Email subject line best practices: why most of them were written for someone else's list

Do email subject line best practices actually work?

Some do, some don't, and the ones that don't are usually the ones that sound the most certain. Here's what I've seen when I look at real sends.

Emoji. Within one category, I looked at 4 brands. Emoji in the subject line helped two of them and hurt the other two. Same category, opposite results. If someone tells you emoji always lift or always hurt, they're telling you about their list, not yours.

Length. At a fitness tech brand, subject lines of 20 characters or fewer got more than 2x the clicks of lines between 41 and 60 characters. Short won, clearly.

But in the pooled data from 4 consumable brands, subject lines of 40+ characters helped on well-segmented lists. My read: when you already know exactly who you're talking to, a longer line has room to say something specific to them. When you're sending to a broad list, short and punchy gets through.

Those two findings don't cancel out. They tell you length depends on who's reading.

What does work across brands?

When I pooled 7 brands and 2,821 campaigns, two patterns held up better than anything else.

News beats brand voice. Subject lines that told people something new did better than lines written to sound clever or on-brand. A snack brand doing about $650K a month sent a restock email with the subject line "We're back baby!" It made $48,771 from 30,498 people, its best campaign ever. That's news, said plainly.

Vague questions are the worst. Subject lines that ask a fuzzy question with no real information performed worst of every pattern. Think something like "Ready for something new?" That's my made-up example, not a real send, but you've seen a hundred of them. It gives the reader nothing to click for.

If you take one thing from this section: say what's new. Restocks, launches, a new flavor, a deadline that's real. People open and click news.

My starting rules for subject lines

I still start every brand with a few rules. They're starting points, not laws. Here's my SOP:

• Keep subject lines under 50 characters. Long enough to say something, short enough to survive on a phone.
• Write preview text at 80 to 100 characters. It's the second subject line. Don't waste it on "View in browser."
• No fake "Re:" or "Fwd:". It might get a click once. It costs you trust every time after.
• Judge winners by clicks. Not opens. More on why below.

Notice that rule one and the data above don't fully agree. That's the point. The rules get you started. Your own tests tell you when to break them.

How to test subject lines on your own list

You don't need a stats degree. You need a simple routine and the patience to run it for a few weeks. Here's how I'd set it up in Klaviyo:

1. Test one thing at a time. Length, or emoji, or news vs voice. Not all three at once, or you won't know what moved.
2. Use a real A/B split on a decent-sized send. Klaviyo's built-in A/B test on campaigns works fine. Tiny segments give you noise, not answers.
3. Pick clicks as the winning metric. Set the test to choose a winner on click rate, not open rate.
4. Write the result down. A simple sheet with the date, the segment, both lines, the click rates and the winner.
5. Look for a pattern before you make a rule. One test is a coin flip. Four or five tests pointing the same way is a pattern for your list.
6. Test by segment. Your repeat buyers might love a short, punchy line while new subscribers need more context. The 40+ character finding on well-segmented lists is a good reason to check.

If you're not segmenting yet, start there first. A subject line test sent to your whole list is testing a blended audience. I lay out where to begin in email segmentation for ecommerce.

Why I judge subject lines by clicks, not opens

Opens used to be the obvious way to score a subject line. They're not reliable anymore.

Across 4 consumable brands, open rate tracked revenue at 0.32 and click rate at 0.65. For one brand, open rate tracked revenue negatively: more opens, less money. Apple Mail auto-opens pushed open rates past 85% on some segments.

So if you crown a winner on opens, you might be picking the line that Apple Mail liked, not the one people liked. Clicks are harder to fake. I go deeper on this in open rate vs click rate.

Where these numbers come from

The pooled numbers come from two samples: 4 consumable brands with 2,897 campaigns and $17.7M in email revenue, and 7 brands with 2,821 campaigns for the subject line patterns. Both are small samples, so read them as directional. The single-brand numbers each come from one brand I've worked with, described by category instead of name.

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WRITTEN BY Jack White

Founder of Bardown Digital, a Chicago-based Klaviyo email and retention agency for Shopify brands. He's worked with brands doing $500K to $100M a year, with $100M+ in email-attributed revenue across them.

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