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

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.

• 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!

• 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:

• 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.:

1. 0 variables → your sales development process
2. 1 variable → sales development process at {{Company}}
3. 2 variables → {{FirstName}}, sales development process at {{Company}}
4. 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.

• 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!

• 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.