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How To Write Email Subject Lines
A subject line has one job and about forty characters to do it in. Everything else in the email is invisible until that job is done.
Quick Answer
subject = one concrete noun + one reason to open, kept under 41 characters
- L
- Length of the subject line in characters
- T
- The truncation threshold for the mail client
Phones cut a subject at roughly 41 characters and tablets at about 60. A six-line set built from a good topic averages 38 characters, and four of the six fit a phone screen in full.
What Is How To Write Email Subject Lines?
A subject line is the only part of an email that every recipient sees. The body, the images and the offer are all behind a decision that takes about a second and is made mostly on forty or fifty characters.
Mail clients cut a subject at a fixed number of characters, and the number is smaller than most people assume. Roughly forty one characters show on a phone and about sixty on a tablet, so the second half of a long subject is simply not read.
The character limit is not a guideline. It is the width of the preview pane on the smallest screen that the client supports, and everything past it is replaced by an ellipsis whether or not it was worth reading.
A subject line has to survive on its own, without the context of the campaign or the sender name. Saying what the email is about beats hinting at it, because a hint that needs the body to make sense is a hint that gets skipped.
Concrete nouns outperform abstractions. A named product, a season or a number is recognisable in a way that a benefit is not, and recognisable is what stops a thumb from swiping past.
A number gives the reader a scale to hold on to. Three things, fifty per cent and the last day all tell you how much attention the email wants, which is a different and more useful signal than an adjective.
A question works when the reader already wants the answer. It fails when the question is merely rhetorical or when the reader has no stake in it, so a question is a bet on the audience rather than a universal trick.
Urgency only works when it is true. A countdown that resets every week trains the list to ignore your deadlines, and the open rate you gain this month is paid for by every campaign after it.
Personalisation buys a fraction of a second, not a conversion. A first name in the subject line is worth very little because everyone does it, while a reference to what the reader actually did is worth a great deal.
All caps and exclamation marks read as shouting and are treated as such by both the reader and the filter. A subject that looks like an advertisement is filtered as one, whether or not it says anything useful.
Certain words are weighted heavily by spam filters no matter how natural they read. Free, act now, guaranteed and a handful of others appear in enough junk mail that their presence moves a message towards the junk folder.
The goal changes the formula. Getting an email opened rewards curiosity and a concrete noun, getting it clicked rewards clarity about what is behind the link, and getting a reply rewards a question that needs a one word answer.
The generator takes the topic, the goal and the audience, and produces six lines with a character count, a verdict on whether the phone will show them in full, and a flag on any spam trigger. It gives you formulas rather than finished sentences so that editing one does not destroy the reason it worked.
Formula
shown = the first T characters, where T is about 41 on a phone and 60 on a tablet
The client shows only the prefix that fits its preview pane. Any part of the subject past the threshold is cut, so the important half has to be at the front.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| s | Subject line | characters | The full text entered into the subject field. |
| T | Threshold | characters | How many characters the client shows before truncating. |
| L | Length | characters | How many characters the subject actually has. |
opens = a concrete noun plus a gap; clicks = a concrete noun plus a promise; replies = a question with an easy answer
Each goal answers a different question for the reader, so the formula that wins for opens loses for clicks. The generator swaps the whole family of lines when the goal changes.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| goal | The reader action | action | Open, click or reply. |
| formula | Line pattern | pattern | The sentence shape that serves that action. |
average length = total characters across the set / number of lines
A set averages out the noise of any single line. A mean below the phone threshold means most of the set will be read whole, which is the figure worth watching.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| n | Lines in the set | count | How many candidates were generated. |
| L_i | Length of line i | characters | Characters in one candidate. |
| L | Mean length | characters | The average across the set. |
How To Calculate How To Write Email Subject Lines
- 1
Write the topic down as a noun, not a benefit
Name the thing: the product, the season, the event. Benefits are what the body is for, and a subject built on a benefit usually reads like every other subject in the inbox.
- 2
Choose the action you want
Opening, clicking and replying need different lines. Picking the action first stops you writing six variations of the wrong thing.
- 3
Generate and count
Each line comes with its character count. The count is the whole point, because it decides whether the second half of the line is ever seen at all.
- 4
Check the phone verdict
Anything past forty one characters will be cut on a phone. Read only the prefix that survives and ask whether it still says what the email is about.
- 5
Clear the spam words
A flagged word does not doom the email, but it moves it towards the junk folder. Rewording is usually cheaper than living with the flag.
