Email Subject Line Tester
Length, spam phrases, caps and preview text, with phone and desktop inbox previews.
About this tool
Type the subject line, the preview (preheader) text and a from name. The tool counts characters and approximate pixels, flags all-caps words, multiple exclamation marks, classic spam phrases, fake RE:/FWD: and missing preview text, notes the things that tend to help, and renders the inbox row at phone and desktop widths so you can see where it truncates.
What actually affects opens
Subject-line tools that award a score out of 100 are selling confidence, not evidence; what can be checked honestly is mechanics. Length: phones show roughly 35–45 characters and desktops 60–70, so the important words go first. Signals: all-caps words, stacked exclamation marks and the old spam vocabulary still count against small senders even though modern filtering is mostly about sender reputation and engagement. Preview text: the inbox shows it next to the subject, and leaving it empty means the first line of the email — often ‘View in browser’ — is what people read. Things that tend to lift opens across studies: a specific number, personalisation that is actually relevant, a question for outreach, and a clear promise the email keeps. None of it substitutes for a test: send two subject lines to 10–20% of the list and let the A/B test calculator tell you whether the difference is real. Nothing is stored or sent. The headline analyzer does the same job for article titles, the email list cleaner protects your sender reputation, and the email signature generator finishes the email.
Frequently asked questions
How long should a subject line be?
Under about 45 characters to survive phone inboxes; up to 60 is fine if the key words come first. The preview shows the cut-off.
Do spam words still matter?
Less than they did — reputation and engagement dominate — but for new domains and small lists the classic phrases still hurt, and they read as spam to humans regardless.
What is preview text?
The snippet shown after the subject in most inboxes (also called the preheader). Set it deliberately; 40–90 characters is the useful range.
Does it predict my open rate?
No, and nothing can. It checks the mechanics; an A/B test on your own list measures the result.