What Email Subject Line Length Works Best?

Alright fellow marketers, let’s get to the bottom of this. Subject lines can be one of the most frustrating components of email marketing. How can something possibly be short, fun, eye-catching, enticing, and compelling all at the same time? I know. It doesn’t make any sense. As a result, marketers are plagued by a few common questions:

• Are shorter subject lines better?
• What is the optimal character count?
• Does subject line length even matter?

Often, the answers to your questions live in the data.

In this blog, I’ll walk you through how I collected and dissected data to determine the ideal word and character count for writing subject lines that beg to be opened (and lead to a click).

Step 1: Data Collection

There is a common misconception that the right question to ask would be: which subject line length correlates to the higher open rate? But that doesn’t factor in downstream metrics such as clicks, click-to-open rate, and unsubscribe rates. The question we are really asking is this:based on word/character count, what subject line length gets us the most engagement (clicks)? Click To Tweet To answer this question, we needed the following campaign attributes and metrics:

• Subject line
• Sent
• Delivered
• Unique opens
• Open rate %
• Clicks
• Click-to-open rate %
• Unsubscribe rate %

I bet you can’t guess what happened next! Using Marketo, I ran a report in capturing these attributes and metrics over the past six months, which gave me a large sample size of ~200 email campaigns and over two million emails sent. The key here is having a large enough sample size to show some statistical significance.

Step 2: Analyzing the Data

With hundreds of lines of data, I needed to aggregate subject line length by word count. Most of our subject lines fell between four and nine words and just to be safe, I included a 10+ words category. Then, I pieced out the data by word count for all the campaign metrics listed above with the immediate goal of aggregating average open rates paired with average click-to-open rates. Using these two metrics, I then built a mock data set to determine which word count length gave me the highest level of engagement.
Here’s the data:

You can see that the highest open rates occurred with four-word length subject lines and the lowest open rates were tied to nine-word length subject lines. But the click-to-open rates didn’t correlate, meaning high open rates did not correlate to high click-to-open rates. Therefore, we needed to look at overall engagement, not just open rates. It makes sense too. It’s more challenging to be descriptive and actionable in four words than it is in nine words. Thus, more people might click an email with a nine-word subject line because they had a better sense of the email’s content from the start.

Step 3: Take the Data and Model It Out

Now we needed to take the word count averages and factor them into a model with constant variables to determine which subject line length generated the best overall engagement rates. For this, I used the following metrics: sent, open rate, opens, click-to-open rate, and clicks.
Here’s the model, using 100,000 emails sent as our constant variable:

The winner was seven words for overall engagement. And if you write a subject line with eight words, well, you’re in for a shock! It’s nearly half the performance as subject lines with only one less word.

I hope that this process has given you better insight into how to evaluate your own subject lines for open rates, click-to-open rates, and overall email engagement. And if you’re wondering what the ideal character count would be, our seven-word subject lines have an average character count of 41 characters. But let’s face it, it’s easier to count words than it is characters. That would drive you absolutely nuts!

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