How to Write Email Subject Lines People Open
To write an email subject line, tell the reader what’s inside in words they’d use themselves, with one specific detail and a reason to open now, in under 50 characters. Then write the preview text as the second line of the same thought. Draft several from different patterns, cut the ones that promise more than the email delivers, and send the clearest.
Below: the job of a subject line, the patterns that work, how to handle the preview text, and how to choose between drafts.
What makes a good email subject line?
It earns the open from the right person and keeps their trust after they’ve read the email. Both halves matter. A subject that gets opens and then disappoints teaches your list to skip the next one.
In practice that means four things:
- Clear: the reader knows what the email is about without opening it.
- Specific: it names a thing or a time, or a situation the reader is in.
- For one reader: the person you wrote it for would feel it’s about them.
- True: the email delivers what the subject promises.
Tension helps too: a cost, a gap, a small surprise. But tension without clarity is clickbait, and clarity without tension is easy to skip. The best subject lines have both.
How long should an email subject line be?
Under 50 characters is a good default. Copix’s hook rules for subject lines set 50 as the limit, because mobile inboxes cut subjects at around 40 to 50 characters. Put the important words first so the subject still works if it’s cut.
Shorter isn’t automatically better. “Update” is short and says nothing. Aim for the shortest version that is still specific.
Which subject line patterns work?
These come from the hook families Copix ranks for email subject lines, each with the trap to avoid. The examples are for shape only.
| Pattern | Formula | Example (for shape only) | Trap |
|---|---|---|---|
| Curiosity gap | The [one thing] that [surprising effect] | the one line I add to every follow-up | A gap the email never closes |
| Result first | [Result] in [time or constraint] | a finished launch email before lunch | Inventing a result you can’t show |
| Mistake | If you [habit], [cost] | if you send on Friday afternoons, read this | Shaming the reader instead of naming the habit |
| Scene | [Time], [place or object], [something about to go wrong] | 6 pm, the invoice is still in drafts | A scene with nothing at stake |
| Sorting question | Do you [situation only your reader is in] | still invoicing from a spreadsheet? | A question everyone answers yes to |
| Count | [Number] [things] that [outcome] | the 3 lines I cut from every sales page | Padding the list to a round number |
The “result first” pattern needs a real result behind it. If you don’t have one, use another pattern rather than inventing a number.
Lowercase subjects often read more personal, like an email from a friend. Title Case reads like a newsletter. Neither is wrong; pick the one that matches the email.
What should you avoid in subject lines?
Openers readers have learned to skip. Copix’s list of banned openers for hooks includes “did you know”, “have you ever”, “are you tired of” and “you won’t believe”, along with others. They all delay the point.
Also avoid:
- Fake reply or forward prefixes. “Re:” on an email that isn’t a reply gets an open once and costs trust after.
- Promises the email doesn’t keep. If the subject says “the template”, the template has to be in the email.
- All caps and stacks of punctuation. They read like spam to people, whatever filters make of them.
- Generic labels. “Our October newsletter” tells the reader the date and nothing about why to open.
How do you write preview text?
Treat it as the second hook. The preview text shows next to or under the subject in most inboxes. Leave it empty and the inbox fills it with the first text in your email, often “View in browser” or a greeting.
Write it to extend the subject, not repeat it. If the subject is “6 pm, the invoice is still in drafts”, the preview might be “here’s the two-minute fix we use every Friday”. Together they form one thought, and each makes the other worth reading.
How do you choose the best subject line?
Write eight to ten from at least four patterns, then score each against the checklist above. Treat “true” as a gate: a subject that fails it is out.
When we ran five sample subjects through the hook scorer in Copix’s review tool, “Did you know your emails get skipped?” scored 3 of 5. It wasn’t specific, and the fix it gave was to drop the opener “did you know” and start on the point. “We fixed the export bug you reported on Tuesday” scored 4 of 5, losing only on tension. The scorer is a heuristic: it gave “Our October newsletter” 4 of 5 too, because a month counts as specific. That’s why two of the seven points, “one reader” and “true”, are left to you.
If your list is big enough, test the top two. Most email tools let you send two subjects to a small share of the list and the winner to the rest.
Can AI write email subject lines?
It can draft many quickly. Give it the email itself and the character limit, describe the reader, and ask for subjects from several patterns. Models repeat the same openers and lean on curiosity gaps, so score the results and rewrite the best one in your voice. Our guides to writing hooks and headlines use the same rubric, and making AI writing sound human covers the tells. For the body of the email, AIDA and the other copywriting frameworks give it a structure.
Do you need a tool to write subject lines?
No. The checklist and the pattern table above are enough for a weekly newsletter. Write ten, score them honestly, send the best. If that’s your volume, keep the tool budget.
A tool helps when an assistant writes many emails and the subjects start to sound alike. Copix gives your AI a write tool. For an email it returns the subject line formulas that fit, with the length limit and the rules for the inbox. Its review tool checks the subject and preview with the email, and grades up to 20 subjects at once when you pass them as lines. See the tools and the plans, or set it up with the guide to Claude for copywriting.
FAQ
How long should an email subject line be?
Under 50 characters works as a default, since mobile inboxes cut around 40 to 50. Put the key words first in case the end gets cut.
Should I use emojis in subject lines?
Only if they fit your brand and your readers. An emoji can’t replace a specific detail, and a subject that relies on one usually has nothing else to say.
Should I personalize subject lines with the reader’s name?
A name can help when the email is personal. On a mass newsletter, readers know it’s automated, so a specific topic does more than a first name.
Do questions work as subject lines?
A question works when only your reader would answer yes, like “still invoicing from a spreadsheet?”. Questions anyone would answer yes to are easy to ignore.
What is preview text?
The short line shown next to or under the subject in the inbox. Write it as a second hook that extends the subject, or the inbox shows the first text of your email instead.