Email Subject Line & Preheader Preview Tester
Test email subject line length and preheader preview text against mobile iPhone Mail, Gmail, and Outlook truncation boundaries to maximise email open rates.
1. Subject Line & Preheader
Recommended preheader length is 40 to 90 characters to avoid empty template code trailing.
No common commercial spam filter trigger words detected in your subject or preheader.
2. Mobile & Desktop Inbox Mockups
iOS Mail & Mobile Lockscreen
Gmail Desktop Inbox Row
About this tool
How to use it
- Enter your Sender or Brand Name (e.g., 'Acme Studio' or 'Sarah from Support').
- Type your email subject line to track character and word count against the 40-to-60 character mobile cutoff threshold.
- Draft your preheader (preview text) to fill the 40-to-90 character buffer displayed next to or below the subject line.
- Inspect the automated Spam Trigger Word audit to catch phrases that trip ISP spam filters (such as '100% free', 'urgent', or 'cash').
- Preview the realistic iPhone iOS Mail and Gmail Desktop inbox mockups to evaluate visual truncation and reader impact.
- Click 'Copy Subject Line' or 'Copy Subject + Preheader' to paste into Mailchimp, Klaviyo, HubSpot, or Substack.
Why it matters
Over 55% of all marketing and newsletter emails are opened on mobile devices where subject lines longer than 45 characters are truncated with an ellipsis (...). Furthermore, leaving preheader text empty causes email clients to display awkward raw template code like 'View in web browser' or 'Unsubscribe'. A pre-flight simulator ensures your hook is 100% readable.
When to use it
Use before broadcasting email marketing campaigns, launching product announcements, publishing Substack newsletters, or sending B2B cold sales outreach.
Good to know
Optimal email subject lines typically range from 6 to 10 words (35 to 45 characters). Placing your core offer or value hook at the very beginning ensures maximum open rates regardless of screen size.
Common questions
What is the best character length for an email subject line?
The ideal length is between 35 and 45 characters (roughly 6 to 9 words). This ensures your entire message displays cleanly without being cut off on mobile devices like iPhones and Android smartphones.
What is an email preheader?
A preheader (also known as preview text) is the short snippet of text displayed immediately following or below the subject line in an email inbox. It acts as a secondary headline to boost open rates.
What happens if I don't set a preheader?
If you do not specify a preheader, email clients will automatically grab the first readable text from your email body, which often results in unsightly text like 'Can't see images? Click here to view online'.
Should I use emojis in my email subject line?
Using a single well-chosen emoji can increase visual contrast in a crowded inbox. However, using multiple emojis often looks spammy and can cause spam filter flags.
Is my confidential newsletter draft or subject uploaded to any server?
No. All character counting, spam checks, and inbox mockups run 100% locally inside your web browser. Zero marketing copy is ever transmitted or stored.