How to A/B Test Email Subject Lines (And Why It’s Crucial for Growth)

Learn how to a/b test email subject lines properly - and why it’s essential for long-term eCommerce growth.

Why You Need to A/B Test Email Subject Lines to Scale Smarter

To scale an eCommerce brand, you need to a/b test email subject lines with intent, not assumptions. Subject lines determine whether your email even gets read. They’re the entry point to your revenue.

Improving open rates by even a few percentage points can add thousands in revenue across flows and campaigns. But without data, you’re just guessing. And guessing doesn’t scale.

Let’s break this down into a repeatable, results-driven process you can start using immediately.

What Is an A/B Test in Email Marketing?

An a/b test email is when you create two versions of an email – usually with different subject lines – and send each to a small percentage of your audience. Whichever version performs better gets sent to the rest.

The idea is simple: let behavior, not opinion, determine your messaging. But to get reliable results, you need a large enough sample size – at least 10,000 people total across multiple sends.

Otherwise, you’re making decisions based on noise, not signal.

The Psychology Behind High-Performing Subject Lines

People don’t open emails logically – they open them emotionally. A subject line that sparks curiosity, urgency, or personal relevance can drive up open rates dramatically.

This taps into behavioral patterns like:

Curiosity Bias – Subject lines that create a gap (“You forgot this…”)
Personalization Effect – Including a name or custom detail improves relevance
Cognitive Ease – Short, punchy phrases are easier to process and more likely to be opened

Testing allows you to see which psychological trigger resonates most with your audience. That nuance is where conversion lives.

Real-World Example: Personalization at Scale

A Shawfire Media client tested two simple subject lines:

“Hey there, quick one for you”
“Hey Alex, quick one for you”

Everything else stayed the same. The version with the name increased open rate by 12% across a 20K list. That small tweak, scaled across multiple flows, meant thousands in added revenue over a quarter.

That’s the power of structured testing.

How to A/B Test Email Subject Lines (Step-by-Step)

1. Define a Clear Hypothesis

Start with a reason. You’re not testing just to test – you’re testing to learn.

Some testable variables:

First name vs. default (e.g., “Hey Alex” vs. “Hey friend”)
Emoji in subject line vs. no emoji
Numbered list (e.g., “3 tips”) vs. written out (“three tips”)
Casual vs. formal tone

2. Test Across Multiple Campaigns and Flows

Don’t limit yourself to one send. Run tests across:

5 – 10 campaigns
5 – 10 automated flows (like abandoned cart or welcome series)

This ensures results are consistent across different contexts.

3. Use a Large Enough Sample Size

To avoid misleading results, aim for 10,000 total recipients across both versions. Anything less risks being skewed by outliers.

If your list is smaller, test consistently over time and look for patterns.

4. Track Results Meticulously

Use a spreadsheet or template to log:

Test version A and B
Open rates, click rates, and revenue
Audience size and segmentation
Notes on timing, offers, tone, etc.

The more precise your tracking, the more useful your data becomes over time. Want a done-for-you tracker? Download it here by joining our email list.

5. Iterate Based on Patterns

One test won’t tell you everything. But over time, you’ll learn what works:

Maybe emojis in the sender name outperform the subject line
Or subject lines starting with numbers get more clicks
Or informal tone wins for new leads, but not existing customers

These patterns are your brand’s edge. Test → track → refine → repeat.

Avoid These Testing Mistakes

Testing too small
You need large, diverse samples to trust your data.
No tracking
If you don’t document what you test, you’ll keep repeating past mistakes.
Chasing open rates without looking at revenue
Open rate is a signal, not a finish line. Make sure your winning subject lines also convert.

Key Takeaways for A/B Test Email Strategy

A/b test email subject lines with clear goals and solid hypotheses
Run tests across flows and campaigns for broader insight
Use 10,000+ recipients for reliable data
Track every variable and result to build your subject line playbook
Use patterns – not one-offs – to guide future sends

Want better email performance without guesswork?

Shawfire Media builds full-service retention strategies for eCommerce brands - and testing is baked into everything we do.

👉 Join our list to get our A/B testing tracker and start refining your subject lines with data.