Email Subject Line Tester & Analyzer
Score a subject line across length, clarity, engagement, readability and spam signals, and see where it gets cut off on a phone.
Paste a subject line and this tester breaks it into five scores — Length, Clarity, Engagement, Readability and Spam risk — each with specific notes on what tripped the deduction, plus a preview of how the line truncates in a narrow mobile inbox. The scoring is deterministic: the same text always produces the same result, and every check is a rule you can read about on this page rather than a black box.
It's for anyone editing a subject line before it goes to a list — email marketers running campaigns, founders sending a newsletter, agencies reviewing client copy. The problem it solves is narrow and real: catching the things you stop seeing after the fifth rewrite, like a line that runs to 78 characters or a stray ALL-CAPS word.
Your subject line
Audience and email type are only used for the optional AI alternatives.
Your score appears here
Type a subject line to get an instant breakdown across length, clarity, engagement, readability, and spam-risk signals.
Need more than a free tool? Ask Kestry Chat Agent about your campaign strategy, marketing questions and customer workflows.
How this email subject line tester works
Five sub-scores are calculated from the text of the subject line, then combined into an overall score out of 100. They're weighted differently, because length and spam signals do more damage when they go wrong than a missing question mark does.
| Sub-score | Weight | What it checks |
|---|---|---|
| Length | 25% | Character count against a 30-50 character target band |
| Clarity | 20% | Word count, vague filler, presence of a concrete noun, opening capitalisation |
| Engagement | 20% | Question, number, personalisation, action verb |
| Readability | 15% | Average word length, punctuation density, emoji count, brackets |
| Spam risk | 20% | Spam-associated phrases, ALL CAPS words, stacked and repeated punctuation |
- Length — full marks between 30 and 50 characters, stepping down through 20-29 and 51-60, then 10-19 and 61-70, and bottoming out beyond that.
- Clarity — deducts for going over nine words, for filler like "stuff" or "update on", for containing no concrete noun, and for opening in lowercase.
- Engagement — builds up from 40, adding credit for a question mark, a digit, personalisation ("you", "your", a merge token) and an action verb such as get, save, learn or discover.
- Readability — deducts for high average word length, more than two punctuation marks, more than one emoji, and brackets in an already short line.
- Spam risk — deducts for each spam-associated phrase, any word of four or more letters in all caps, stacked punctuation like !! or ?!, and every exclamation mark after the first.
Engagement is the one to read with a raised eyebrow. Because it builds up from 40 rather than down from 100, a clean declarative line with no question, no number and no "you" scores low there while being perfectly good. Transactional and announcement emails routinely land in the 40s and should.
Read the findings under each sub-score rather than the number: "78 characters will truncate" is actionable, "score 47" isn't. Rules can also be satisfied without the writing improving — add a question mark, a digit and the word "your" and Engagement leaps to 95 on a worse line. Treat the score as a proofreading pass, not an editor.
Scoring runs entirely in your browser as you type. The optional "suggest alternatives" button is the exception: it sends your subject line — plus preview text, audience and email type if you filled them in — to an AI model to write variations.
What makes a good email subject line?
A subject line has one job: make the next second of a person's attention worth spending. It competes with thirty other lines in a list view, all asking for that same second. Clarity beats cleverness almost every time, because a reader scanning an inbox is deciding whether the email is relevant, not admiring the writing.
- Say what's inside — "Your March invoice is ready" beats "An important update" because the reader can decide instantly whether it matters.
- Front-load the specific words. The first three or four carry the line on a phone.
- Be concrete. Numbers, names and dates outwork adjectives: "3 pricing pages we rebuilt" says something "exciting changes" doesn't.
- Match the promise to the contents. Curiosity that isn't paid off inside produces one open and a long tail of ignored ones.
- Sound like a person the reader knows. Corporate and marketing register both read as bulk mail, the category people delete unopened.
Curiosity and clarity aren't opposites, but the trade-off is real: a vague line can earn opens from an audience that already trusts the sender and collapses with one that doesn't. If your list is cold, err toward clarity.
Subject line length and mobile truncation
The tester targets 30 to 50 characters — long enough to say something specific, short enough to survive the narrowest common display. The preview panel cuts your line at 41 characters, a reasonable stand-in for a phone held in portrait; beyond that point, the grey text is what a reader may never see.
There is no single truncation limit to design against. What's displayed depends on the client, the device, the orientation, the font size the reader chose, and whether they use a list view or a preview pane. Desktop clients show considerably more than a phone; a reader with large accessibility text sizes sees less.
- Write the line you want, ignoring length.
- Check whether the first 40-odd characters still make sense alone. If the subject only works once the last three words arrive, it fails on mobile.
- Move the specific noun, number or name earlier — usually you can cut the run-up rather than the substance.
