Email subject lines: what actually gets your emails opened

The short answer: keep subject lines under 12 words and under 90 characters, since longer ones see meaningfully lower open rates. Numbers in a subject line tend to lower open rates. A dollar sign helps, but skipping the number altogether performs best. Emoji placement matters more than whether you use one at all: leading with two emojis at the start performs best, while bookending or burying them mid-subject-line costs opens. And most “spam words” are safe to use. Only seven tested phrases showed a real drop in open rates: “work from home,” “make money,” “limited time,” “guaranteed,” “hurry,” “don’t miss out,” and “act now.”

That’s based on an analysis of over 3 million campaigns and 14 billion emails sent through Flodesk. The rest of this post breaks down what the data shows, what it means for how you write subject lines, and how to test what works for your specific audience using Flodesk’s built-in A/B testing.

Most email senders spend the majority of their time on the email body: the design, the copy, the offer. The subject line gets whatever’s left over.

That’s a mismatch. A beautifully designed email that nobody opens is just a beautiful file sitting unread. Your subject line is the gatekeeper. Getting it right, even marginally, compounds over every campaign you send.

The good news: subject lines aren’t a mystery. They follow patterns. And once you know what the data says, you can stop guessing.

Before we get into the data, one honest caveat: open rates have been a murkier metric since Apple introduced Mail Privacy Protection (MPP) in 2021, a feature that pre-loads email tracking pixels for Apple Mail users so that some opens get recorded even when a subscriber doesn’t actually open the email.

This affects open rate benchmarks across the industry. It doesn’t make open rates useless. At scale, they’re still the most practical signal available. But it’s worth knowing that the numbers you see aren’t a perfect count of human eyeballs. We’ll point out where this is especially relevant as we go.

We looked at over 3 million campaigns (more than 14 billion emails sent) and categorized subject lines by their structural patterns. Here’s what stood out.

Long subject lines are a common pattern in our data, and they consistently underperform.

Subject lines of 12 or more words see open rates averaging 6% lower than standard-length subject lines. Subject lines of 90 or more characters show a similar dip, running about 4% lower on average.

The likely reason: clarity. A short subject line makes a specific promise. A long one dilutes it, and often gets cut off in the inbox preview anyway. Attention spans are short, so your subject line should match.

The takeaway: stay under 12 words and 90 characters. If you can trim toward 6 to 9 words, even better, but the meaningful drop-off begins at 12 words and 90 characters.

Numbers feel like a safe, concrete way to add specificity to a subject line. The data says otherwise.

Subject lines without a number see open rates up to 14% higher than subject lines with one. Numbers tend to read as promotional or list-like, which may cost you the open.

Can’t resist a number? Pair it with a dollar sign. Numbers paired with a currency symbol outperform plain numbers by up to 12%. A price feels like real information. A bare number often reads like filler.

The takeaway: leave numbers out where you can. If a number is essential to the subject line, anchor it to a dollar amount rather than leaving it bare.

Emoji in subject lines is one of the most debated tactics in email marketing Our data shows the presence of an emoji matters far less than where you put it.

Leading with two emojis at the very start of a subject line is the strongest pattern in our data, with open rates up to 11% above average.

Emojis won’t hurt your open rate on their own, but burying them mid-subject-line or bookending your subject line (one at the front, one at the end) can cost you up to 31% in opens.

The takeaway: if you’re using emoji, lead with them. Two at the very start is the best-performing pattern we found. Avoid scattering emoji through the middle of a subject line or bookending it front and back.

Sparkles is the most-used emoji in Flodesk subject lines, appearing in 230,000+ campaigns, followed by Party Popper at 126,000+. Both are proven inbox scene-stealers that build anticipation before the click.

In case you were wondering: bookending isn’t even the worst way to use two emojis. Scattering them through the middle of a subject line performed worse than bookending in our data, making it the single lowest-performing emoji pattern we tested. If you’re set on using two, start strong rather than filling the middle.

We included all-caps subject lines in the analysis. The sample was too small to draw reliable conclusions, so we’re not making any claims here. What we can say: the senders using all-caps subjects are a very small minority (under 0.02% of campaigns), which suggests most people already instinctively avoid it.

