Why Does the Subject Line Matter So Much?
In email marketing the subject line is the first thing the recipient sees, and often the only thing. In the inbox list nothing is visible except the sender name, the subject line and, if there is one, the preview text (preheader); the decision to open the email or not is made largely on those three elements. However carefully the content was put together, if the subject line is cut off in a meaningless place, or the preheader field is left empty and fills up with some random text, that first-impression opportunity is lost.
Together, those three elements form the only impression that takes shape in the recipient's mind before the body of the email is ever opened. The subject line and preheader therefore deserve at least as much care as the body copy, not less; however valuable the content is, an email that is never opened never delivers that value to the other side.
Truncation Lengths by Client
Every email client cuts the subject line at a different width. These cut-off points are approximate estimates based on character count, because email client fonts are not standardised the way they are in Google's search results.
| Client | Subject line | Preheader |
|---|---|---|
| Gmail desktop | Truncated at roughly 70 characters. | Appended in the space left after the subject line, up to roughly 100 characters in total. |
| Gmail mobile | Truncated at roughly 33 characters. | Not displayed separately. |
| Outlook desktop | Truncated at roughly 60 characters. | Shown on a separate line, up to roughly 90 characters. |
These numbers are practical approximations, not a pixel-perfect specification. The real cut-off point varies with screen width, the font in use, the zoom level and the version of the client; clients like Gmail and Outlook also make small changes to their interfaces over time. Treat these figures as a pre-send sanity check rather than a firm guarantee.
What Is Preview Text (the Preheader)?
The preheader is the grey preview text that appears immediately after the subject line in most inbox listings. If no preheader is explicitly set, clients usually pull the first line of the email body automatically — which often results in something useless like "View this email in your browser".
The Risk of Emoji and Special Characters
Emoji and special characters can render inconsistently across clients: some clients strip the emoji entirely, some show a fallback glyph, and some render it normally. Senders who use emoji in the subject line would do well to test emoji-heavy lines separately; otherwise an empty box or a question mark may appear where the emoji was meant to be.
What to Watch For When Writing a Subject Line
Knowing the truncation lengths is not enough on its own; the subject line itself has to be built to stand out in the inbox. Some basic principles:
- Put the most important information in the first few words — the part before the cut-off point is visible in every client, the part after it may not be.
- Avoid subject lines written entirely in capitals; they are hard to read and can be treated as a spam signal by some inbox filters.
- Excessive exclamation marks or a pile of special characters undermine the impression of a professional, trustworthy sender.
- The subject line and the actual content of the email must be consistent with each other; wording that is unrelated to the content and exaggerated purely to earn a click erodes recipient trust over the long term.
- Testing two different subject line versions of the same campaign on a small group of recipients, then observing which performs better, clarifies over time which tone works for your audience.
Strong and Weak Subject Line Examples
To make the truncation lengths and preheader rules concrete, here are a few examples:
| Example | Assessment |
|---|---|
| "News!!!" | Vague and unclear in its relation to the content; the recipient has no idea what to expect, and the multiple exclamation marks can create a spam impression. |
| "Your order has shipped — tracking number inside" | Carries concrete information and creates an expectation; even if it is truncated, the first part stands on its own. |
| "LAST CHANCE! CLICK NOW!!!" | All capitals and heavy punctuation; hard to read, and it undermines the impression of a trustworthy sender. |
| "Your July invoice is ready, payment due 15 July" | Even the part before the cut-off point conveys the essential information (invoice and date), so it keeps working even when truncated on mobile. |
How the Subject Line and Preheader Work Together
The subject line and the preheader should be thought of not as two independent sentences but as two parts of a single message. The subject line draws attention; the preheader completes it and supplies the reason. If the subject line is "Your invoice is ready", for example, the preheader should carry additional information such as "Payment due 15 July" rather than repeating the same words. Writing exactly the same sentence in the preheader as in the subject line means throwing away a limited amount of display space.
Common Mistakes
- Leaving the preheader field empty and letting the client pull a random line from the email body.
- Placing the most important word of the subject line just beyond the character limit, in other words in the part that gets cut off.
- Writing subject lines based only on the desktop view, without ever testing how they look on mobile.
- Sending an emoji-heavy subject line on the assumption that it renders the same way in every client.
- Writing exactly the same text in the preheader as in the subject line, spending display space on repetition.
Test Before You Send
Checking your subject line and preheader text against different client views before you send catches most of these mistakes early. The tool below shows these character-count-based truncation estimates side by side for Gmail desktop, Gmail mobile and Outlook desktop, so you can be sure both fields look the way you expect before the send goes out.