looking at laptop for best email subject lines

Last week marked the official launch of the summer sale season for merchants targeting North America — and that means it’s prime time for sending sale emails!

To celebrate, we dove into the data to see what works best when it comes to nailing subject lines. We looked at 3,237 email campaigns sent to a total of 111,694,760 email addresses between Monday, May 29 and Thursday, June 1.

We dug into the data and found out a lot about what’s going on with email subject lines. For example, 168 campaigns used “Memorial Day” and got a 19% open rate, beating the average. And even one opportunistic merchant worked “covfefe” into their subject line — unfortunately, that one did not perform well.

But let’s continue with the data.

Across the board, here’s what we saw:

  • The average open rate was 17%
  • The average list size was 34,506
  • The average subject line length was 42 characters

The top five subject lines from last week were:

  • A Big Thank You!
  • We have never given this much off before!
  • The hamsters are working their magic!
  • Welcome to Death Sentences Reading Club

Then we went a little further and started looking at results by the size of the campaign (number of recipients campaign was sent to). By breaking our data down by list size, we were able to compare what big and small senders are doing and what they’re seeing for open rates.

List size Number of campaigns Avg. open rate Best open rate
100–1k 556 24% 83%
1k–5k 643 19% 79%
5k–10k 355 15% 62%
10k–25k 536 14% 53%
25k+ 871 14% 49%

Let’s take a look at the 5 best practices we found — starting with the relationship between audience targeting and open rates.

Best practice #1: Send to a targeted segment of your email list

The first thing that jumps out from this data set is the inverse correlation between the size of the list and the open rate: the more people you send to, the less likely it is that any given recipient will open your email.

But is this just the law of large numbers at work, or is there something else going on here? We recently published our ecommerce industry benchmark report that showed how segmentation increases the responsiveness of your email list, so my first stop was to see how many top-performing campaigns in the 25k+ bucket were sent to targeted segments (as opposed to everyone on the whole list).

After all, we can see from the max open rates that some merchants were able to get nearly 3x the average response, even though they sent to a large population.

So it’s not too surprising to see that 10 out of the top 10 top performing campaigns in the 25k+ bucket were sent to segments.

9 out of 10 of those segments were “active” segments, meaning that the recipients have all engaged with emails from that company at least once in the past 30 days. Now this means they could’ve taken any of the following actions: opened email, clicked, purchased, or was active on site.

My favorite segment took it one step further and targeted active subscribers who might be interested in a pair of designer pants. The company did this by segmenting based on what people were viewing on their website. If they viewed the specific collection that the pants were in, they would enter into the segment. This segment was sent to 30k+ recipients and received a little over a 40% open rate.

Now that’s targeted.

Best practice #2: Keep subject lines short and sweet

Does keeping your subject line short and sweet really matter? It sure does. We took a look at the 10 lowest-performing campaigns in our data set and saw that, on average, their subject lines had an average of 82 characters — that’s right, nearly twice the average length of a subject line!

And among the 10 best-performing campaigns, the average subject line was 33 characters — a decrease of more than 20% from the average, and a whopping 60% shorter than the length of the average campaign in the bottom 10.

All told, there were 46 campaigns sent with subject lines under 10 characters — and the average open rate for those campaigns was 21%, a full 4 points above the general average for our data set.

Need some inspiration for concise subject lines? Here were a few of our favorites, all with open rates that outperformed the average:

  • Uh oh…it’s all over in 4 hours
  • Tick-tock! Sale ends at midnight
  • Everyone is wrong about “basics”
  • TGIT

Best practice #3: Don’t be afraid of exclamation points!

Exclamation points… love them or hate them, they’re a staple in ecommerce email subject lines. There were a total of 1,173 exclamation points across 3,237 campaigns in our data set. And as it turns out, the excitement paid off: the average open rate for this group was 21%, 4 points over the average.

Some of our favorites include:

  • Don’t Miss Out On This Major Deal!
  • It’s finally here!
  • We have never given this much off before!

Just be careful. Excessive use of exclamation points in a subject line can potentially trip up spam filters, especially if they’re paired with common words that spam filters look for (free, limited time offer, special promotion).

Best practice #4: Emojis for the win

Emojis for the win! Out of the 3,237 campaigns sent, 152 used emojis — and their average open rate was 20%, 3 points over the average.

So there it is, ladies and gentlemen — emojis work  ?

Some of my favorite subject lines with emojis include:

  • ⌛️Time is of the essence! Do Not Wait!⌛️
  • Which cat are you? ❤️??? (4 NEW tees!)
  • ⏰ Time to wake up! ⏰ (+NEW Star Wars tee!)
  • ? Be a hero! ? (+NEW Marvel tee!)

And not only are these fun, they’re effective. These 4 subject lines finished in the top 10 in performance, with an average open rate of 48%.

If you want to learn more about emojis, check out our guide: 3 do’s and don’ts of using emojis in emails.

Best practice #5: Test, test, test

Testing is a good idea, no matter the circumstance. Any of the best practices described above can and should be tested to help you better understand what resonates with your audience.

3 out of the 10 best-performing campaigns sent to lists of more than 25k tested their email sends, with the top performer getting an open rate of 62%.

And if you average the open rates from all three of the campaigns, you get an open rate of 39%.

My favorite A/B test came from a toilet paper company. They tested their subject line to see if an emoji would have an affect on the open rate:

  • A: Wipe with us at work! ??
  • B: Wipe with us at work!

Can you guess which one won? The one with the toilet emoji, of course!

The bottom line

Now I’m not saying you’re going to see a 62% open rate right out of the gate. But if you try out some (or even all) of these five best practices, you’ll be giving yourself a head start to achieving a higher open rate and better engagement.

10 out of 10 of the top senders were segmenting their list. If you’re sending to an engaged segment of your email list, you’ll immediately see an increase in your engagement.

You only have a short amount of time to grab your reader’s attention, so keep your subject lines on the shorter side.

Exclamation points and emojis work — just don’t overdo it. And most importantly, test, test, test. Your audience is always changing — and there’s always room to do better.


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