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About this tool
Score email subject lines for spam risk, readability, urgency and clickability, preview inbox truncation across Gmail, Outlook and Apple Mail, and A/B test two subject lines side by side.
The Email Subject Line Tester grades a subject line out of 100 using nine weighted metrics — length 20%, spam safety 15%, readability 15%, personalisation 10%, urgency 10%, power words 10%, capitalisation 10%, numbers 5% and emoji 5% — and shows how the line truncates in Gmail, Outlook and Apple Mail. It is for email marketers and anyone scheduling a campaign who wants a reproducible read on a line before it goes out. Every score is rule-based and deterministic: readability uses the real Flesch Reading Ease formula, and spam risk comes from a 75-phrase trigger dictionary plus penalties for exclamation marks, ALL-CAPS words and repeated symbols.
Open Email Subject Line Tester on AltFTool — it loads instantly in your browser.
Type into the Subject line box (placeholder "Type or paste your email subject line…", capped at 300 characters), press Paste to pull the line from your clipboard, or Try an example to cycle through the six built-in samples. The line below counts "N characters · N words" and flips to "Likely to truncate on most inboxes" past 60 characters.
There is nothing to submit — scoring runs live as you type. The overview shows the score gauge, the grade letter and its label (A+ Excellent, A Great, B Good, C Needs Work, D High Risk), a "N characters · N words · Clickability N/100" line and the nine weights under Scored on, while the tabs beneath hold Metrics, Highlights, Suggestions, Variations, Inbox Preview, Insights, Compare (A/B) and History.
Under Export, click PDF, JSON or TXT — the button reads Exporting… while it runs — to download the metric breakdown, suggestions and variations as email-subject-analysis-<subject-slug>.pdf, .json or .txt, the slug being your subject lowercased, hyphenated and cut to 40 characters. Clear empties the box so you can start on the next line.
The nine metric weights are exposed in the interface and sum to 1, so you can see exactly why a line scored what it did and which metric to fix first.
Spam, urgency, power and personalisation matches are highlighted directly in your subject with spam taking priority on overlaps, so you edit the exact words that cost points.
The full metric breakdown, suggestions and generated variations export as PDF, TXT or JSON, so a score can be attached to a campaign brief or diffed later.
30 to 50 characters scores a perfect 100 on the length metric here; outside that band the score drops about 3 points per character of distance, with a floor of 10. That range sits inside every major client's truncation point except Gmail Mobile, which cuts at roughly 40 characters.
Each match against the 75-phrase spam dictionary costs 12 points, capped at 60. On top of that: one exclamation mark adds 5 penalty points, two add 10, three or more add 15; each ALL-CAPS word longer than two letters adds 10 up to a cap of 30; two or more dollar signs add 10; and repeated symbols like !! or $$ add another 10.
It applies Flesch Reading Ease — 206.835 minus 1.015 times the word count minus 84.6 times syllables per word — treating the subject as a single sentence, then clamps the result to 0-100. Short phrases can push the raw formula above 100 or below 0, which is why the clamp exists.
Approximately 70 characters in Gmail Desktop, 40 in Gmail Mobile, 60 in Outlook's reading-pane list and 90 in Apple Mail on iPhone or iPad. These are widely-cited approximations rather than pixel measurements — real truncation shifts with font, zoom and window width, so treat them as a planning guide.