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 with the words a phone cuts off, 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, with the words each phone cuts off, 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.
How long should an email subject line be?
Short enough that the reason to open shows on a phone. That's about the first 35 characters.
I measured all 56,262 subject lines in the tester's phone inboxes, drawn 390 pixels wide. The Gmail app showed 47% of them in full and cut the rest after about 36 characters.
| Inbox, on a phone | Subject lines shown in full | Where the rest got cut |
|---|---|---|
| Gmail app | 47% | After about 36 characters |
| iPhone Mail | 65% | After about 46 characters |
Length itself didn't decide which email got more reactions. When 2 subject lines from the same newsletter differed by 10 or more characters, the shorter one won 49.7% of 22,678 head-to-heads: a coin flip.
| Subject line length | Got more reactions | Head-to-heads |
|---|---|---|
| 1 to 20 characters | 49% | 5,905 |
| 21 to 30 characters | 49% | 10,235 |
| 31 to 40 characters | 50% | 11,461 |
| 41 to 50 characters | 52% | 9,357 |
| 51 to 60 characters | 50% | 7,225 |
| 61 to 70 characters | 51% | 4,979 |
| 71 characters or more | 49% | 5,868 |
So write the line as long as it needs to be, and put the words that make people open first. Half the lines in the set were 39 characters or fewer.
The tester shows where each phone cuts your line, and which words get cut off.
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.
Keep it long enough to fill its line. When it's short, Gmail fills the rest with the first words of your email, and the tester tells you when that happens.
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 one 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.
Do emoji help a subject line?
Not in this data. Subject lines with an emoji got more reactions in 36% of 1,040 head-to-heads inside the same newsletter.
Only 3.8% of the 56,262 lines had one.
The emails are newsletters, measured by reactions. For a sale or a launch, test an emoji on your own list before you use it everywhere.
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.
How to write a subject line that gets opened
Put the reason to open in the first 35 characters, where every phone shows it.
Then write preview text that gives a second reason, long enough to fill its line.
In the data, what won most often inside the same newsletter was a negative word, a number at the start, and "I" or "my". What lost most often was a colon or dash splitting the line in 2, an exclamation mark, a word in capitals, and emoji.
Use the score to pick between the 2 lines you like best, then let your list decide with an A/B test.
Who is the email subject line tester for?
It's for anyone about to send an email:
- Email subject line generator – people who need more lines to test
- A/B test calculator – marketers comparing two lines
- Headline analyzer – writers working on headlines
- Email countdown timer – anyone announcing a deadline
More tools like these are on my free tools page.
Final thoughts on email subject lines
Put the reason to open in the first 35 characters, where every phone shows it.
Then write preview text that gives a second reason, long enough to fill its line.
In the data, what won most often inside the same newsletter was a negative word, a number at the start, and "I" or "my". What lost most often was a colon or dash splitting the line in 2, an exclamation mark, a word in capitals, and emoji.
Use the score to pick between the 2 lines you like best, then let your list decide with an A/B test.
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 be long enough to say what's inside, with the reason to open in the first 35 or so characters.
In the tester's phone inboxes, the Gmail app cut 53% of 56,262 real subject lines, most after about 36 characters.
Length itself didn't decide which email got more reactions: the shorter of 2 lines won 49.7% of the time.
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 a subject in all 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.
In 56,262 newsletter emails, subject lines with an emoji got more reactions in 36% of 1,040 head-to-heads inside the same newsletter.
Test one on your own list before you use it everywhere.
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.
Use "Re:" only when the email really is a reply.
Gmail's sender guidelines ask senders not to start a subject with "Re:" or "Fwd:" otherwise.
Put the reader's name in a subject line only if your email platform fills it in for every reader.
A tag left as typed, like {first_name}, shows up in the inbox as it is, and the subject line tester flags it.
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.
What you type 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.
The email signature generator builds your signature, and the email countdown timer puts a live countdown in a launch email.
The A/B test calculator tells you when a test has a winner, and all my free tools are in one place.
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.
More free tools
X
Bluesky
LinkedIn
Threads
Mastodon
Instagram
TikTok
Pinterest
YouTube
Amazon KDP
Apple Books
Amazon
Spotify
Apple Podcasts
Free AI newsletterThe most actionable AI newsletter for founders
Every week, get proven AI strategies, curated tools, and step-by-step systems to grow your audience, create better content, and build a profitable creator business.
No fluff, no filler, no BS. Just five minutes each week that might level up your online business and life.
P.S. Sign up now to get free access to my ultimate AI tools guide for creators.
