We Studied 200k+ Emails. Here’s How to Write Subject Lines that Actually Work.

Have you wondered how many things should go in the right direction for your prospect to open a cold email? 

From an SDR’s side, several factors could impact the chances of prospects opening your cold email, like the subject line, what kind of prospects you’re reaching out to, the credibility of your organization, the email address, the time at which the email reaches the prospect or whether it even lands on their primary inbox.

Among these, having a good subject line is one of the most important factors in getting prospects to open the cold email. A study shows that 33% of recipients open an email based only on its subject line.

Since we know that you have a lot on your plate as an SDR, we took it upon ourselves to discover how you can write better subject lines for your cold emails.

We studied 2,344 unique subject lines from the thousands of cold email campaigns that comprised over 255,000 emails sent using Klenty. Here, we’ll share our study’s findings and examples to inspire you to craft subject lines that boost your open rates.

Here are 5 data-backed ways to write better subject lines

Image of 5 data-backed tips to write better subject lines

1) Include numbers in the subject lines 

We found that sales emails that had numbers in the subject lines had an average open rate of 20%. On the other hand, emails with subject lines didn’t have an average open rate of 12%. 

Image of reminding to include numbers in subject lines 

According to the Nielsen Norman Group, numbers catch the digital readers’ eyes. Why? Because we love data. And data points are usually represented in numerals. So, when people scan different subject lines in their inbox, a number is proven to get their attention.

Here are a few examples:

  • 26 seconds (or less).
  • It takes 2 minutes to get started… and we can save you time!
  • 3 Secret recipes to meet {goal}
  • 3 days until X expires…
  • 2 actionable tips…
  • {Prospect name}, 1 question…
  • 3 secret recipes to unlocking {problem}
  • [Download] 3 new resources
  • Guess who ranked us #1…
  • Tickets for {event name} selling out in 3,2,1…

2) Have currency values in subject lines

Emails that included currency values in its subject lines had an average open rate of 29%, while the subject lines that didn’t had 13% average open rate.

Image of currency values in subject lines

While having currency values in subject lines boosts open rates as per our study, you have to be careful as using a currency sign alone could potentially trigger spam filters. 

Here are a few examples:

  • Unlock your contract for £9.95!
  • [Case Study] $25,000/mo in recurring revenue
  • Top 10 under $10

3) Have a question mark 

As with including numbers, we found that emails with question marks in subject lines had an average open rate of 20%, while emails with subject lines without question marks had an open rate of 12%. 

Data Image presenting the open rates having a question mark in your subject lines.

Questions immediately arouse curiosity in your prospects’ minds, prompting them to open your email. Remember that the question you ask in the subject line must be relevant to the prospect. 

Here are a few examples:

  • What makes a good story, {Prospect Name}?
  • Guest post?
  • Coffee date?
  • Getting back in touch?
  • How do you get your team involved?
  • Had questions on {name of blog}. Can we connect?
  • How to reduce employee turnover this year?
  • How are you dealing with {Pain Point}?
  • Thinking about {specific goal}?
  • Are you making these mistakes?

4) Use emojis

Using emojis also had results similar to numbers and question marks. Those emails with emojis in subject lines had an average open rate of 20%, while emails with subject lines without emojis have an average open rate of 12%. 

Data Image presenting the open rates having emojis in your subject lines.

While using emojis in subject lines can indeed be attention-grabbing, use your discretion while deciding to use these as not all prospects might appreciate these little icons. A Nielsen Norman Group study says that emojis in subject lines could potentially diminish your email’s (and possibly your organization’s) value in your prospect’s eyes. So, use an emoji only if you’re sure it would truly add value to your subject line.

Here are a few emoji subject lines examples:

  • {Prospect Name}, the beginning of a new challenge? 🔥
  • {Prospect Name}, it’s the end? 😿
  • 😱 {Company Name} & one-click integrations!

5) Personalize subject lines using prospect’s name

Personalizing the subject line using the prospect’s name is perhaps the most common way of personalization as it is the easiest data to collect. And it is effective in boosting open rates for a reason: People naturally become curious when they see their names in unexpected places, like a cold email that lands in their inbox, so they tend to open the email.

Image of personalize subject lines using prospect’s name

Our analysis found that by personalizing the subject line with the prospect’s name, the average open rate goes up to 39%. On the other hand, emails that do not contain the prospect’s name (but might still contain other kinds of personalization) have an open rate of just 10%.

Pro tip: Go beyond first-name personalization to see the best results. Consider other factors like your prospect’s location, pain point, the tool they use, favorite ball game, etc., while crafting your subject line. 

Here are a few examples:

  • Stop, {Prospect Name}.
  • {Prospect Name} x {Company Name}
  • {Prospect Name}, We Missed You!
  • {Prospect Name}, if you’re a rockstar recruiter, let’s talk.
  • Check it out, {First name}
  • {Prospect’s Name}, here is a personal note
  • {Prospect name}, congrats on your promotion!

Less is more: how one-word subject lines can still be effective

Here’s a surprising data point we found. * drum roll * 

Emails with one-word subject lines have an open rate of 34.47%!

According to Marketo, the optimum subject line length is 7 words. But, one-word subject lines can still be effective with prospects. Here’s an example of a firm reporting an increase in their website traffic by 83% by using a one-word subject line, which managed to garner an above-average clickthrough rate.

Image of how one-word subject lines can still be effective

While a subject line with only one word can make the prospects curious, it runs the risk of triggering spam filters. Coming up with a one-word subject line that could get your prospects to open your cold email but doesn’t trigger any spam filters could be a fun challenge!

Here are a few one-word subject lines examples:

  • {Prospect Name}…
  • Reconnecting
  • Subscription
  • Re:
  • Contributor?


The above findings will help you develop innovative approaches to subject lines and boost the open rates of your cold email campaigns. But, keep in mind that no trend in sales is set in stone and will work forever. So, take our data-backed findings and test them using A/B testing or other tools to see which of these works best for you.

Comments are closed.