Examples
Example 1: Six lines for an autumn sale mailing
- Topic
- autumn sale
- Goal
- Get it opened
- Audience
- Existing subscribers
| Step | Calculation | Result |
|---|---|---|
| Topic as a noun | the topic, lowercased | autumn sale |
| Audience clause | appended where the line names the reader | for subscribers |
| Shortest line | Is autumn sale worth it? | 24 |
| Longest line | The autumn sale questions you asked for subscribers | 51 |
| Median of the six lengths | (33 + 40) / 2 | 36.5 |
| Mean of the six lengths | (40 + 48 + 24 + 51 + 33 + 31) / 6 | 37.8 |
Result: The six lines for an open goal come out at 24, 31, 33, 40, 48 and 51 characters, with a median of 36.5 and a mean of 37.8. Four of the six sit under the forty one character phone threshold, and the two that do not only lose the tail. Nothing in the set trips a spam filter.
Example 2: Switching the goal to a click
- Topic
- autumn sale
- Goal
- Get it clicked
- Audience
- Existing subscribers
| Step | Calculation | Result |
|---|---|---|
| Shortest line | See the autumn sale range | 25 |
| Longest line | Your autumn sale shortlist for subscribers | 42 |
| Median of the six lengths | (30 + 32) / 2 | 31 |
| Mean of the six lengths | (25 + 29 + 32 + 42 + 30 + 35) / 6 | 32.2 |
Result: Switching the goal from an open to a click swaps the whole family of lines: the lengths come out at 25, 29, 30, 32, 35 and 42 characters, with a median of 31 and a mean of 32.2. The click set is shorter on average because every line now names what is behind the link rather than leaving a gap.
Calculator
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Common Mistakes
Writing the good line in the second half
A phone shows about forty one characters. Anything past that is replaced by an ellipsis, so a subject that lands its point at character fifty has not landed it at all.
Using the same line for opens and for clicks
An open rewards a gap the reader wants closed, while a click rewards saying plainly what is behind the link. A line that does one rarely does the other, and using the wrong one wastes the better position.
Stacking urgency on every send
A deadline that resets every week stops being believed. The open rate gained from a fake countdown is borrowed from every campaign that follows it.
Shouting in capitals
All caps and exclamation marks are read as shouting by people and weighted as spam by filters. Both effects cost more than the emphasis gains.
Padding with filler words to sound polite
Please, kindly and just take up the space that the concrete noun needed. Trimming them is the cheapest length reduction available and costs nothing in tone.
Trusting a spam score over the actual words
Filters look at the whole message, not one word. A flagged word is a prompt to reword, not a verdict, and a clean checklist is no guarantee of the inbox either.
Testing only on yourself
Your own client, your own screen and your own familiarity with the campaign are three biases at once. Check the length on a phone, not on the compose window where it always fits.
FAQ
How long should an email subject line be?
Under forty one characters if you want it shown in full on a phone, and under about sixty for a tablet. Anything longer is cut, so put the part that matters at the front.
Do emoji help or hurt?
They help when they carry meaning the words cannot, and hurt when they are decoration, because they take the space that the concrete noun needed and they read as marketing.
What is the best length for a marketing email?
There is no single best length. The right length is the shortest one that names the thing and gives a reason, which usually lands between twenty five and forty characters.
Should I put the company name first?
Only if people open for the name rather than for the content. The name is usually displayed separately by the client anyway, so spending the subject on it duplicates what is already shown.
How do I test which subject line works?
Send the same body to two random halves of the list with different subjects, and compare opens on a sample large enough that the difference is bigger than the noise. Changing two things at once tells you nothing.
Why do my emails go to spam even with a short subject?
The subject is one signal among many. Sender reputation, link tracking domains, image-to-text ratio and complaint rates all count, and a good subject cannot rescue a bad list.
Is a question mark a good idea?
It works when the reader already wants the answer and the question is the reason they subscribed. It fails when the question is rhetorical, so try it and measure rather than assuming.
References
- [1]Campaign Monitor, Email subject line length, measured — https://www.campaignmonitor.com/resources/guides/email-marketing-guides/
- [2]Mailchimp, Spam filter triggers to avoid — https://mailchimp.com/help/avoid-spam-filters/
- [3]Campaign Monitor, Sending and testing subject lines — https://www.campaignmonitor.com/resources/knowledgebase/