Subject line tester
Count a subject line's characters and see how much of it each of five inbox clients shows.
Subject line tester, the tool
What each client shows
- Gmail app, Pixel 933 charactersNothing typed yet.
- iOS Mail, iPhone 1641 charactersNothing typed yet.
- Outlook desktop55 charactersNothing typed yet.
- Apple Mail, macOS65 charactersNothing typed yet.
- Gmail on the web70 charactersNothing typed yet.
Flags
Flags appear as you type.
Flags are prompts to reread. Google’s sender guidelines list no words to avoid.
Where the widths come from
We found no email client that publishes how many characters of a subject line it shows. The five numbers in the checker are from Martech Zone mobile subject line counter, updated May 11, 2026, whose own note says the count changes with device, client version and the font size a reader sets. Clients cut by width, so narrow letters fit more and wide capitals fewer.
| Client | Characters shown | Against the first 30 |
|---|---|---|
| Gmail app, Pixel 9 | 33 | |
| iOS Mail, iPhone 16 | 41 | |
| Outlook desktop | 55 | |
| Apple Mail, macOS | 65 | |
| Gmail on the web | 70 |
The scale runs to 80 characters. The mark is the first 30, where we recommend the words that identify the message go. Google’s sender guidelines add one rule: no Re: or Fwd: unless the message is a reply or a forward.
More free tools
- Email size checkerMeasure your email's HTML and see how close it is to the size where Gmail clips it.
- Email contrast checkerCheck the contrast ratio of a text color on a background against WCAG 2.2 AA and AAA.
- Spam word checkerFlag the promotional phrases in your copy so you can reread them before you send.
Further reading
Last verified . The sources named on this page were read that day.