Draft A
Enter a subject containing letters or numbers to see its checks and previews.
Email subject line tester
Use this email subject line tester to compare three drafts with their own preheaders. See sourced inbox previews, expand a sample name and inspect every editorial point before a real A/B test.
Local draft checks. No AI call, upload or opening-rate prediction.
This email subject line tester compares three subjects with separate preheaders, sample personalisation and sourced Gmail, Outlook and Apple Mail display scenarios. Every editorial point shows its rule, source and checked date. Provider guidance informs reply, forward and deceptive badge reviews. Scores predict neither opens nor delivery. Optional saved comparisons stay on your device.
Compare subject lines and preheaders
Each draft has its own preview text. The score counts editorial checks passed; it cannot choose an A/B winner or predict opens, clicks, spam placement or delivery.
Recognised first-name fields such as {{first_name}}, *|FNAME|* and [NAME] expand before counting. Unknown fields remain flagged. Test your platform and a missing-name fallback separately. Sample name limit: 80 text units.
Subject 30 adopts the top of Litmus generic mobile range of 25 to 30. Preview 40 is our choice within Email on Acid mobile range of 30 to 80. Changing a budget uses your assumption for this scenario. Other inbox previews keep their published defaults.
Litmus display guidance Checked 2026-10-12Email on Acid preview guidance Checked 2026-10-12These character budgets illustrate cuts. Apple Intelligence can show a summary instead of your preheader. Outlook can hide previews or show one to three lines. Campaign Monitor guidance cites 2021 device data; it is not a current app measurement.
Apple Mail summaries and previews Checked 2026-10-12Outlook message preview settings Checked 2026-10-12Campaign Monitor display guidance Checked 2026-10-12Draft B
Enter a subject containing letters or numbers to see its checks and previews.
Draft C
Enter a subject containing letters or numbers to see its checks and previews.
What this score measures
AIGROW editorial method, dated 2026-10-12: display fit 25 points, distinct preheader within its budget 10, no false reply or forward prefix 20, no unresolved supported tokens 25, no whole-field capitals 10, and no repeated question or exclamation marks 10. A pass earns all points for that check; a flag earns zero. Missing preheaders and uncased scripts skip their checks. The displayed score is earned points divided by applicable points, times 100, rounded.
Capitalisation means at least four cased letters with the whole field uppercase. Repeated punctuation means at least two adjacent question or exclamation marks. These thresholds and point weights are our editorial choices, not measured predictors. Emoji and badge reviews add no points. Counts use Unicode graphemes after whitespace normalisation and sample-name expansion, so joined emoji count once.
Gmail asks for accurate subjects and actual reply or forward prefixes. It also warns against graphic imitation of verified identity. Yahoo asks for accurate, non-deceptive headers. Neither guide supplies our old promotional spam-word list. We do not label words such as free as automatic spam triggers. We cannot check authentication, reputation, consent, complaints, links or message-body filtering from a subject alone.
Gmail sender guidelines Checked 2026-10-12Yahoo sender guidance Checked 2026-10-12Display figures were checked on 2026-10-12. They are sourced guidance and disclosed assumptions, not measurements of current inbox apps. Font, sender name, screen width, app version and preview settings change truncation. Custom budgets allow 1 to 200 visible characters. Send real inbox tests and run an A/B campaign with your own data before choosing a winner.
Campaign Monitor display guidance Checked 2026-10-12Litmus display guidance Checked 2026-10-12Email on Acid preview guidance Checked 2026-10-12Each subject and preheader accepts 2,000 UTF-16 text units. Optional history saves at most five three-draft comparisons in this browser only. It saves when you choose Save, and Delete removes this tool history. Other legacy writing-tool history is separate. No draft or sample name is sent to a server.
How to use this tool
- Write the subject and preview
- Enter up to three subjects with a separate preheader for each. Each field accepts 2,000 UTF-16 units. Counts use visible Unicode graphemes after whitespace normalisation and sample-name expansion. Empty preheaders may fall back to message-body text. The selected message type tells the tester whether a reply or forward prefix is appropriate.
- Read sourced display scenarios
- Sources checked 12 October 2026. Campaign Monitor supplies Gmail 70 and iPhone portrait 41; Litmus supplies generic desktop 60 and mobile 25 to 30. Email on Acid supplies mobile preview 30 to 80, webmail 40 to 120, Outlook 50 and iOS Mail 84. Each scenario names adopted assumptions separately from client-specific guidance. Adjust budgets and send real inbox tests.
- Inspect every point and sender setup
- AIGROW editorial weights dated 12 October 2026: subject fit 25, distinct preheader fit 10, reply or forward honesty 20, no unresolved tokens 25, capitalisation 10 and punctuation 10. Skipped checks remove available points. Emoji and badge reviews have no points. No word list or score predicts spam or opens. Save up to five comparisons locally, restore them or delete this tool history.
Questions before you start
See how AI assistants describe your business
Run the free visibility scan to check which sources assistants use. If you need regular monitoring, compare the plans before choosing one.