An email subject line generator that briefs your own AI with what won in real tests, then checks every line it writes.
Type your sender name, subject and preview text. Get a score, see them in Gmail, Outlook and iPhone Mail, check them against Gmail's rules and SpamAssassin's, and compare 2 lines.
The email
Ask your AI
Describe the email above to brief your AI.
Paste its lines back
What you type stays in this browser. The checks run on your device.
This free email subject line generator writes subject lines with your own AI, then checks every one.
Describe the email, and it builds a prompt with what won in real tests. Your AI writes the lines, you paste them back, and each is checked for spam rules, merge tags and fake replies.
Here's how it works, and what makes a subject line get opened.
What is an email subject line generator?
An email subject line generator writes subject lines for an email from what it's about and who it's for.
This one uses your own AI, so it costs nothing, and it gives your AI what won in real tests to work from.
Where the score comes from, and how far to trust it
I read 56,262 emails from 579 public newsletters on Substack's leaderboards, each with its subject line and how readers reacted. On Substack, a post's title is its email subject unless the writer changes it.
Then I compared emails from the same newsletter, sent to the same readers within 45 days. In pairs where 1 got at least twice the reactions, this is how often a subject with each part won:
| Part | Got more reactions | Head-to-heads |
|---|---|---|
| A negative word like "never" or "stop" | 63% | 4,957 |
| Starting with a number | 60% | 2,716 |
| "I" or "my" | 59% | 5,840 |
| Opening with how, why or what | 58% | 5,745 |
| An emotional word like "amazing" | 58% | 379 |
| "You" or "your" | 54% | 6,257 |
| "This", "these" or "here's" | 53% | 2,592 |
| A superlative like "best" or "ever" | 53% | 2,044 |
| "We" or "our" | 53% | 2,834 |
| A quote in quotation marks | 52% | 1,788 |
| A question | 49% | 4,828 |
| A number | 45% | 11,193 |
| A word in capitals | 44% | 5,707 |
| An exclamation mark | 42% | 2,129 |
| A colon, dash or bar splitting it in 2 | 41% | 9,925 |
A model trained on those pairs gives the score: where your line's wording lands among all 56,262 subject lines. On 107 newsletters it never saw, the line it scored higher won 56% of the time, when a coin flip gets 50%.
That's a real edge, but a small one. The topic matters more than the wording, and reactions come from people who opened and read, so they measure the whole email. Use the score to pick between lines you like, then test the top 2 on your list.
The emails are newsletters. Launch, promo, welcome and cold emails get the same score, and the checks count as much for them, until I have their own proven lines.
Spam: the subject is a small part
Gmail looks at the sender first. Its sender guidelines ask every sender for SPF or DKIM, a spam rate under 0.3%, and no fake "Re:" or "Fwd:". Bulk senders also need DMARC and one-click unsubscribe.
The subject adds a little on top. SpamAssassin, the open source spam filter, scores these subject habits, and calls an email spam at 5 points:
| What the subject has | SpamAssassin rule | Points |
|---|---|---|
| The whole subject in capitals | SUBJ_ALL_CAPS | 0.5 |
| "!" and "?" together | PLING_QUERY | 0.1 |
| S.p.a.c.e.d-out letters | GAPPY_SUBJECT | 0.1 |
| "As seen" | SUBJ_AS_SEEN | 2.711 |
| Starting with a dollar amount | SUBJ_DOLLARS | 0.1 |
| "Your family" | SUBJ_YOUR_FAMILY | 2.91 |
| "Lose" with pounds or weight | SUBJECT_DIET | 1.927 |
| Starting with "Buy" | SUBJ_BUY | 0.594 |
So a subject alone rarely sinks an email. A sender with a bad reputation sinks every subject.
How to test subject lines
Send line A to half your list and line B to the other half, and count clicks, not opens. Apple's Mail Privacy Protection hides whether people opened.
The A/B test calculator tells you how big each half needs to be, and when the winner is real.
Email Subject Line Generator FAQs
Questions about the email subject line generator? Here's what to know.
An email subject line generator writes subject lines for an email from what it's about and who it's for. This one briefs your own AI and checks what it writes.
The email subject line generator uses your own AI, such as ChatGPT or Claude, with a prompt it builds from your email and from real test results. Nothing is generated on my site.
Test 2 subject lines at a time on your list, so each half is big enough to show a real winner.
Yes, the email subject line generator is free, with no sign-up. It runs on your device.
No. What you type in the email subject line generator stays in your browser, and the checks run on your device.
The email subject line generator is for creators and marketers who send newsletters, launches and welcome emails.
To get the rest of the email right, the email signature generator builds your signature and the A/B test calculator tells you when a test has a winner. See all my free tools.
Navid Moazzez
AI business strategist & AI OS builder
Navid Moazzez helps creators and founders master AI and build their own AI Operating System (AI OS) to automate their business and life.
Navid.me is reader-supported. When you buy through links on this site, I may earn an affiliate commission. Learn more.
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