Glossary
Subject line
Last reviewed 18 September 2026
How long should an email subject line be?
A subject line is the line an inbox shows beside your name, and the length that matters is not the total but the front of it: put what is interesting in the first 30 to 40 characters, which is roughly what an iPhone held upright will show.
An inbox truncates at a pixel width in a proportional typeface rather than at a character count, so every figure quoted in characters is a translation that moves with the letters you chose. Outlook on a desktop shows roughly 60 and Gmail in a wide browser window roughly 70, and the shortest of the three is your working budget, because it is the one your reader is most likely holding.
A course subject line does not have to sell anything, which is a freedom worth using. The reader confirmed their address for this and the sequence is the reason the message is there, so a plain description of the day beats anything clever, and a “Day 2 of 5:” prefix costs about eleven characters for a real gain in orientation.
See also
- Preheader
A preheader is the line of text an inbox shows after the subject when there is room, taken from the top of the message body, and it is a separate opportunity rather than an extension of the subject line.
- Call to action
A call to action is the single thing an email asks the reader to do, stated plainly and linked once, which in a course email is usually the step that day’s lesson was teaching rather than a request to buy anything.
- Email course
An email course is a finite sequence of lessons delivered one email at a time, in order, on a clock that starts when each reader joins, and it ends on a day both of you knew about in advance.
Read next
This term is covered properly in Subject lines for a 5-day email course: the Day N of M pattern, which is a longer piece on the subject.
Every other term is on the glossary index , the longer writing is in the guides, and what this site itself does is on the home page.
Thinking of writing one of these?
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Accounts are opened a few at a time rather than by signing up. Leave your address and you will be written to when the next ones open.
One message, when there is room. No course emails, no newsletter, and the address is not passed on. Ask and it is deleted; what is kept, and for how long, is in the privacy policy.