Definition
Subject line length refers to the number of characters visible in an email subject line before the email client truncates it. Different email clients display different numbers of characters based on their interface design, font sizing, and device type. Gmail on desktop displays approximately 60–70 characters, while Gmail on mobile truncates after 30–40 characters. Outlook desktop shows approximately 60 characters, and Apple Mail displays 50–60 characters. These limits apply to the combined preheader text when it is appended to the subject line by some email clients.
Research on optimal subject line length consistently shows that shorter subject lines outperform longer ones for mobile open rates. Data from multiple industry studies suggests that subject lines of 30–50 characters achieve the highest open rates on mobile devices, where the character limit is most restrictive. On desktop, subject lines of 40–60 characters perform similarly well. Subject lines shorter than 20 characters risk being too vague, while those exceeding 70 characters are truncated on nearly all clients, hiding the most compelling copy.
Emoji and special characters add complexity to subject line length calculations because they do not occupy a single character width. An emoji may render as one visible character but consume multiple bytes in the subject source, and different email clients display emojis at different widths. Some clients count emojis as 1–2 character widths while others count them as 3–4. Security characters such as curly quotes and accented letters also vary in display width across clients. Testing subject line appearance in the most common clients is essential before deployment.
Best Practices
Keep the primary call to action and key information within the first 30 characters of the subject line. This ensures visibility on mobile where truncation is most aggressive.
Write subject lines that are 30–50 characters for campaigns where mobile constitutes more than 50% of anticipated opens. Test shorter and longer variations to find the optimal length for your specific audience.
Avoid critical information at the end of the subject line after character 60. Desktop truncation at this point will hide closing words regardless of the device's actual screen width.
Account for preheader text in total visible length. Many email clients concatenate preheader text after the subject line in the inbox view, effectively increasing the visible subject area but also increasing truncation risk.
Test subject line rendering in Gmail mobile, Apple Mail, and Outlook before each campaign. Each client renders characters differently and a subject that fits Gmail mobile may truncate in Apple Mail.
Use emojis strategically and test their display in target clients. An emoji adds personality but consumes 2–4 character positions of display width, reducing available space for copy.
Related Glossary Terms
Frequently Asked Questions
Gmail mobile typically displays 30–40 characters of the subject line before truncation. The exact number varies by device width, font size settings, and whether the preheader text is appended. Testing on actual mobile devices is more reliable than relying on published character counts.
Not always. The relationship between subject line length and open rate depends on audience, industry, and content quality. Longer subject lines sometimes outperform shorter ones on desktop, especially for B2B audiences. The key is not absolute length but placing the most compelling information within the visible area for the client your audience uses most.
Emojis are wider than standard characters in most email clients. A single emoji may consume the same visual space as 2–4 text characters, reducing the number of visible tokens. Some clients also fail to render certain emojis, showing a blank box instead. Use emojis early in the subject line if you must use them at all.
Preheader text appears after the subject line in many email clients' inbox views, effectively extending the visible area. Gmail shows approximately 60–100 additional characters of preheader text, while Outlook shows less. The combined subject plus preheader should be optimised so the most important message is visible in the subject alone.
RFC 5322 limits a header line to 998 characters, and subject lines should stay under 250 characters to be safe with most MTAs. However, the practical limit is not technical but visual: subject lines over 100 characters are truncated by every major email client.