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The Email Subject Line Tester scores subject lines for expected open rate, spam risk, mobile truncation, and emotional pull. Scoring is based on patterns observed across 50M+ sent emails for our clients.
Your Klaviyo flows are leaking revenue
Your preheader text appears here alongside the subject...
38 chars — optimal for desktop and mobile
No spam trigger words detected
2 power words — good emotional signal
Uses "you/your" — personal voice lifts open rates
Want email driving 35% of revenue?
Our Email & Retention service builds 12+ Klaviyo flows plus SMS.
Email servicesInput current metrics — no signup, no data saved to servers. Everything runs in your browser.
Results update live as you change inputs. See verdicts, benchmarks, and context, not just raw numbers.
Follow the "What this means" recommendations. Share with your team or bring to your next strategy review.
Test subject lines before sending campaigns
A/B variant generation for email sends
Audit subject line patterns across flows
Screen for spam triggers in bulk sends
Subject lines drive 40% of email open rate variance. A subject that scores well on clarity, specificity, and curiosity consistently outperforms clever or clickbait alternatives. Our tester runs subject lines against a scoring model trained on actual open-rate data, flags spam-trigger words and phrases, and checks mobile preview truncation. Output is a composite score plus specific improvement suggestions.
Our Email & Retention service takes this to the next level — full audit, strategic roadmap, and senior operators executing the work. Starts at $6k/month, quarterly contracts.
Yes — 100% free, no signup, no email wall. We built it because our operators needed it and decided to share.
No. All calculations run entirely in your browser. Nothing is sent to or stored on any server.
Reach out — we will share an embed snippet for teams that want to use this inside their own workflows.
Yes. No signup required, no data collected, nothing sent to our servers. Everything runs in your browser. We publish free tools because they build trust with operators who might eventually hire us — that's the entire business model.
The math is correct and the formulas are standard. The output is only as good as the inputs you provide — garbage in, garbage out applies here like anywhere. We recommend calibrating with at least 3 months of real data before making major decisions based on the results.
The tool lives at a stable URL that you can share with your team. We don't currently offer embeds, but the output is shareable via the copy button on calculator results. For white-label or embedded versions, contact us directly.
Three reasons. First, they help operators make better decisions even if they never hire us. Second, they let us show our thinking publicly — the frameworks behind these tools come straight from our client engagements. Third, they attract the kind of operators who later become great clients.
The free tools cover standard scenarios. For custom analysis — multi-channel attribution, cohort LTV modeling, MMM — we do this as part of paid engagements. Our CRO & Analytics service starts at $6k/month and includes custom dashboarding plus BigQuery setup if needed.
When in doubt, read the source. These are the documents and datasets we reference for our email work.
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Patient organic growth that compounds month over month.
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More revenue from the traffic you already earn.
DTC brands doing $1M–$50M. Acquisition, retention, margin.
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Minimums shown · Stage-adjusted pricing · No 12-month lock-ins · Money-back guarantee
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