- Read the truncated version out loud. Half-sentences that stop mid-phrase read as broken, not intriguing.
Common subject line mistakes
ALL CAPS and stacked punctuation
A capitalised word of four letters or more costs 20 points on spam risk; "!!" and "?!" take a further deduction, as does every exclamation mark after the first. Three exclamation marks don't convey three times the enthusiasm — in a list of lines that aren't shouting, the shouting one looks less credible rather than more urgent.
Spam-associated wording
Phrases like "act now", "100% free", "risk-free", "click here" and "limited time" each take a deduction. The honest reason to avoid them isn't a filter with a banned-word list — modern filtering leans far more on sender reputation and engagement than on vocabulary. It's that these phrases are the native language of mail people don't want.
Vagueness
"Quick update on things" trips two clarity checks at once: filler wording and no concrete noun. Vague lines feel safe to write because they can't be wrong, which is exactly why they can't be interesting.
False urgency
A deadline that isn't real is the fastest way to teach a list to ignore you. "Last chance" every fortnight stops meaning anything by the third send, and the readers who notice are the engaged ones you can least afford to lose. Real deadlines, stated with the actual date, keep working.
Clutter
Brackets, multiple emoji and long words slow down a line that has under a second to be read. One emoji can anchor the eye; three read as decoration. These deductions are small individually and add up quickly.
Email subject line examples: before and after
| Weaker | Stronger | What changed |
|---|---|---|
| Newsletter #47 | Inside: 3 pricing pages we rebuilt this month | Names the contents instead of the format — nobody opens an email because it is issue 47 |
| ACT NOW - 50% OFF EVERYTHING!!! | Your 50% code expires Sunday night | Drops the caps and stacked punctuation, and swaps manufactured urgency for a real, stated deadline |
| Quick update on things | Your March invoice is ready to download | Replaces filler with the actual subject of the email, so the reader knows whether to act |
| We are so excited to announce our brand new spring product line | New for spring: 12 lightweight jackets | 63 characters down to 38, with the specific detail moved to where mobile shows it |
| risk-free trial, no obligation - click here | Try it free for 14 days, no card needed | Removes three spam-associated phrases and states the same offer in the words a person would use |
| you won't believe what we found in our data | What we found in 400 checkout flows | Capitalised opening, a concrete number, and a promise the email can actually keep |
None of these are templates to copy. The pattern worth taking is the direction of travel: from the format to the contents, and from what you feel about the news to what the reader gets from it.
Preview text: the second half of the subject line
Preview text — the preheader — is the snippet most inbox clients show after or under the subject line. It's pulled from the first text in the email body unless you set it explicitly, which is why so many campaigns lead with "View this email in your browser", or with nothing at all when the email opens on an image.
Treat it as a continuation, not a summary. If the subject raises a question, the preview starts answering it; if the subject states the news, the preview gives the reason to open. Repeating the subject word for word wastes the only other line of copy the inbox gives you.
- Subject: "Your March invoice is ready to download" — Preview: "Total is 240, due 31 March. One click to pay."
- Subject: "What we found in 400 checkout flows" — Preview: "Three fixable mistakes, and the one nobody expects."
- Subject: "New for spring: 12 lightweight jackets" — Preview: "Plus what we changed after last season's returns."
How much preview text is shown varies as widely as subject line length does, so put the load-bearing words first here too. The field in this tool isn't scored; it's passed to the AI suggestions so alternatives come back as a pair.
How to improve open rates (honestly)
Subject lines are the most visible lever and rarely the most powerful one. If open rates are falling, check these before another round of rewriting — roughly in the order they tend to matter.
- Sender reputation and authentication — SPF, DKIM and DMARC configured correctly. Major mailbox providers have tightened requirements for bulk senders in recent years, and a sender failing these hits delivery problems no copy will fix.
- List hygiene — remove hard bounces immediately and sunset subscribers who haven't engaged in six to twelve months. Mailing dead addresses depresses engagement rates, which feeds back into filtering.
- Consent and acquisition source — a purchased list damages reputation faster than anything copy can repair, and complaint rate is the metric that punishes it.
- Segmentation — relevance beats phrasing. An ordinary line sent to the people it applies to outperforms a cleverer one sent to everybody.
- Sending frequency — both extremes hurt. Too often brings fatigue and unsubscribes; too rarely and recipients forget who you are and report you.
- Send time and from name — test timing on your own list rather than trusting a published best hour, and protect the from name: it's what a reader recognises first, often before the subject.
A/B testing subject lines
Test one variable at a time, and give each variant enough recipients that the result isn't noise. On a few hundred sends, a two-point difference in open rate tells you nothing. Log what you learn across tests, because one result on one send is a data point, not a rule.