How Long Should an Email Subject Line Be? What the Data Says

Ask ten marketers how long a subject line should be and you'll get ten confident answers: under 50 characters, six to ten words, as short as possible. The data is less tidy. Several very large studies exist, and they don't fully agree with each other.

The short answer

There is no universal best length. Several large studies found that shorter subject lines earned more opens, but others found strong results at both extremes, and the effect varies by industry. A sensible approach is to write a line short enough that its key point survives truncation on a phone, specific enough to give a reason to open, and then test lengths on your own list.

What the studies found

Study	Size	Reported pattern
MailerMailer (2013 data)	About 1.2 billion emails	Open rates generally fell as subject lines grew: 16.8% at 4-15 characters versus 10.7% at 40-50, with a slight rise beyond 50
Retention Science (2014 report)	260 million+ emails, 540 campaigns	6-10 words did best at 21%, five words or fewer got 16%, and 11-15 words got 14%
Yes Lifecycle Marketing (Q2 2017)	7 billion+ emails	Under 21 characters got an 18.5% open rate; 21-60 characters was lowest at 13.8%; 61+ characters reached 14.8%
Inntopia (resort and travel emails)	250 million+ emails	A U-shaped curve: the shortest and longest lines did best, with the low point near 55 characters

The Yes Lifecycle Marketing report adds two useful details. Only about 5% of the subject lines it examined were under 21 characters, so brevity was rare, and length made a noticeable difference for retail and technology but not for financial services, publishing or hospitality and travel.

Why the results disagree

Different senders and industries. A pooled average hides the fact that a retailer and a bank may behave differently, as the Yes data suggests.
Different units. Some studies count characters, others words, so the ranges can't be lined up exactly.
Different email types. Newsletters, promotions and transactional messages are read in different moods.
Curiosity versus detail. A very short line can work as a teaser, a long one can spell out a concrete reason to open, and the middle can do neither well. This may explain the U-shaped result, though that is an interpretation rather than a proven cause.
Correlation, not proof. Short subject lines may be used more by senders whose audiences already know and trust them, so length may not be the only reason for the difference.

One more caution: these analyses are several years old. Some predate Apple's Mail Privacy Protection, introduced in 2021, which can record opens that no person made. Don't treat the percentages above as benchmarks for today. The useful part is the shape of the patterns, not the exact numbers.

What to do with this

Front-load the point. Put the most important words first, so truncation can't cut the part that matters.
Preview on a phone. Many phones show only a limited portion of the line. Roughly 30 to 40 characters is a commonly cited rule of thumb, but it varies by device and app, so send a test to yourself.
Match length to the job. A short line suits a familiar sender and a teaser. A longer line suits a message whose price, date or benefit is the reason to open.
Don't pad or chop to hit a number. A clear 62-character line beats a vague 30-character one.
Treat subject line and preview text as a pair. Many inboxes show both, so the second can finish what the first starts.

Same message, three lengths

Illustrative examples for a free-shipping offer, not tested results.

Short (26 characters): "Free shipping ends tonight"
Medium (43 characters): "Your free shipping ends tonight at midnight"
Long, front-loaded (70 characters): "Free shipping ends tonight: plus the three best sellers still in stock"

All three open with the same key fact, so even if the longer ones are cut off on a phone, the point survives. Which one wins depends on your audience, which is why testing beats guessing.

How to test length on your own list

Split the audience randomly and send the versions at the same time.
Change only the length, keeping the message and offer the same.
Give small lists a chance to show a real pattern: a difference of a few opens is noise, so repeat the test over several sends before drawing conclusions.
Judge by clicks as well as opens, since a curious short line can win opens without leading to action, and privacy features can inflate opens.

If your list is too small for a clean A/B test, a prediction tool such as our Email Campaign Predictor can estimate how a rewritten subject line is likely to perform before you send. Use it alongside testing, not instead of it.

Frequently asked questions

Is a shorter subject line always better?

No. Several large studies found shorter lines earned more opens, but others found strong results at both the shortest and longest lengths, and the effect varied by industry. Test length on your own list rather than assuming.

How many characters show on a phone?

It varies by device, app and screen settings. Around 30 to 40 characters is a commonly cited rule of thumb, so put the key point first and send a test email to your own phone to see what actually displays.

Should I measure subject line length in characters or words?

Truncation depends on display width, so characters are the more practical measure. Words are easier to think about while writing. Either works as long as you test the result.

Does preview text matter as much as the subject line?

In many inboxes the preview text appears right after the subject line, so the two read as one message. Writing them together usually works better than treating them separately.