Paste a subject line and get an instant 0–100 score for open-rate potential and spam risk — plus a live inbox preview showing exactly where it gets cut off. No signup, nothing stored.
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The best-tested subject line still needs a deliverable email platform behind it. Kit (formerly ConvertKit) is our pick for creators — free up to 10,000 subscribers.
Start free on Kit →Aim for 30–50 characters. Most mobile inboxes cut off around 35 characters and over half of all opens happen on mobile, so front-load the important words. This tester shows the exact cut-off point in the preview above.
The score combines 11 signals correlated with open rate and deliverability: length, word count, spam-trigger words, ALL-CAPS, excessive punctuation, emoji use, personalization, power/emotion words, urgency, numbers, and question phrasing. It runs entirely in your browser — nothing is uploaded.
One well-placed emoji can lift opens, but two or more often trips spam filters and looks noisy on desktop. The tester flags when you cross that line.
Money and pressure words are the worst offenders — "free", "guarantee", "act now", "$$$", "winner", "risk-free". The checklist above lists any it finds in your subject line so you can swap them out.