DievioEmail Subject Line Tester
Compare email subject lines for character count, word count, punctuation and unresolved personalization. Preview how a shortened subject might look on mobile.
How to use this tool
- Enter one subject line per line.
- Compare length, preview and text checks.
- Choose a subject to test with your actual audience.
A worked example
Input
{{first_name}}, quick idea for {{company}}Output
Unresolved merge tag · mobile preview may truncate
Preview personalization with actual recipient rows before comparing the final rendered subject.
What the result means
Compare readability and specificity, then test your chosen subject with your actual audience. The mobile preview is illustrative, not a rendering of every email client.
Actual display length depends on the email client and font. This tool does not predict open rates or spam-filter decisions.
Frequently asked questions
Can this predict the open rate?
No. It checks properties of the text and displays an illustrative preview. Opens depend on audience, sender identity, timing, client behavior and measurement limitations.
Why is the mobile preview limited to 35 characters?
It provides a quick view of how a shortened subject might read. The exact number of visible characters varies by font, device, email client and inbox layout.
Is this free, and are my inputs uploaded?
Use this tool without an account, subscription or credits. Processing and file exports run in your browser; the tool does not upload your inputs or send emails. Clear inputs to remove the current data. Inputs are not saved when you reload the page.
Keep working with free tools
Cold Email Copy Checker
Check a cold email for length, multiple calls to action, common high-pressure phrases, missing merge fields and a missing opt-out. Get transparent editing suggestions.
Open tool →Mail Merge Preview
Preview an email template against your own CSV rows before sending. Find unknown variables and empty cells, with optional fallback values for missing personalization.
Open tool →Outreach Volume Calculator
Work backward from a meeting goal to the number of unique prospects and maximum emails required. Use your reply and meeting rates, outreach period and sequence length.
Open tool →