What’s a good email subject line that can flawlessly carry out desired messages? Well, you might say it depends, but our research shows certain rules should be observed in order to write the best subject line.

Omnisend checked every email sent in 2017, and after combing through the data we fleshed out 4 main questions to ask concerning subject lines.

  • Does personalization really affect open rates?
  • Do exclamation marks help or hurt conversions?
  • Should you offer discounts as percentages or dollar amounts?
  • How long should your subject lines be?

1. Does personalization have any effect on email marketing open rates?

When we talk about personalization, we mean adding the recipient’s name in the subject line (usually just the first name). We wondered whether personalized subject lines would catch recipients’ attention and encourage them to open emails more often.

The data was surprising: out of all emails sent, only 3.5 percent contained a personalized subject lines.

When comparing the open rates, we see personalized email subject lines have only a 0.2 percent increase in open rates (18.1 percent personalization open rate vs. 17.9 percent non-personalized open rate), which is not statistically significant.

Therefore, the data seems to show that personalization has no real effect on email marketing open rates.

2. Do exclamation marks help or hurt your email marketing conversions?

We took a look at 30 of our most successful email subject lines for newsletter campaigns and realized something interesting – 14 cases contained an exclamation mark. That’s almost half.

We began to wonder if including exclamation marks in subject lines would have any impact on the open rates. We broke this down into the two categories: newsletter campaigns and automated email workflows.

A. Campaigns

For regular newsletter campaigns, 42 percent of all emails sent had subject lines with exclamation marks, and 58 percent had none. We saw that subject lines without exclamation marks had an 18 percent open rate vs. 17 percent for subject lines with exclamation marks – a full percentage point better open rate on average.

We broke it down further to the amount within subject lines. Data shows the more exclamation marks, the lower your open rate will likely be. Average open rates for subject lines with 1 exclamation mark is 17.5 percent. It goes down to 16.7 percent for 2 exclamation marks, and finally 16.5 percent for 3 or more exclamation marks.

B. Automation workflows

95 percent of all automated emails that Omnisend marketers send out contain exclamation marks. That is one marked difference between automation and newsletter campaigns. In total, automated email subject lines with 1 exclamation mark have an open rate of 29 percent, while 2 or more raises that open rate to 35 percent.

We discovered even more interesting things when we checked out this dynamic between sending single emails and series of emails.

Sending one email. Automated emails with 1 exclamation mark had a 47 percent open rate, but only a 42 percent open rate for 2 or more exclamation marks. Therefore, this data suggests that for single-email automation workflows, 2+ exclamation marks in your subject lines will have a positive effect on your open rates.

Still, it’s worth noting that the number of subject lines with 2+ exclamation marks are about 0.7 percent of those subject lines with exclamation marks.

Sending a series of emails. Automated emails with 1 exclamation mark had an open rate of 24 percent. However, this time 2 or more exclamation marks had a negative effect on open rates, as those resulted in only a 21 percent open rate on average. Therefore, when you have a series of emails in your automation workflow, 2+ exclamation marks will bring down your open rates.

3. Should you offer discounts as percentages or dollar amounts?

The question here is simple – does it sound better when you get a $15 discount or a 10 percent discount? We decided to look at the data to see how open rates differ for subject lines containing the dollar ($) sign vs. those that contain the percentage (%) sign.

10 times as many email marketers used the % sign in their subject lines vs. those that used the $ sign. Nonetheless, the $ sign fares better in terms of open rates.

Subject lines containing the $ sign had a 29 percent open rate on average, compared to the 25 percent open rate for subject lines containing the % sign. Therefore, using dollar signs has a bigger impact on recipients.

4. How long should your email subject lines be?

We always thought that shorter subject lines should be the preferred standard. This actually came to us as a surprise, but we noticed that Adidas likes using pretty long subject lines. This one has 213 characters:

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

Naturally, we wondered what affect the length of subject lines has on open rates. According to our email marketing data, the best number of characters to use is 21-30, followed closely by 61-70 and 81-90. Subject lines between 21-30 characters also happen to be the most popular within our system.

A chart of Omnisend's research regarding total sent vs. open rate


While there is no cookie-cutter solution on how to write a perfect email subject line, we have noticed certain tendencies that could help companies produce more captivating and productive subject lines to reach desired email marketing goals.

The bottom line is that subject lines should continue being relatively short and compact, preferably avoiding exclamation marks and percentage signs when talking about money. Data also shows that subject line personalization through using first names doesn’t bear any significant increase in open rates.

This is a guest blog post provided by Omnisend. Although Three Girls Media was compensated to publish this guest post, it meets our editorial standards and provides value to our readers.

Special Offer:

Sign up for a complimentary consultation during July and receive a Social Media Management Guide valued at $475!  We offer a 30-minute phone consultation with our CEO, and can answer your questions and discuss your specific marketing needs - no strings attached. Call 408-218-2391 or contact us today to arrange your consultation!