Free tool · Lab 10
Email subject line tester
Type your subject line, preheader and sender name and see how they look in a Gmail-style inbox, on mobile and on desktop, with the part that gets cut off marked. You get an explained score, an A/B comparison and six starting variants.
Cut-off thresholds · approximate, editable
Mobile · inbox, approximate cut-off
Desktop · inbox, approximate cut-off
Score for subject A · rule-based
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The score is the sum of points across 11 criteria (maximum 100). Each criterion’s maximum is shown in the chart. It measures how well rules of thumb are followed, not the real results of a campaign.
This tool applies rules of thumb and does not test real deliverability: it cannot tell whether an email lands in the Inbox, in Promotions or in Spam, and it does not predict open rate. The preview is approximate, because every email app displays things differently. Your text stays in your browser.
Variant generator · 6 templates based on Subject A
These are rule-based templates, not text written by an AI model: adapt them to what your email actually promises. Each variant is scored with the same rules so you can see the effect.
How the tester calculates the score
The tester applies rules of thumb, not a statistical model. Each criterion earns a fixed number of points, and the total gives a score from 0 to 100. The rules are the ones anyone preparing a campaign checks:
- Length (16 points): the comfortable range is 25–60 characters; shorter gets vague, longer gets cut off. The cut-off thresholds (about 35 characters on mobile, 65 on desktop) are editable estimates.
- Capitals (10) and punctuation (10): words written entirely in capitals, multiple exclamation marks and repeated symbols lose points.
- Emoji (5): one is acceptable; three or more clutter the subject.
- Risk words (15): a list in Romanian and English with two tiers: words strongly associated with spam (−7 points) and common promotional words (−3 points).
- Urgency (8) and clickbait (10): urgency without a verifiable deadline (date, time, stock) loses points, as does phrasing like “you won’t believe”.
- Personalisation (6) and specificity (5): a merge tag or first name earns points, and so does a concrete number; generic wording such as “newsletter” or “new guide” does not.
- Preheader (15): 8 points if it is filled in (40–100 characters) and 7 if it adds new information instead of repeating the subject.
Read the score like this: 90–100 very good, 70–89 good, 50–69 fair, below 50 weak. A question in the subject and the sender name appear as notes with no points. In the A/B comparison, a gap under 5 points is treated as a tie.
What it does not do: it does not test real deliverability and does not predict open rate. Whether an email lands in the Inbox, in Promotions or in Spam depends on domain reputation, SPF, DKIM and DMARC authentication, list quality and how recipients react. A healthy list starts with consent and GDPR, and the emails worth sending automatically are covered in the guide to essential email automations. The final verdict between two subjects comes only from an A/B test on random segments of your list. Writing principles are in the article on email subject lines, and we can take over the full execution through our email marketing and automation service.
Frequently asked questions
About email subject lines and preheaders
How many characters should an email subject line have?
There is no universal limit. As a rule of thumb, 25–60 characters are comfortable. On mobile you usually see around 30–40 characters and on desktop 60–70, but it depends on the app and the screen. That is why the main idea is worth putting at the start of the subject.
What is a preheader and why does it matter?
It is the short text shown next to or under the subject in the inbox. If you do not set one, the app takes the first lines of the email, often “View in browser”. Used well, it continues the subject’s idea with a benefit or a detail instead of repeating it.
Does the tester tell me whether my email will land in Spam?
No. Deliverability depends on domain reputation, SPF, DKIM and DMARC authentication, list quality and how recipients react. The tester only flags wording and symbols that are often associated with unsolicited messages, so you can avoid them from the start.
Can I use emoji in a subject line?
Yes, but in moderation. One emoji can stand out in a busy inbox, yet it looks different on each device, and several make the subject look like an advert. Always check how it looks in the apps your audience uses most.
How do I choose between two subject lines?
The score helps you remove obvious mistakes, but it does not say which subject will be opened more. Send version A to one random segment of your list and version B to another, both large enough, and compare the results. Then send the winner to the rest of the list.
Are the subject lines I type saved?
No. Everything is calculated locally in your browser, and the text is not sent anywhere or stored. If you use the button to send the result to us, the summary goes into the contact form only after you submit it yourself.
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