Here’s where conventional wisdom really falls apart.

Most “spam words to avoid” lists circulating online are long, outdated, and mostly wrong. We tested more than 70 phrases commonly blamed for triggering spam filters or spooking subscribers, across millions of Flodesk campaigns.

Most of the phrases we tested showed no meaningful impact on open rates. Words like “discount,” “free,” and “sale now” performed in line with the platform average, contradicting the conventional advice to avoid them.

A handful of phrases did show a real, consistent drop. Ranked worst to least-worst, here are the actual offenders, each lowering open rates by 5 to 20 percentage points:

These phrases tend to lean into spammy, phishing-adjacent, or urgency-driven language, the kind readers (and inboxes) have learned not to trust.

As before, correlation isn’t causation. Senders using these phrases may have other list health factors at play. But the pattern held across a much larger set of phrases this time, which makes it a stronger signal than before.

The takeaway: most everyday marketing language is safe to use. The exceptions are the seven phrases above. If your subject line leans on generic urgency or a vague promise of money, it’s worth rewriting.

Preview text (also called preheader text) is the snippet of copy that appears after your subject line in most inboxes. It’s prime real estate, and most senders aren’t using it strategically.

In our data, about 97% of campaigns used what we’d classify as standard preview text. Repeated punctuation in preview text showed similar open rates to normal preview text but slightly lower click rates, a minor negative effect, but worth avoiding.

The bigger opportunity isn’t what to avoid; it’s what to use it for. Preview text that extends or complements your subject line adds a second hook. Instead of letting it default to “View in browser” or the first line of your email body, write it intentionally.

A few approaches that work:

Preview text is short (typically 40–130 characters depending on the email client), so keep it tight and make it count.

Data tells you what patterns correlate with performance. It doesn’t write your subject lines. Here’s a framework for doing that well.

The most effective subject lines do one of two things: they tell the reader exactly what’s inside (clarity), or they create a specific open loop that the email resolves (curiosity). Trying to do both usually produces a vague, generic subject line that does neither.

Clarity works well for: offers, announcements, content updates, and anything with a clear “what’s in it for me.” Curiosity works well for: storytelling emails, behind-the-scenes content, or anything where the payoff is the experience of reading.

Vague subject lines underperform. “Big announcement” does less work than “We’re launching something new on Friday.” “A few things I’ve been thinking about” does less work than “Why I stopped discounting, and what happened next.”

Specificity creates a promise. The email either keeps it or breaks it. Over time, keeping promises builds the open habit with your audience.

First-name personalization in subject lines can lift open rates, but only when it feels natural rather than mechanical. “Hey [first name], don’t miss this” reads as a template. Using personalization tied to something meaningful (a location, a recent purchase, a segment behavior) is where it earns its keep.

Your subject line sets an expectation. If it’s playful and the email is a hard sell, the disconnect lands badly. Read your subject line alongside the first two sentences of your email before sending. Do they feel like they belong together?

All the patterns above are starting points. What actually works for your audience is something only your data can tell you, and that’s where A/B testing comes in.

Flodesk’s built-in A/B testing lets you send two subject line variations to a portion of your list and automatically delivers the winning subject line to the rest. No manual tracking, no spreadsheets.

How it works:

The winner is determined by open rate. Keep the MPP caveat in mind here: Apple Mail pre-loading can inflate open rate counts for Apple users, which means results won’t be a perfect read. That said, at the volume of a real A/B test, open rate is still the most practical signal you have available.

What to test:

Test one variable at a time. If you change the length, the tone, and add an emoji all at once, you won’t know what drove the result. Start with the element you’re most uncertain about.

Good variables to start with:

For step-by-step setup instructions, see the Flodesk help center guide to A/B testing subject lines.

Running consistent tests and learning from them over time is much easier when your platform is built for it. Flodesk’s subject line A/B testing is built into the send flow. No third-party tools, no manual audience splitting. You set it up once per campaign and let the results do the work.

Beyond testing, Flodesk gives you the design tools to make sure the email that follows your subject line is worth opening: beautiful templates, flexible layouts, and email automations that keep your list engaged between campaigns.

Before you hit send, run through these: