Everyone knows you can use ChatGPT to write email subject lines. The problem is most people use it badly, get generic results, and conclude that AI isn’t that useful for this kind of work.
The tool isn’t the issue. The prompts are.
I’ve spent enough time experimenting with AI-assisted subject line writing to have opinions on this. ChatGPT is genuinely useful for breaking through writer’s block, generating variations quickly, and surfacing angles you wouldn’t have considered on your own. But left to its defaults, it produces exactly what you’d expect: safe, forgettable copy with too many exclamation points and a suspicious fondness for the word “sizzle.”
The difference between mediocre output and useful output is almost entirely in how you brief it.
What ChatGPT Is Good At (and What It Isn’t)
Before getting into the how-to, it’s worth being clear-eyed about what you’re working with.
ChatGPT is fast at generating volume. If you need ten subject line variations quickly, it’s faster than most human writers. It’s also good at applying specific patterns consistently: urgency framing, curiosity gaps, question formats, benefit-led lines. Ask it to produce one of each and it will.
What it doesn’t know is your audience. It has no idea that your subscribers skew toward email professionals who are skeptical of hype. It doesn’t know your brand voice, your historical performance data, or that your list responds better to plain and direct than to clever wordplay. You have to supply all of that context. The model can only work with what you give it.
Think of it as a well-resourced but context-free copywriter. The more you brief it, the better it performs.
How to Write Prompts That Actually Work
Have you ever typed “write me email subject lines for our summer sale” into ChatGPT and gotten results that were technically fine but somehow completely lifeless? That’s what happens when you give the model nothing to work with.
A better approach is to give it the same brief you’d give a human copywriter:
- Who you’re writing for: who the audience is, how familiar they are with your brand, what they care about
- What the email is about: the offer, the announcement, the goal
- Your brand’s tone: direct, conversational, minimal, a bit wry; be specific
- What to avoid: spam-adjacent words, excessive punctuation, clickbait patterns
- Constraints: character count, whether you’re using personalization tokens
Here’s the difference in practice.
Weak prompt: “Write email subject lines for our summer sale.”
Strong prompt: “Write 8 email subject lines for an email marketing audience. The email announces our biggest summer sale: 30% off all plans for 72 hours. Our tone is direct and slightly wry — no hype, avoid exclamation points unless truly warranted. Steer clear of words like ‘amazing’, ‘incredible’, or ‘exclusive’. Give me one variation each of: a direct offer line, a curiosity-based line, a question, and an urgency line. Keep each under 50 characters.”
The output from the second prompt will be materially better. Not because ChatGPT got smarter, but because you removed the space that generic defaults rush to fill.
Prompt Templates Worth Keeping
A few starting points you can adapt to your own campaigns.
For a promotional email: “Write 8 subject lines for [audience description]. The email promotes [offer]. Tone: [your tone]. Avoid: [words or styles to avoid]. Give me one variation each of: direct offer, curiosity gap, urgency, and question format. Max 50 characters each.”
For a newsletter or content email: “Write 6 subject lines for an email containing [brief content summary]. Audience: [description]. Aim for subject lines that feel personal and conversational rather than promotional. No clickbait.”
For a re-engagement campaign: “Write 5 subject lines for a re-engagement email to subscribers who haven’t opened in 90 days. The email offers [your hook]. Tone should be honest and low-pressure. Avoid anything that sounds desperate or accusatory.”
For building an A/B test: “I’m A/B testing subject lines for [email description]. Give me two variations that take clearly different approaches: one that leads with the benefit, one that leads with curiosity. Make them distinct enough that the test will be meaningful.”
Things to Watch For in the Output
ChatGPT has a few persistent tendencies when writing subject lines that are worth knowing about.
It defaults to exclamation points. If you don’t explicitly ask it not to use them, expect several. Tell the model your preference at the start of your prompt and it will follow the instruction.
It gravitates toward clickbait patterns. “You won’t believe…”, “This one trick…”, these show up because they’re overrepresented in the text it was trained on. If your brand doesn’t speak that way, say so explicitly. “Avoid clickbait phrasing” is a legitimate instruction and it works.
It may generate spam trigger words without flagging them, particularly in promotional contexts. Always run your final candidates through a spam checker before sending, especially for offer-heavy emails.
And the most important thing to remember: ChatGPT cannot tell you which line will actually perform with your audience. That’s still your job, your data, and ideally an A/B test.
The Human Layer Still Matters
The best use of ChatGPT for subject lines isn’t to hand over the work. It’s to give yourself more raw material to apply judgment to.
Generating 10 to 15 options from a well-constructed prompt, then cutting down to the two or three worth testing, is faster and usually more effective than writing from scratch. The model handles volume. You handle selection and refinement.
Edit the output. Take the strongest element from one line and the tone from another. Use what it generates as a starting point, not a final answer. I’ve rarely published a ChatGPT subject line verbatim — but I’ve regularly used one as the seed for something better.
That combination, AI for volume and human for judgment, tends to produce stronger results than either approach alone.
ChatGPT isn’t a shortcut to great subject lines. It’s a faster way to get to the options worth thinking seriously about. The marketers who get the most out of it are the ones who give it the most specific brief, not the ones who type the least.
Alterable helps email marketers add real-time personalized content to their campaigns — countdown timers, dynamic products, location-based images, and more.
