What is an email subject line tester?
A subject line has about a second to earn an open. It is also cut off differently depending on where it is read: a phone shows far fewer characters than a desktop client. An email subject line tester helps you see that before you press send.
With this email subject line tester you type your Email Subject Line, an optional Preview Name for the sender, and optional Preview Text (preheader). The page updates as you type. You see three inbox mock-ups (Gmail desktop, Outlook desktop and mobile), a character bar with a 40 to 60 character target, and a score ring with a factor list. If it finds words from its spam list or its power word list, it shows them as tags.
Be clear about what the score is. It is a rule-based checklist written into the page, not a model trained on your audience, and it does not predict open rates. The inbox previews are approximations that cut text at fixed lengths. Real clients vary by screen width, font and settings. Use the tool to catch obvious problems, then A/B test with real subscribers.
How to test a subject line
- Type your subject into Email Subject Line. The counter and bar show length against the 40 to 60 character target.
- Change Preview Name to the sender name your subscribers will see.
- Add your Preview Text (preheader). The page suggests 40 to 90 characters.
- Look at the Gmail, Outlook and Mobile previews to see where the subject and preheader are cut off.
- Read the score, the Analysis Factors list and any spam or power words found.
- Edit the line, and the results change immediately. Repeat until the factors look right to you.
What this tester checks
Three inbox mock-ups
Subject text is cut at 70 characters for Gmail, 65 for Outlook and 45 for mobile. Preview text is cut at 50, 60 and 40.
Score from 0 to 100
The score starts at 50 and adds or subtracts points for length, spam words, all-caps words, a question mark, a number, personalisation, emoji, preview text and power words.
Spam trigger word list
Words such as free, winner, act now, discount and click here are listed as tags if they appear in your subject.
Power word list
Words such as exclusive, proven, discover, how to and new are shown as tags when found.
Personalisation detection
Placeholders like {{ }}, %first_name% or [name] earn bonus points, so you can test merge-tag subjects.
Live updates
Everything recalculates on each keystroke. There is no button to press.
Who uses it, and for what
- Newsletter writers. Compare three versions of next week's subject line and keep the one with the cleanest factor list.
- Small shop owners. Check a sale announcement for words that look like spam, and see how it appears on a phone.
- Marketers planning A/B tests. Draft two variants with different lengths, then send them to a split of your list to see real results.
- Copywriters. Check how a preheader continues the subject line. For naming ideas, try the Newsletter Name Generator or the Title A/B Tester.
Tips for better subject lines
- Put the key words in the first 40 characters. Mobile cuts the subject at about 45 in this preview.
- Write the preheader on purpose. Without one, the client shows the first lines of your email body instead.
- Spam words are matched anywhere inside the text, so save also matches saved, and win matches window. Read the flagged list with that in mind.
- A flagged word is not a ban. Spam filters look at many signals, including your sender reputation and email content.
- Do not rely on caps or several emojis. The tool deducts points for three-letter or longer all-caps words and for more than three emojis.
- Test the real thing. Send both versions to small groups and compare open rates after a day.
Is it safe? Privacy and limits
Everything runs in your browser as you type. The subject line, preview name and preheader are not sent anywhere by this tool and are not saved. The inbox previews are mock-ups with fixed cut-off lengths, and the sender address shown in the Outlook preview is a placeholder. The score is a simple checklist, not a prediction of opens or deliverability, which depend on your list and sender reputation.
Email subject line tester FAQ
It lets you see and check a subject line before you send it. This one shows how your subject and preheader might appear in three inbox styles, counts characters against a 40 to 60 target, and scores the line with a fixed set of rules. It also lists any spam trigger words and power words it finds. It does not send email or contact your email platform, so it only reviews the text you type.
This tool treats 40 to 60 characters as ideal, 30 to 75 as acceptable and anything else as too short or too long. That is a rule of thumb, not a standard. Mobile screens show far fewer characters than desktop, and the preview here cuts at 45 for mobile, 65 for Outlook and 70 for Gmail. Put the most important words first, so that a cut-off does not hide your point.
The score starts at 50. Length 40 to 60 adds 15, and 30 to 75 adds 5, otherwise 10 is deducted. No spam words adds 15, and each spam word deducts 8. No all-caps words adds 5, a question mark adds 5, a number adds 5, personalisation adds 10, one emoji adds 5, and preheader text of 30 or more characters adds 10. Power words add up to 10. The result is limited to 0 to 100.
No. Despite the label, it is only a checklist of common copywriting advice. It has no knowledge of your audience, sender name, send time or past results. A subject that scores 85 can still perform badly with your list, and a plain 50 can do well. Use the score to catch obvious issues such as shouting or a missing preheader, and use real A/B test data to judge performance.
They are words and phrases often found in unwanted email, such as free, winner, cash, act now and click here. The tool has a built-in list and flags any that appear in your subject. Modern spam filters look at much more than words: sender reputation, authentication, links, formatting and complaints. So a flagged word does not mean your email will go to spam, but over-using such words can hurt trust.
The checker matches text inside words, not whole words. So save is flagged in saved, win in window and free in carefree. The same applies to power words, where short words such as you or new can match inside other words. If a flag looks wrong, check whether the word is a part of a longer one. Treat the lists as prompts to re-read the line, not as hard rules.
No. They are simplified mock-ups with fixed cut-off lengths: the subject is shortened to 70, 65 and 45 characters, and the preheader to 50, 60 and 40. Real clients cut text by pixel width, so results vary by screen size, font, settings and whether the email is opened in an app. Use the previews to see roughly what gets trimmed, and send yourself a test email to check the real display.
Yes. It is free, has no signup and no limit on how many lines you test. It runs in your browser, and nothing you type is stored by the tool. If you are thinking about titles for other content, you may also like the Title A/B Tester. Always send a test email to yourself before a full send, since only your real email platform shows the final rendering.