Email Subject Line Tester
Paste your subject line. Get an instant score and specific fixes — before you hit send.
0
out of 100
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Get started with Kit →How the score works
Open rates live and die on the subject line — it's the one thing every subscriber sees before deciding to open or ignore. This tool checks the factors that consistently move opens, based on how inbox providers and readers actually behave:
- Length (30–50 characters) — long subjects get truncated on mobile, where most email is read.
- Spam triggers — words like "free", "guarantee", and "act now" raise filter risk and lower trust.
- Power & curiosity words — specific, emotional, or curiosity-driven language earns the click.
- Personalization — a {first_name} token or "you/your" lifts relevance.
- Numbers — concrete numbers ("7-day", "3 ways") outperform vague claims.
- Restraint — ALL CAPS and !!! read as shouting and hurt deliverability.
The score is a directional guide, not gospel — the only true test is your own A/B test. But fixing the flags below will almost always help.
Subject line formulas that score well
- Curiosity gap: "The mistake costing you 40% of your opens"
- Number + benefit: "3 emails that doubled our reply rate"
- Personal + specific: "{first_name}, your draft is ready"
- Contrarian: "Stop A/B testing your subject lines"