Definition
Subject line truncation occurs when an email subject line exceeds the number of characters an email client is willing to display, causing the end of the line to be cut off and replaced with an ellipsis ("..."). Because the visible length differs by device, client and screen size, a subject line that fits on desktop may be truncated on mobile.
This matters because the most important information in a subject line usually sits at the end. When that tail is hidden, the recipient only sees part of your message — and often the weakest part.
Average Subject Line Length by Client
| Email Client / Device | Approximate Visible Characters |
|---|---|
| Desktop email clients (wide) | 60-75 characters |
| Mobile clients (portrait) | 30-45 characters |
| Gmail app (mobile) | 35-45 characters |
| Apple Mail on iPhone | 30-40 characters |
| Outlook mobile | 25-35 characters |
| Webmail in a narrow pane | 45-55 characters |
Note that character limits are not fixed. They vary with screen width, email client and font size, so these figures are useful ranges rather than hard rules.
How to Avoid Losing Your Message to Truncation
- Front-load the key information: Put the hook, value or offer in the first 30-40 characters so it survives even the narrowest mobile previews.
- Keep subject lines to 40-50 characters: This is the safest range for the modern, mobile-heavy inbox.
- Do not rely on the final words for meaning: If the tail holds the main point, the message is lost for mobile users.
- Test the preview on mobile: Render the email at a phone width before sending to see exactly where truncation cuts in.
- Let the preheader carry the tail: If a longer subject is unavoidable, make the preview text complete the meaning rather than repeating it.
Common Subject Line Truncation Stats
| Approach | Typical Outcome |
|---|---|
| Subject under 40 characters | Nearly always fully visible on mobile |
| Subject 50-60 characters | Fully visible on most desktops, cut on mobile |
| Subject over 70 characters | Cut on almost every device |
Related Glossary Terms
Above the Fold in Email
Above the fold in email refers to the content visible to a recipient before they scroll, a critical area for first impressions and primary calls to action.
Accessible CTA (Email)
An accessible CTA is a button or link coded with sufficient colour contrast, visible focus states, descriptive screen-reader text, and a minimum touch target of 44×44 pixels.
Adaptive Design
Adaptive design in email uses predefined layouts that adjust to specific device or client widths, rather than fluidly resizing to every possible screen.
AI-Generated Content in Email
AI-generated content in email is copy, images, code or subject lines produced by artificial intelligence tools to speed up campaign production and testing.
Alt Text
Alt text is the written alternative to an image in an email, displayed when images are blocked, slow to load, or consumed by screen readers.
Autoplay Video in Email
Autoplay video in email refers to the limited ability to play video content automatically when an email is opened, using specific client-supported techniques.
Frequently Asked Questions
A length of 40-50 characters is the safest for the modern inbox. This range is short enough to avoid truncation on most mobile clients while still long enough to communicate a clear message. If you must go longer, front-load the essential information in the first 30 characters.
Mobile clients display fewer characters because the screen width is narrower than a desktop inbox. On a portrait phone screen, a client may only show 30-45 characters before cutting off the rest. Since a large share of opens happen on mobile, designing for that constraint matters.
Yes. If truncation cuts off the most compelling part of your subject line, recipients lose the reason to open. Studies and ESP testing consistently show that concise subject lines that survive truncation perform better than longer ones that are cut mid-thought.
Not necessarily. The goal is to keep the meaningful message visible, not to hit an arbitrary length. A 55-character line that carries the hook early can outperform a 25-character line that is vague. Test subject lengths for your specific audience to find the sweet spot.