What this email subject line tester checks
An email subject line tester scores a subject line on the things that decide whether it gets opened: whether it fits on a phone screen, whether it contains words that spam filters and readers distrust, whether it shouts, and whether it gives the reader a concrete reason to open. This free subject line checker also scores the preview text, because Gmail and Apple Mail show the two together and a good subject with an empty preview wastes half the space.
The score is deterministic. The same line always gets the same number, and every deduction is listed below. It is not a prediction of open rate, which depends on your list, your sender reputation and the day you send. Treat it as a pre-flight check that catches the mistakes people make when they are in a hurry.
The scoring rules, in full
Every line starts at 100. These are the assumptions this tool uses, and the deductions.
| Check | Rule | Deduction |
|---|
| Mobile cut-off | About 40 characters is what most phone inboxes show before the ellipsis | 8 over 40, 15 over 55, 25 over 70 |
| Too short | Under 15 characters rarely says anything | 10 |
| Spam trigger words | A list of 28 phrases such as free, guaranteed, act now, click here | 10 each, up to 30 |
| Shouting | More than half the letters in capitals | 15 |
| Exclamation marks | None allowed, except one for a promotion | 5 each, up to 15 |
| Fake reply | Starts with Re: or Fwd: on a first-touch email | 20 |
| Emoji | Allowed, except on cold outreach and transactional email | 8 in those cases |
| Cold outreach with no hook | No number and no question | 5 |
| Preview text | Empty, under 30, over 110, or a repeat of the subject | 4 to 8 |
The 40-character mobile figure is an assumption. Inbox apps differ by device width and font size, and none of them publish a fixed number. If your list is mostly desktop, the cut-off matters less.
What actually gets an email opened
- Specificity beats cleverness. "3 changes that cut our no-show rate" opens better than "Big news inside" because the reader knows what they get.
- The sender name does half the work. A person's name from a domain the reader recognises matters more than any subject line. Fix that first.
- Preview text is a second subject line. Use it to finish the thought, not to repeat it.
- Numbers and questions earn a second look. Not because of magic, but because they force you to be concrete.
- Fake urgency costs you the next email. "Last chance" on something that is not the last chance trains the list to ignore you.
Testing a subject line properly
A score tells you a line is not broken. Only a send tells you it works. Split the list, send two subject lines to a small slice each, wait a few hours, then send the winner to the rest. Most email tools do this automatically. Keep a record of the winners, because after ten sends the pattern in your own list is worth more than any general rule, including the ones on this page.
Questions people ask
How long should an email subject line be?
Short enough to read on a phone, which is roughly 40 characters before the inbox adds an ellipsis. The tester uses 40 as its assumption. Longer lines are not wrong, as long as the point is in the first 40 characters.
Do spam words really block delivery?
Modern filters weigh sender reputation and engagement far more than individual words, so one word rarely blocks a send. The bigger cost is the reader: words like free and guaranteed signal a pitch and lower opens. The tester flags them for both reasons.
Is this a spam score?
No. It does not test your sending domain, authentication, list hygiene or content-to-image ratio, which are what spam filters actually score. Use a deliverability tool for that. This checks the subject line itself.
Should I use emoji in subject lines?
For newsletters and promotions, an emoji can help a line stand out and the tester allows it. For cold outreach and transactional email it reads as out of place, so the tester deducts there.
Why does the preview text count?
Because inboxes show it next to the subject on desktop and under it on phones. An empty preview means the inbox grabs the first line of the email, which is often "View this email in your browser". The tester deducts for empty, too short, too long or a repeat of the subject.
Can I test many subject lines at once?
Run them one at a time; the link in your address bar carries the inputs, so you can bookmark or share each result. Batch scoring is not offered because the point is to read the checks, not to rank fifty lines.
Last reviewed 14 September 2026. Formulas and benchmarks are published on the page so you can check them. This tool gives estimates, not quotes.
