Check the length, clarity and tone of your subject line, see how it may look in an inbox, and compare up to five options side by side.
Checked as you type, inside your browser. Your text isn’t sent to Atomic Mail or anyone else, and nothing is saved.
Start typing a subject line. Your score, suggestions and an inbox preview appear here.
How much of a subject line fits depends on the app, screen width, font and settings. This preview shows roughly where text may be cut, not exactly how Gmail, Outlook or Apple Mail will display it.
The tester checks your subject line as you type. No account, no limit on tests, and nothing leaves your browser.
Enter the subject line exactly as you plan to send it, including emoji and merge tags like {{first_name}}. Add a preheader if you have one.
You get a score out of 100 with points for length, word count, formatting and tone. Each check says what it found and how to fix it.
Switch to Compare up to 5 to see several versions side by side, then copy the one you like or download all of them as a CSV.
Every point is explained. The score uses fixed, published rules, so the same subject line always gets the same result.
| Check | What it looks at | Points |
|---|---|---|
| Length | Character count. Full points from 20 to 60 characters, fewer for very short lines and for lines long enough to be cut off. Emoji and accented letters count as one character each. | 25 |
| Word count | Full points for 3 to 9 words. Also flags a word repeated twice in a row and longer words used more than once. | 20 |
| Formatting | Words in capital letters (short acronyms are fine), repeated punctuation like !!, more than two emoji, runs of symbols like $$$, extra spaces and invisible characters. | 20 |
| Clarity and tone | A Re: or Fw: prefix on something that isn’t a reply, lines too general to say what’s inside (“Quick question”), and pressure or hype wording. Wording checks run for English only. | 20 |
| Preheader | Only scored when you add one: its length, whether it repeats the subject, and leftover boilerplate like “View in browser”. | 15 |
Original examples to adapt, grouped by purpose. Click Test on any line to load it into the tester and see how it scores.
Say what the email is about and what you need, so it can be found again later.
For applications, official requests and anything addressed to someone you don’t know.
Put the topic, the length and a proposed time in the subject so people can answer quickly.
Remind the reader what you discussed instead of writing just “Following up”.
Specific and plain beats clever. Mention something relevant to them, not you.
Lead with the connection or the shared context so a stranger knows why you’re writing.
Name the best item in this issue rather than the issue number.
State the offer and the real deadline once. Capitals and exclamation marks add nothing.
Confirm the signup and point to the first useful step.
There’s no single ideal length. What matters is how much of your line people actually see, and whether the important words are in that part.
30–40
Inbox lists usually show about this much on one line, depending on the app, screen width and text size. Some apps add a line of preview text below.
60+
Wider layouts show more, but the subject shares the row with the sender name and preheader, so long lines still get cut.
No limit
RFC 5322 sets no practical limit on subject length. Long header lines are simply wrapped. The real limit is what the inbox shows.
The preheader, also called preview text, is the short line many inboxes show after or under the subject. If you don’t set one, the app shows the first text of your email instead.
Give the next useful detail, like a date, an amount or the reason to open.
“View this email in your browser” at the top of a template often becomes the preview. Hide it or move it.
Around 40 to 100 characters is a practical range. Many apps show less, so the start matters most.
Rarely on their own. Modern filters mostly judge who is sending and how recipients respond. The subject is one signal among many.
SPF, DKIM and DMARC prove your domain allowed the message. Without them, even a perfect subject line can be filtered or rejected.
Check it belowComplaint rates, bounces, opens and replies teach providers whether people want your mail. This matters far more than individual words.
Biggest factorShouting, misleading prefixes and stacked hype add risk, mostly because they annoy people into marking mail as spam. No single word gets you blocked.
What this tool checksThe tester helps you write clear options. A real A/B test with your own audience tells you which one works.
Test two versions that differ in one way, such as a question against a statement, or the date at the start against the end. Otherwise you won’t know what made the difference.
Split a meaningful part of your list at random and send both versions at the same time. Tiny differences on small lists are usually noise.
Apple Mail Privacy Protection preloads images, which inflates open rates. Clicks, replies and conversions are more reliable signals of a better subject.
It checks a subject line against writing rules: length, word count, formatting such as capital letters and emoji, misleading or pushy wording, and how well the preheader complements it. You get a score with an explanation for every point, plus an illustrative inbox preview.
Yes. There’s no account, no email required and no limit on how many subject lines you test or compare.
No. The checks run in your browser. Your text isn’t sent to Atomic Mail, isn’t included in analytics, and isn’t saved. When you close the page, it’s gone.
85 or more means the line is clear, well formatted and not pushy by these checks, and 70 to 84 is good. Treat the score as a writing guide. It can’t predict open rates, because those depend on your audience and sender reputation.
There’s no fixed rule. Phones often show about 30 to 40 characters and desktop inboxes more, so put the most important words first. The tester gives full length points from 20 to 60 characters and warns when a line is likely to be cut off.
It flags pressure and hype wording such as “act now” or “100% free”, plus capital letters, repeated punctuation and fake “Re:” prefixes. These are advice, not a blacklist: spam filters judge your sending domain and reputation far more than individual words.
One emoji can work for consumer email if it fits your brand. They display differently across apps, and more than two make a line harder to read, so the tester flags three or more.
Length, word count, emoji and formatting checks work in any language, including right-to-left scripts. The wording checks are written for English and are skipped for other languages, and the result tells you when that happens.
Not yet. Use the examples above as a starting point, write two to five versions, and compare them side by side. A generator is planned, and it will only be added in a way that keeps your text private.
Free, with no signup. They all run in your browser.
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