B2B Email Marketing: How to Write the Perfect Subject Line (With Our Data)

Kit Fox

Confession–our headline might be a tad misleading. There’s actually no such thing as a universally “perfect” subject line. If there was, your inbox would become a monotonous wasteland, with every marketer using precisely the same tone, format, length and style.

Here’s the good news: With time, testing, and some creativity, you can craft the (nearly) perfect subject line for your audience. Notice the emphasis. Your email list is absolutely unique, filled with a cohort of people that don’t just vary from other industries–but from your competitors. That means the subject lines that will work for you will only work for you.

All you need is find them.

In this blog we’ll tell you how, using our testing process and data. We’ll explore why subject lines matter, why they’re more of a science than an art, and how you can build out your own testing process to find your perfect (or as perfect as possible) subject line formula.

Feature image for email marketing blog.

How to Find (Your) Perfect Subject Line

Why Subject Lines Matter

The vast majority of email inbox interfaces present the user with the sender’s email, timestamp, subject line, pre-header, and (sometimes) the first few lines of content. That means there are only two reasons you likely open any given email.

  1. You trust the person it’s from.
  2. You believe it’s going to contain useful information.

As B2B marketers, it’s enormously difficult for us to be part of the first cohort. We can certainly earn a spot as a trusted contact over time with high brand authority, but we can never compete with bosses, coworkers, or family.

That means we have to rely on option 2, convincing the recipient that our email is useful. That puts significant pressure on the subject line. It’s your front door–the split-second of attention you get to persuade them to come inside. That means subject lines are the number one (though not the only) factor that will make or break any email campaign.

Why Subject Lines Aren’t... Everything

In the complex, niche B2B industries where we work (sectors like contract manufacturing or biotech), the sales cycle can simmer for months. We look at subject lines, then, as part of an email strategy that compounds brand authority and action over time–not an urgent firesale of instant action. You won’t see siren emojis or “LAST CHANCE” or many of the quick-click ALL-CAPS language launched by consumer e-commerce campaigns.

There’s only one metric that subject lines can even affect—email open rates. For us, that’s only the first rung of a tall ladder that leads toward lead conversion. The click-thru-rate is a far more useful indicator of success because it’s a clear sign of intent and interest. And the content and call-to-action within the email affect CTR far more than the subject line itself. It’s why you’ll see a more cautious, more authoritative, less click-bait style tone in our emails as opposed to the ones sent by D2C brands.

How to Find Your Perfect Subject Line

But you still need your subject line to persuade your contacts to open the email and take action. This means you do need to implement a process to test, refine, and find the subject line format that works best for your specific audience. Your answer: The A/B test.

Every reputable Email Service Provider (ESP) includes an A/B testing tool where some portion of your contacts get one version of an email while another portion get a slightly-tweaked version. Success is measured by a specific metric (usually open rate or click-through-rates) based on the variable you test.

For subject line A/B tests, you’ll focus on open rate–the best indicator of whether or not your words were effective.

Here are the 5 fundamentals of a strong A/B test:

1. Only Test One Variable at a Time

Like a well-executed scientific study, you will only get accurate results from your A/B test when you focus on a single change. If you’re testing subject line length, don’t adjust the time you send the email. If you’re trying to see how a specific verb affects the CTA button within the email, don’t touch the subject line. To truly determine causality, you need to be methodical and incremental in your approach.

2. Take Time

Don’t ever take results from a single send as gospel. External factors, from deliverability to timing to a specific company’s holiday schedule could skew your numbers. Trends are much stronger indicators then specific points. But to understand a trend, you need to let your tests take time. The precise length will depend on your list size and send volume. In general, we let tests run for at least 4 specific sends to the same list before changing our format.

3. Limit Characters

Every inbox provider limits the amount of characters displayed on a user’s screen–and that varies based on their device and browser. In general, your subject lines should be between 30-50 characters to ensure they aren’t cut off. Even when complying with that length, try and keep the action up front, limiting connector or extraneous words.

This is a bad subject line: We’re excited to announce our newest product!

This is a better subject line: Our newest product launches today.

The former delays the meatiest part of the subject line (the new product). The latter uses active voice and puts the most important elements up front.

4. Don’t Overpromise

Creating false urgency, or stuffing superlatives into the subject line that aren’t backed up in the email body, only increases mistrust with your audience. There’s no hack or single formula that will quickly and dramatically increase open rates. The only thing that truly works? Offering useful content to your audience–consistently. Your subject line is only the front door. It doesn’t need flashing lights and a buzzer to ensure folks come inside.

5. Don’t Overreact

Your data should serve as a set of general guidelines, not ironclad law. Once you have a rough sense on an ideal length, structure, and tone, move on. A/B testing CTA’s in the body of the email, delivery time, and send frequency are just as valuable (if not more so) than focusing solely on the subject line.

How We Found Our (Almost Perfect) Subject Line

We spent three months tinkering, testing, and optimizing the subject lines in our email nurture series. Based on our send volume of roughly one email a week, we spent a month focused on each variable. Here’s what we found.

Test 1: Question v. Statement


A: Why is Nobody Opening Your Emails?
B: Email Techniques to Energize Your B2B Audiences

Open Rate Results:


Findings: With less than 1% difference in open rate, this is not statistically significant for us to take any immediate action. Which is why we moved on to a completely different test.

Test 2: Topic v. No Topic


A: Market Research: How to Spy on Your Competitors for (Mostly) Free
B: How to Spy on Your Competitors for (Mostly) Free

Open Rate Results:


Findings: A 6.5% open rate boost is a clear indicator that our audience wants to (at a super quick scan) to know exactly what our email will cover.

Test 3: Topic/Question v. Topic/Statement


A: Content Marketing: Interview Tips from a Journalist
B: Content Marketing: How can these journalism tactics help you interview SMEs?

Open Rate Results:


Findings: With less than 1% difference in open rate, this is not statistically significant for us to take any immediate action. What is significant? The huge jump in open rate against the previous tests. This tells us we’re doing something right. The result has altered our subject lines to always include the topic of the email first, then err on the side of including a question (but only if that question is strong, and clearly answered in the email content).


Have we found our subject line panacea? Absolutely not. But our three-month test has given us very clear subject line guidelines and best practices that we’ll continue to use, refine, and optimize over time.

You likely will never find the perfect subject line for every single email. But, with time and a methodical approach, using the guidelines outlined above, you can get close.

Ready to elevate your B2B marketing?

We help leading business-to-business brands hit their marketing goals. Get in touch to learn how Altitude Marketing can help you reach your peak performance.

Kit Fox

Kit Fox is Altitude’s lead internal brand storyteller and content creator. Before joining Altitude in 2024, Kit spent a decade in the publishing industry, where he served as an editor for, Runner’s World. Men's health, and Men’s Journal, special projects director for Hearst Magazines, and director of membership for Lehigh Valley Public Media. He is the co-author of “Mighty Moe: The True Story of a Thirteen-Year-Old Running Revolutionary.”