How the preview estimates truncation
Inbox apps cut text based on how wide it renders on screen, which depends on the font, the device and the user's text-size settings. Since that can't be known in advance, this tool uses simple "width units": most characters count as 1, while emoji and full-width characters (like Japanese or Chinese) count as 2 because they usually take more space.
| Preview | Subject budget | Preview text budget |
|---|---|---|
| Small phone | ~30 units | ~40 units |
| Typical phone | ~40 units | ~90 units (about two lines) |
| Desktop row | ~70 units | ~50 units after the subject |
These are rules of thumb, not exact limits. Use them to spot obvious problems, then confirm with a real test send.
Tips for subject lines that survive truncation
- Front-load the point. Put the offer, the topic or the reason to open in the first 30–40 characters.
- Use preview text as line two. Add new information instead of repeating the subject.
- Check merge tags. A broken
{{first_name}}in the inbox is worse than no personalization. - Go easy on emoji. One can add personality; several may render differently across devices.
Measuring the impact? Open rates are less reliable than they used to be because of privacy features in some mail apps, so pair them with clicks. Tag your links with the UTM Builder to see which subject lines drive real traffic.