Subject Lines: What Our Latest Research Tells You


Karolina PetraskieneContent Marketer at Omnisend

Thursday, November 15, 2018

When it comes to email marketing, subject lines are the backbone of a successful campaign. Without a good subject line, your emails won’t be opened, and your efforts will be wasted. So how can you optimize them for maximum impact?

Article 5 Minutes
Subject Lines: What Our Latest Research Tells You

Sifting through our customers’ email data from 2017, we stumbled upon some really interesting discoveries.

From over 128,000 emails sent by 7,200 brands, this data gave us the opportunity to put forth and answer these four fascinating email marketing questions:

  1. What effect do exclamation marks have on conversions?
  2. Is it better to offer discounts as a percentage or dollar amount?
  3. Does personalization have any impact on open rates?
  4. What’s the optimal length for a great subject line?

Here’s what we found out:

Question 1. What effect do exclamation marks have on conversions?

So, how did we arrive at this dilemma in the first place?

We were compiling our best performing email subject lines – 30 in total – when we noticed that 14 of the top 30 contained exclamation marks. Now, that has touched the curiosity nerve.

We began to wonder if including exclamation marks in subject lines would have a tangible effect on email open rates and eventually, on conversions. So we broke down the emails into two natural categories - automated workflows and newsletters - and dug deeper.


From all the regular newsletter campaigns, 42% of the subject lines contained at least one exclamation mark and 52% didn’t.

We saw that subject lines with exclamation marks had a 17% open rate, while those without exclamation marks enjoyed an 18% open rate – a full percentage point better open rate on average.

We looked even deeper, checking whether the open rate changed with a higher number of exclamation marks in a subject line. This has led us to discover that the more exclamation marks you use in a subject line, the lower your open rate is likely to be.

Subject lines containing one exclamation mark average a 17.5% open rate. Those with two exclamation marks have a 16.7% open rate, while three exclamation marks lower the open rate even further to 16.5%.

Automated workflows

Surprisingly, 95% of all automated emails that Omnisend customers send out contain exclamation marks.

Even further, automated email lines containing one exclamation mark achieve an open rate of 29%, while those with two or more exclamation marks boost the open rate to 35%!

However, this shouldn’t be taken at face value, as there are quite significant differences between the number of subject lines with just one exclamation mark vs. those with two or more.

When we broke the emails down into single-email automation workflows vs. workflows containing a series of emails, we came across even more interesting results.

Single-email automation

A single automated email with a subject line that contains one exclamation mark had a 47% open rate and those with two or more exclamations marks had a lower open rate of 42%.

In other words, it’s better to include only one exclamation mark in a subject line for single-email workflows.

A series of emails

Similarly, automated emails with one exclamation mark in the subject line achieved an open rate of 24%. Those with two or more exclamation marks had an open rate of 21% on average, demonstrating a negative effect of a higher number of exclamation marks.

Like with single-email automation, it’s best to avoid adding more than one exclamation mark.

Question 2. Is it better to offer discounts as a percentage or dollar amount?

Have you ever wondered how differently people respond to seeing discounts in dollar signs (enjoy $15 off) vs. percentages (get 10% off)? The fact is, these two methods generate different results.

So we looked at the data to determine the impact of discounts in dollars vs. percentage on email open rates.

What we discovered is that there are 10x more subject lines that employ the % sign compared to those using the $ sign. However, the $ sign fares better when it comes to open rates.

Subject lines containing the $ sign had a 29% open rate and those with the % sign had a 25% open rate. That’s a 4% difference.

Question 3. Does personalization have any impact on open rates?

When we talk about personalization in this context, we mean including the recipient’s first name (or sometimes their full name) in the subject line.

Although personalization is all the rage among eCommerce marketers at the moment, we wondered whether personalized subject lines would have any real effect on people’s decision to open emails.

It’s a subject that can quickly divide marketers into two camps: those who argue that personalization is the only way forward vs. those who believe subject line personalization has become too spammy to garner any benefits for brands.

So we looked at the data for answers and found a few surprises: only 3.5% of all emails sent contained a personalized subject line.

When comparing the average open rate for those emails, we discovered that personalized email subject lines only generate 0.2% increase in open rates (18.1% personalization open rate vs. 17.9% standard open rate), which has no statistical significance.

Question 4. What’s the optimal length for a great subject line?

How long should a subject line be?

Looking at one of our top performing brand’s emails, we’ve noticed that they favor long subject lines. For instance, the example below has 213 characters in it:

“COMING SOON: Adidas Originals Equipment Support EQT "White/Turbo-Red" Pack & EQT Support BOOST Ultra "Chinese New Year" / Stussy - Spring/Summer '17 Collection / New Balance - 247 & Sport Style Clothing Collection”

Phew! That’s a very long subject line. So does it actually work?

We decided to compare the most popular subject line length (11-50 characters long) with much longer subject lines.

As the graph below demonstrates, longer subject lines do not deliver better results.

Subject line length

Although the subject lines with character counts of 51-90 have a 40.5% average open rate compared to the common character counts of 11-50 that achieve an average open rate of 33.8%, their sample size is much smaller.

The data tells us that subject lines between 21-30 characters have the best open rate, and they also happen to be the most popular option.

Karolina Petraskiene

Karolina Petraškienė is a blogger and email enthusiast at Omnisend, the ecommerce marketing automation platform dedicated to helping ecommerce stores build strong, lasting relationships with their customers.


Join the conversation...