[Reply Research] How to Write Better Subject Lines, According to Data

[Reply Research] How to Write Better Subject Lines, According to Data


Effective cold outreach is a form of art… with a great deal of science thrown in! 

If you’ve ever wondered why some emails drive results while others don’t, you’re not alone. Helping other sales teams ace outreach for the past 6 years, we at Reply have always wanted to crack the code and find a recipe for a perfect cold email. 

That’s why we’re happy to announce the launch of Reply Research, a series of studies analyzing different aspects of cold outreach based on first-hand data from millions of emails sent using Reply.

So, let’s slip into our lab coats and dig deeper into the data, starting with the hidden patterns and actionable insights for effective cold email subject lines

4 data-backed insights to help you write better subject lines

Just like every big brand has a catchy slogan, every successful cold email starts with a good subject line. It gives the recipients an idea of what’s inside that email while also triggering their interest to learn more.

Being the first thing someone sees when your email lands in their inbox, the subject line is also one of the main criteria they will consider before opening it. So here comes the main question: What makes a cold email subject line openable?

To figure this out, our data scientists have analyzed 712K first-step emails with unique subject lines against these 4 key parameters: 

  • length
  • letter case 
  • variables 
  • emojis

Let’s take a closer look at the results and what they mean for outreachers. 

Disclaimer: The average performance metrics for the analyzed emails that we will be using as the benchmarks in this research are 31.69% open rate and 3.02% reply rate. 

If your emails already outperform these results, congrats! But the sky’s the limit, right? Why not try to do even better? 😉

benchmark results

How to Write Better Subject Lines (According to Data)

Our collection of data-backed insights on effective cold email subject lines in a convenient PDF format

How long should your subject line be?

One thing I’ve always been curious to find out is: Could the length of your subject line influence the open rates?

On one hand, many people check their inbox on their smartphones. So it totally makes sense to keep your subject lines short enough to be displayed in full on mobile (usually, 25-30 characters).

On the other hand, does this really have a direct effect on your open rate?

The answer is yes! According to our data, open and reply rates vary based on your subject line length.

open and reply rate by number of words

  • Subject lines with 1, 2, 3, and 4 words get the highest possible open rate. While the difference isn’t that stark, it’s still pretty clear that the fewer words — the better.
  • Using longer subject lines with 8+ words will most likely lead to the lower open and reply rates. So, don’t get carried away!

subject lines results

  • If we’re being more precise, subject lines containing 3 words generate 3% more opens than the average. A perfect example of such subject line would be: {{My Company}} for {{Company}}?
  • On the other hand, emails with no subject lines tend to generate almost 36% fewer opens than average. Why? Probably because such emails are easy to overlook or seem suspicious.
  • Remarkably, the average reply rate declines if the subject line has 5 or more words, but an average open rate is the same whether you use 5 words or 11+ words in the subject line.

What’s the takeaway here? 

💡 Always keep your subject lines short, ideally no more than 4 words, (but don’t leave it blank!) to get the highest open and reply rates.

Pro tip: If you’re looking for some inspiration, we have 4 subject line formulas (+ examples) ready to use.

Should you capitalize your subject lines?

As any other SDR Leader, I am not just sending outreach emails, but also receiving plenty of them from other salespeople. Recently, I’ve started noticing an interesting trend with many subject lines containing only lower case words, e.g., “sales development consulting for reply.io” or “reply.io lead generation services.

Since it did catch my attention (and people seemed to be doing it on purpose), I got curious: Could the letter case used in your subject line really influence the open rate?

To find that out, we analyzed 3 variants of the same subject line containing:

  1. all lower case words → sales development process at reply.io
  2. most lower case words → Sales development process at Reply.io
  3. most upper case words → Sales Development process at Reply.io

Long story short, it could indeed influence the open rate, yet correlation isn’t that significant, according to the research 👇:

subject lines by letter case

  • The subject line containing all lower case words (variant #1) generated 3.46% fewer opens. The average reply rate here was lower by 11%.
  • Using the subject line with mostly lower case words (variant #2) didn’t affect our open and reply rates, keeping them close to the average (just some 0.22% higher).
  • For the subject line containing most upper case words (variant #3), the open rate was 2.46% higher and reply rate — 10.93% higher. This means we have the winner!

