Email Subject Line Tester
Paste a subject line and its preview text. The tester shows how they will truncate on a phone, flags the mechanical problems that cost opens (length, caps, punctuation, emoji, spammy phrasing, broken personalisation) and tells you what to change. It is a heuristic checklist, not an open-rate prediction, and it says so on every result.
Your email
Heuristic score
0
/ 100
A checklist score based on the rules below, not a forecast of opens. It cannot see your list, your sender reputation or your offer.
Characters
0
Words
0
Emoji
0
Signals found
Inbox preview
Approximate renders at a common 390pt-wide phone size with default text settings. Real cut-offs shift with the phone model, font-size settings, the length of your from name and whether a date or attachment icon is shown. Use this to see what gets cut, not as a pixel-perfect guarantee. On iPhones and Macs with Apple Intelligence message summaries switched on, Mail can show a short AI-written summary instead of your preview text (Apple Support), which is one more reason to make the subject line carry the message on its own.
IPHONE MAIL (APPROX.)
GMAIL APP (APPROX.)
DESKTOP WEBMAIL, ONE LINE (APPROX.)
What to change
Score breakdown
Every subject line starts at 100. These are the deductions applied. Nothing is hidden.
Everything runs in your browser. Nothing you type is sent to us or stored. The only thing that tells you which of two good subject lines wins is an A/B test on your own list; see our guide to A/B testing email subject lines.
Want a real audit on these numbers?
Drop your store URL and email. We'll review your actual Klaviyo account (read-only) and send you a written 90-day roadmap, free, within 24 hours.
How the tester works (and what it can't do)
A subject line tester is useful for one job: catching mechanical mistakes before a send goes to 50,000 people. It is not an oracle. We built this one to be explicit about which rules it applies and why, so you can disagree with a rule when you have a reason to.
What it checks
- Length and truncation. Characters and words, plus an approximate render of what survives on iPhone Mail, the Gmail app and a one-line desktop inbox. Full marks between 20 and 50 characters; points come off as the line gets very short or very long.
- Spam-style phrasing. A short list of phrases that read like scams, get-rich pitches or phishing (for example "act now", "risk-free", "you've won", "verify your account"), plus fake "Re:" and "Fwd:" prefixes, which Gmail's sender guidelines tell senders not to use unless the message really is a reply or forward. We deliberately don't flag ordinary retail words like "free shipping" or "sale".
- Health-claim risk. Words such as "cure", "miracle" or "lose weight fast" get flagged because, for supplement and wellness brands, they are a regulatory problem before they are a deliverability one.
- ALL CAPS, punctuation and emoji. One capitalised word for emphasis is fine; whole lines are not. More than one exclamation mark, repeated "!!" or "?!", and more than one emoji all cost points.
- Personalisation. It detects Klaviyo, Mailchimp and bracket-style tags, previews them with and without a first name, and warns when a Klaviyo tag has no
defaultfallback. - Urgency, curiosity and specificity. It looks for deadline language, open questions and concrete details (numbers, percentages, prices). One clear reason to open is good; three stacked urgency phrases reads as pressure.
- Preview text. Missing, duplicated or boilerplate preview text ("View in browser") is flagged, because it wastes the second most visible line in the inbox.
What it can't see
Your sender reputation, your authentication, how engaged the segment is, what you sent last week, or whether the offer is any good. Those decide most of the result. Mailbox providers filter mainly on reputation and engagement, so a perfect score will not rescue a send to a disengaged list, and a subject line that scores 70 can win on a loyal one. If mail is landing in spam, start with our spam-folder diagnosis and the deliverability checklist, not the subject line.
Write better lines to test
The tester is only as good as the options you give it. For ecommerce-specific lines organised by occasion (sale, launch, restock, welcome, cart, win-back, newsletter), see our ecommerce email subject lines guide, the abandoned cart subject lines breakdown, and the Black Friday and holiday subject lines list. For the copy underneath, read our email copywriting guide.
Subject line tester FAQs
How does this subject line tester score my subject line?
It starts every subject line at 100 and deducts points for specific, visible reasons: length outside the range that displays fully on phones, ALL CAPS words, stacked punctuation, more than one emoji, spam-style phrases, a missing personalisation fallback, missing or duplicated preview text, and having nothing specific or intriguing to open for. Every deduction is listed next to the score, so you can see exactly why it moved.
Can a subject line tester predict my open rate?
No, and any tool that claims to is guessing. Open rate depends on your sender reputation, how engaged your list is, who you send to, and the offer, none of which a tester can see. Apple Mail Privacy Protection also inflates opens. Use this tool to catch mechanical problems before you send, then let an A/B test on your own list decide between two good options.
What is the ideal email subject line length?
There is no single ideal, but anything the reader cannot see does no work. On a typical phone, roughly the first 30 to 45 characters of a subject line are visible before it truncates, depending on the app, screen width and font size. Put the words that matter in that first stretch, and use preview text to carry the rest of the message.
Do spam trigger words still send emails to spam?
Rarely on their own. Gmail, Yahoo and Microsoft filter mainly on sender reputation, authentication, complaint rates and engagement. Words are a weak signal compared with those. But phrases that read like a scam or a get-rich pitch still hurt, partly through filters and mostly through readers who ignore or report them. This tool flags them so you can make a deliberate choice.
Should I use emojis in email subject lines?
One relevant emoji can help a subject line stand out in a busy inbox, and plenty of ecommerce brands use them well. Two or more usually read as noise, some render differently across devices, and they take up space in the part of the line people actually see. Test one emoji against none on your own list before making it a habit.
What is preview text and why does it matter?
Preview text (also called preheader text) is the snippet most inboxes show next to or below the subject line. If you leave it blank, the inbox pulls the first text it finds in the email, often "View in browser" or an image alt tag. Treat the subject line and preview text as one sentence split in two: the subject earns attention, the preview gives the reason to open.
Subject lines are the cheapest lever in email and one of the smallest. If your opens look fine but revenue per recipient doesn't, the problem is usually further down: flows, segmentation or deliverability. Our email design and copywriting team writes and tests the whole message, and the free Klaviyo audit shows where the real gap is.