An email subject line tester that scores your line against real emails, shows it in real inboxes, and says how often the score is right.
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 kind of email
Your line
What you type stays in this browser. The checks run on your device.
This free email subject line tester scores your subject line and shows it in Gmail, Outlook and iPhone Mail before you send.
The score comes from 56,262 real emails. It also checks the line and its preview text against Gmail's sender rules and SpamAssassin's subject rules, finds merge tags left unfilled, and puts 2 lines side by side.
Here's what makes a subject line work, what the data says, and how to test your own.
What is an email subject line tester?
An email subject line tester checks a subject line and its preview text before you send, and shows how they'll look in an inbox.
Preview text: the second subject line
Preview text is the line after the subject in an inbox. Without it, the inbox shows the start of your email, often "View in browser" or an unsubscribe line.
Write it as a second reason to open, not a repeat of the subject. The tester flags both mistakes.
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.
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.
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 Tester FAQs
Questions about the email subject line tester? Here's what to know.
An email subject line tester checks a subject line and its preview text before you send, and shows how they'll look in an inbox.
An email subject line should put its point in the first few words, because phones cut it where it runs out of room. The subject line tester shows where yours gets cut.
Preview text is the line after the subject in an inbox. Without it, the inbox shows the start of your email, often "View in browser".
SpamAssassin's subject rules score a few habits, like the whole subject in capitals (0.5 points) or "As seen" (2.7 points), out of the 5 points that mark an email as spam. The sender's reputation matters much more.
On 107 newsletters it never saw, the subject line tester's higher score won 56% of the time, when a coin flip gets 50%. The topic matters more, so use the score to pick between lines you like and test the top 2 on your list.
Only when the email really is a reply. Gmail's sender guidelines ask senders not to start a subject with "Re:" or "Fwd:" otherwise.
Send line A to half your list and line B to the other half, count clicks rather than opens, and check the result in an A/B test calculator.
Yes, the subject line tester is free, with no sign-up. It runs on your device.
No. What you type in the subject line tester stays in your browser, and the checks run on your device.
The subject line tester is for creators, newsletter writers and marketers who want every send to get opened.
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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