As you can see, every little nuance can make a real difference when it comes to cold outreach.

💡 So, if you’re looking to increase your open and reply rates, here’s a trick: Instead of coming up with the new subject lines, try capitalizing most words within the existing one and see what happens!

How many variables should you use in a subject line?

As you might know, I am really into personalization at scale. I myself got into experimenting with different types of variables in cold emails.

So, the next assumption I was eager to test was: Could adding more variables help me generate a higher open rate?

This time, we picked 4 subject line variants, based on the number of used variables, e.g.:

  • 0 variables → your sales development process
  • 1 variable → sales development process at {{Company}}
  • 2 variables → {{FirstName}}, sales development process at {{Company}}
  • 3 variables → {{FirstName}}, {{Company}}'s {{Year}} sales development

Here’s how the number of variables used in the subject line correlated with the average open rate.

variables in subject line

  • The subject lines containing 2 and 3 variables turned out to be the top performing ones, generating more opens than the average (by 4.04% and 12.15% respectively). That’s the magic of hyper-personalization 🪄!
  • Interestingly enough, the subject lines with 0 or 1 variable had almost the same open rates. 
  • But what’s even more surprising, the reply rate for the subject line containing 1 variable (by far the most popular option in my experience) would be 6.6% lower than the average. Probably, because this kind of subject line looks pretty generic. 

Based on this data, we can assume that it’s generally OK to send cold emails with no personalized variables in the subject line as long as the messages are relevant. In other words, you could use a simple subject line without any variables when reaching out to a small audience segment or prospects matching a specific ICP if you tailor it to their interests or needs. 

For example, if you’re reaching out to the sales leaders currently growing their teams, you can go with “Noticed you are hiring new SDRs” for the subject line and still have them open your email because it’s relevant to them.

💡 If you’d like an extra touch of personalization (and a few more opens for your emails), go ahead and add two or more variables to your subject line!

Should you use emojis in the subject line?

Despite selling B2B, we always try to build and maintain friendly, informal relationships with our prospects. Yet, using emojis in cold outreach might seem too extreme. To clear up any doubt, let’s turn to the data.

So, our next question is: Could emojis really help your cold emails get more opens?

And without further ado, the answer is YES!

emojis in subject line

  • Subject lines containing emojis generated 19.7% higher open rate.
  • Reply rate for subject lines with emojis was 6.95% higher.

Obviously, the 20% increase in engagement makes a huge difference. I think that’s because emojis aren’t really common in cold email subject lines and will help your email stand out in the inbox. 

💡 So I strongly encourage you to try this approach for some of your campaigns, or at least A/B test this subject line variant to make sure it works for your audience as well as it does for ours. 

How to Write Better Subject Lines (According to Data)

Our collection of data-backed insights on effective cold email subject lines in a convenient PDF format

Cold email best practices to put the insights to good use

Based on the data insights listed above (plus some others that we will address next time), we’ve put together a short list with cold email subject line best practices. Feel free to steal it 🙂.

  • Keep your subject lines short and mobile-friendly (ideally, 1-5 words)
  • Add a few personalization variables (2-3 are usually enough)
  • Capitalize all or most words in your subject line
  • Use emojis whenever appropriate
  • Prioritize timing and relevance in your subject lines
  • Always A/B test your subject lines

And remember: While a 30% open rate is ok, 50%+ is amazing! If your results are lower, your subject line is most likely to blame. Follow the tips listed above to fix it!

P.S. More Reply Research issues are coming your way soon 😉. Subscribe to our newsletter below or follow us on LinkedIn to be the first to get the fresh portion of data-backed cold outreach tips.

Inline Feedbacks
View all comments

Ready to get started?

Create your free 14-day account now

Reply.io, All rights reserved © 2022. sales@reply.io Contact sales +1 855 747 99 53 (toll free)