The A/B test results were humbling. Same email, same audience, same send time. The only difference was the subject line. Version A: "Q4 Newsletter - Company Updates and News." Version B: "The change we're making to your account."
Version B had 3x the open rate.
Subject lines are the most important eight words you'll write. They determine whether your carefully crafted email gets read or ignored. They're also the hardest to get right—you're competing with dozens of other emails for attention in a crowded inbox.
These formulas aren't magic, but they're patterns that consistently perform. Adapt them to your context, test them with your audience, and build a library of what works for you.
Curiosity formulas
The incomplete thought. Leave something unsaid that the reader needs to open to discover. "The one thing we changed that doubled conversions" works because readers want to know what that one thing is.
The unexpected combination. Pair two things that don't obviously go together. "What spreadsheets taught me about customer service" creates curiosity through incongruity.
The question they can't answer. Ask something the reader doesn't know but wants to. "Do you know your actual email open rate?" works if they don't—and they probably don't.
The counterintuitive claim. Challenge conventional wisdom. "Why we stopped A/B testing subject lines" makes readers wonder what you learned that contradicts standard practice.
Urgency formulas
The deadline. Real deadlines create real urgency. "Price increases Friday at midnight" is specific and actionable. Fake urgency ("Act now!") is transparent and damages trust.
The limited availability. Scarcity drives action. "Only 12 spots left for the workshop" works when it's true. Manufactured scarcity backfires when readers catch on.
The time-sensitive information. Some information has a shelf life. "Your account changes tomorrow—here's what to know" is urgent because the information becomes irrelevant after the deadline.
The consequence of inaction. What happens if they don't open? "Your subscription expires in 3 days" makes the cost of ignoring clear.
Value formulas
The specific benefit. Vague benefits don't compel action. "Save 3 hours per week on email" is specific and measurable. "Improve your productivity" is forgettable.
The how-to promise. People want to learn. "How to write emails that actually get replies" promises specific, actionable knowledge.
The list of things. Numbers set expectations. "7 email mistakes that hurt deliverability" tells readers exactly what they'll get and how much time it will take.
The resource announcement. New resources are inherently valuable. "New guide: Email authentication for developers" is straightforward but effective when the resource is genuinely useful.
Personal formulas
The direct address. Using the recipient's name or company increases relevance. "[Name], your weekly report is ready" feels personal, not mass-produced.
The insider reference. Reference something specific to the recipient. "Following up on your question about DKIM" shows you're paying attention, not blasting a list.
The mutual connection. Shared context creates trust. "Sarah suggested I reach out" leverages existing relationships.
The acknowledgment. Recognize something about the recipient. "Congrats on the Series A—here's how we can help" shows you've done your homework.
Transactional formulas
The clear action. Transactional emails should be unmistakable. "Your order has shipped—track it here" tells recipients exactly what the email contains.
The status update. Keep recipients informed. "Your support ticket #1234 has been updated" is boring but effective—recipients know exactly what to expect.
The confirmation. Confirm actions clearly. "You're registered for the webinar on March 15" removes uncertainty.
The alert. Security and account alerts need to be taken seriously. "New login to your account from Chicago" is clear and appropriately urgent.
What makes these work
The effective formulas share common traits.
They're specific. "7 email mistakes" beats "common email mistakes." "Save 3 hours per week" beats "save time." Specificity creates credibility and sets clear expectations.
They promise value. Every subject line implicitly answers "why should I open this?" The answer should be clear and compelling. If you can't articulate the value, the subject line won't work.
They're honest. Clickbait subject lines might get opens, but they destroy trust. If the email doesn't deliver what the subject line promises, you've won a battle and lost the war.
They're appropriate to the relationship. A casual subject line to a new prospect feels presumptuous. A formal subject line to a long-time customer feels cold. Match the tone to the relationship.
Testing subject lines
Formulas are starting points, not guarantees. What works for one audience might fail for another. Testing reveals what actually works for your specific context.
A/B test subject lines whenever possible. Most email platforms support this natively. Test one variable at a time—if you change both the subject line and the send time, you won't know which caused the difference.
Track more than open rates. A subject line that gets opens but not clicks might be misleading. A subject line with lower opens but higher click-through might be more effective overall.
Build a swipe file. When you see subject lines that make you open emails, save them. Analyze what made them work. Adapt the patterns for your own use.
Common mistakes
Too clever. Puns and wordplay that require thought to understand get skipped. Clarity beats cleverness.
Too long. Mobile email clients truncate around 30-40 characters. Front-load the important words.
Too vague. "Quick question" and "Following up" say nothing. Be specific about what the email contains.
Too salesy. ALL CAPS, excessive punctuation (!!!), and spam trigger words (FREE, ACT NOW) hurt deliverability and trust.
Too similar. If every email has the same subject line format, readers tune out. Vary your approach.
Frequently asked questions
What's the ideal subject line length?
Under 50 characters ensures full display on most devices. Under 30 characters is safer for mobile. But length matters less than content—a compelling 60-character subject line beats a boring 30-character one.
Should I use emojis in subject lines?
It depends on your audience and brand. Emojis can increase open rates in some contexts and hurt them in others. Test with your specific audience. When in doubt, skip them for professional communication.
How do I avoid spam filters with subject lines?
Avoid ALL CAPS, excessive punctuation, and known spam trigger words. But content matters more than specific words—spam filters evaluate the whole message, not just the subject line. Focus on sending valuable email to engaged recipients.
Should I personalize subject lines with the recipient's name?
Personalization can increase opens, but only if it feels natural. '[Name], your order has shipped' works. '[Name], you won't believe this deal!' feels manipulative. Use personalization when it adds value, not as a trick.