Subject Line Tester
Score your subject line before you send. Checks length, spam triggers, and mobile truncation.
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How this tool works
Paste your draft subject line into the tester above. The tool runs a rule-based analysis in your browser — no data is sent to a server until you optionally export results.
Each subject line is scored from 0–100 across four dimensions: length (ideal 30–50 characters for mobile preview), clarity (specific nouns vs vague hype), curiosity (open loops without clickbait), and spam risk (trigger words that Gmail and Outlook penalize).
The score updates instantly as you edit. Suggested rewrites appear when a dimension falls below threshold — use them as starting points, not copy-paste replacements. Your voice still matters.
Before sending a campaign, test 2–3 variants with a small segment (5–10% of your list) and compare open rates over 24 hours. This tool tells you which variants are structurally sound; your audience confirms which one wins.
Methodology & benchmarks
- Length scoring uses the 30–50 character band recommended by Litmus and Mailchimp deliverability guides — mobile clients truncate around 40 characters on iOS Mail.
- Spam trigger detection checks against a curated list of 80+ high-risk words (FREE, act now, limited time, $$$, etc.) compiled from SpamAssassin rules and creator-newsletter bounce reports.
- Curiosity scoring rewards specific numbers, questions, and unfinished thoughts; it penalizes ALL CAPS, excessive punctuation (!!!), and emoji clusters above two characters.
- Benchmark context: creator newsletters average 38–45% open rates (InfluencersKit aggregate, 2025). Subject lines scoring 70+ correlate with above-median opens when paired with consistent sender reputation.
Frequently asked questions
What is a good subject line score?
Aim for 70 or higher before sending. Scores in the 80–90 range typically have optimal length, no spam triggers, and a clear hook. Scores below 50 usually need a rewrite — either too long, too vague, or loaded with spam words.
Does a high score guarantee more opens?
No. Subject line structure is one factor. Sender reputation, send time, list hygiene, and whether subscribers recognize your name matter equally. Use this tool to eliminate obvious mistakes, then A/B test the top candidates.
Why does my subject line get flagged for spam words?
Email filters scan for promotional language patterns. Words like 'free', 'guarantee', and 'urgent' raise spam scores even in legitimate newsletters. Replace them with specific value propositions — e.g. '3 tactics I used to hit 50% opens' instead of 'FREE tips inside!!!'.
How long should a newsletter subject line be?
30–50 characters displays fully on most mobile clients. Desktop clients show 60–70 characters. Front-load the hook — put the most compelling words first, because truncation cuts from the end.
Can I test subject lines for automated sequences?
Yes. Welcome emails and nurture sequences benefit most from testing because they run repeatedly. Score each email in the sequence, then monitor per-email open rates in your ESP analytics after 2–4 weeks of data.
Is this tool free?
Yes. The subject line tester is free with no signup required. Optional email export (to save results) is gated for lead capture only — the scoring itself runs entirely in your browser.