About the Email Subject Line Tester
This email subject line tester checks a subject line against the habits that hurt opens and deliverability. Paste your subject (and preheader, if you use one) and it counts characters and words, shows how the line looks when a phone cuts it off at about 40 characters and a desktop inbox at about 60, and flags spam trigger words, ALL-CAPS words, repeated exclamation or question marks, too many emoji and fake “RE:” or “FW:” prefixes.
It is built for email marketers, founders sending newsletters and anyone writing cold outreach who wants a quick second opinion before hitting send. Each check is transparent — you can see exactly which rule cost points — so the score is a checklist, not a black box.
No tool can predict open rates: your sender reputation, list quality and relationship with subscribers matter far more than any single word. Modern filters judge the whole message and its sending history, so a flagged word is a caution, not an automatic block. Use the score to catch avoidable problems, then A/B test the subject lines that pass.
With the default inputs, the subject line score (out of 100) is 100. Change any value above to recalculate instantly.
How to use the email subject line tester
- 1Type or paste your subject line.
- 2Optionally add the preheader text that appears after the subject.
- 3Check the mobile preview to make sure the key words appear before the cut-off.
- 4Fix any flagged spam words, ALL-CAPS words, punctuation or emoji.
- 5A/B test two strong versions with a slice of your list before the full send.
Formula and method
The score starts at 100 and subtracts points for each problem. Length: 30–50 characters costs nothing; 21–29 or 51–60 costs 10; 20 or fewer, or 61–70, costs 20; over 70 costs 30. Each spam trigger phrase found as a whole word costs 10 (up to 30), each ALL-CAPS word of four or more letters costs 5 (up to 15), more than one “!” or “?” costs 10, more than two emoji costs 10, and a fake “RE:”/“FW:” prefix costs 10.
Characters are counted as Unicode code points, so an emoji counts as one. Mobile and desktop previews cut the line at 40 and 60 characters, typical of phone and desktop inboxes; actual truncation depends on the email client, screen width and font. Verdicts: 85+ Excellent, 70–84 Good, 50–69 Fair, below 50 Needs work.
Worked examples
Clear promotional subject line
At 49 characters the subject sits in the 30–50 sweet spot, contains no flagged phrases, caps or excess punctuation, and scores 100. On a phone it is cut after “20% off en…”, so the offer is still visible.
Spammy all-caps subject
Three spam phrases (“free”, “act now”, “limited time”) cost 30 points, two caps words (FREE, GIFT) cost 10, three exclamation marks cost 10 and three emoji cost 10, leaving 40 out of 100.
Fake reply prefix
Eighteen characters is very short (−20), and the “RE:” prefix pretends to be a reply (−10), giving 70. Fake reply prefixes can erode trust and are prohibited in some regions’ marketing rules.
Long subject that will be truncated
At 99 characters the line loses 30 points for length and 10 for “free”, scoring 60. Mobile readers see only “Sarah, your cart is waiting (and so is f…”, so the offer and deadline are hidden.
Frequently asked questions
How long should an email subject line be?+
Many marketers aim for roughly 30–50 characters. Phones often show only about 30–40 characters and desktop inboxes around 60, so the most important words should come first regardless of total length.
Do spam trigger words really send email to spam?+
Not on their own. Modern filters at Gmail, Outlook and Yahoo weigh sender reputation, authentication (SPF, DKIM, DMARC), engagement and complaints far more than individual words. Heavy use of hype words, caps and symbols still adds risk and can look untrustworthy to readers.
Should I use emoji in subject lines?+
One relevant emoji can help a line stand out, but results vary by audience, and some clients render emoji differently. Avoid replacing key words with emoji and avoid strings of them, which look promotional.
What is preheader text?+
The preheader is the short preview snippet shown after or below the subject in most inboxes. If you do not set one, the inbox shows the first text in your email, which is often “View in browser” — a wasted chance to add a reason to open.
Is it OK to start a subject with RE: or FW:?+
Using RE: or FW: when the email is not actually a reply or forward is misleading. In the US, the CAN-SPAM Act prohibits deceptive subject lines in commercial email, and it damages subscriber trust even where it is not illegal.