Free Email Subject Line Tester (+ Guide)

What is an email subject line tester?

An email subject line tester checks a subject line and its preview text before you send, and shows how they'll look in an inbox.

How long should an email subject line be?

Short enough that the reason to open shows on a phone. That's about the first 35 characters.

I measured all 56,262 subject lines in the tester's phone inboxes, drawn 390 pixels wide. The Gmail app showed 47% of them in full and cut the rest after about 36 characters.

Length itself didn't decide which email got more reactions. When 2 subject lines from the same newsletter differed by 10 or more characters, the shorter one won 49.7% of 22,678 head-to-heads: a coin flip.

So write the line as long as it needs to be, and put the words that make people open first. Half the lines in the set were 39 characters or fewer.

The tester shows where each phone cuts your line, and which words get cut off.

Preview text: the second subject line

Preview text is the line after the subject in an inbox. Without it, the inbox shows the start of your email, often "View in browser" or an unsubscribe line.

Write it as a second reason to open, not a repeat of the subject. The tester flags both mistakes.

Keep it long enough to fill its line. When it's short, Gmail fills the rest with the first words of your email, and the tester tells you when that happens.

Spam: the subject is a small part

Gmail looks at the sender first. Its sender guidelines ask every sender for SPF or DKIM, a spam rate under 0.3%, and no fake "Re:" or "Fwd:". Bulk senders also need DMARC and one-click unsubscribe.

The subject adds a little on top. SpamAssassin, the open source spam filter, scores these subject habits, and calls an email spam at 5 points:

So a subject alone rarely sinks an email. A sender with a bad reputation sinks every subject.

Where the score comes from, and how far to trust it

I read 56,262 emails from 579 public newsletters on Substack's leaderboards, each with its subject line and how readers reacted. On Substack, a post's title is its email subject unless the writer changes it.

Then I compared emails from the same newsletter, sent to the same readers within 45 days. In pairs where one got at least twice the reactions, this is how often a subject with each part won:

A model trained on those pairs gives the score: where your line's wording lands among all 56,262 subject lines. On 107 newsletters it never saw, the line it scored higher won 56% of the time, when a coin flip gets 50%.

That's a real edge, but a small one. The topic matters more than the wording, and reactions come from people who opened and read, so they measure the whole email. Use the score to pick between lines you like, then test the top 2 on your list.

The emails are newsletters. Launch, promo, welcome and cold emails get the same score, and the checks count as much for them, until I have their own proven lines.

Do emoji help a subject line?

Not in this data. Subject lines with an emoji got more reactions in 36% of 1,040 head-to-heads inside the same newsletter.

Only 3.8% of the 56,262 lines had one.

The emails are newsletters, measured by reactions. For a sale or a launch, test an emoji on your own list before you use it everywhere.

How to test subject lines

Send line A to half your list and line B to the other half, and count clicks, not opens. Apple's Mail Privacy Protection hides whether people opened.

The A/B test calculator tells you how big each half needs to be, and when the winner is real.

How to write a subject line that gets opened

Put the reason to open in the first 35 characters, where every phone shows it.

Then write preview text that gives a second reason, long enough to fill its line.

In the data, what won most often inside the same newsletter was a negative word, a number at the start, and "I" or "my". What lost most often was a colon or dash splitting the line in 2, an exclamation mark, a word in capitals, and emoji.

Use the score to pick between the 2 lines you like best, then let your list decide with an A/B test.

Who is the email subject line tester for?

It's for anyone about to send an email:

• Email subject line generator – people who need more lines to test
• A/B test calculator – marketers comparing two lines
• Headline analyzer – writers working on headlines
• Email countdown timer – anyone announcing a deadline

More tools like these are on my free tools page.

Final thoughts on email subject lines

Put the reason to open in the first 35 characters, where every phone shows it.

Then write preview text that gives a second reason, long enough to fill its line.

In the data, what won most often inside the same newsletter was a negative word, a number at the start, and "I" or "my". What lost most often was a colon or dash splitting the line in 2, an exclamation mark, a word in capitals, and emoji.

Use the score to pick between the 2 lines you like best, then let your list decide with an A/B test.