Cold email subject line tester
Test up to 10 subject lines side by side. Each gets a grade built on cold email reply data, and you see exactly where Gmail and iPhone cut it off.
Runs in your browser. Nothing you type is sent or saved.
How this subject line checker grades cold email
Two scores, each starting at 100 and losing points for what we find, averaged into a grade from A to F. Spam risk uses the same detectors as our cold email spam checker; reply odds uses what cold email reply data shows. Every rule is right here.
Spam risk
50% of your gradeWhat filters see
The spam checker's own detectors, run on the subject alone, where filters weigh every word hardest.
- Blockers{{first_name}}caps grade at D
Unfilled merge tags, spintax, a fake Re: or Fwd:, link shorteners, disguised characters.
- Trigger phrasesact nowup to 40 pts
240 phrases, weighted by severity. A strong one caps the grade at C.
- ShoutingFREE!!!up to 25 pts each
Words or whole lines in capitals, !!! and ?!. Acronyms like SDR are fine.
- Money signs50% offup to 30 pts
Prices, % and $ signs. Next to "save" or "earn", they count most.
- Emoji and links🚀🔥up to 20 pts each
Two or more emoji, or a link. One emoji is only a note.
Reply odds
50% of your gradeWhat people see
What gets a busy person to open a cold email and answer it, from studies that measure replies, not newsletter open rates.
- Length9 wordsup to 35 pts
Best at 3 to 4 words. Points off past 6 words or 40 characters.
- Fits a phoneseries b hiri…up to 12 pts
Cut off past about 36 characters in the Gmail app.
- No clichesQuick questionup to 45 pts
46 lines that read as a template.
- No pitch or baitour platformup to 45 pts each
Selling, ROI words, and curiosity bait the email can't back up.
- First lineHope this finds you wellup to 15 pts
A greeting cliche, an introduction or the subject repeated in the preview.
- About themPriya,up to 20 pts
Tell us who it's for and we check it names their company, not only a first name. Without that, a no-points note when nothing seems to point at them.
Then the grade
- F0-59Likely filtered or ignored
- D60-69Fix the blockers
- C70-79Easy to ignore
- B80-89Almost there
- A90-100Ready to send
Hard limits, however good the rest is
- Any high-severity issueB at best
- A strong trigger phraseC at best
- Any blocker, two strong trigger phrases, or two serious spam issuesD at best
- Two blockers, two disguised words, a phishing phrase, or high spam riskF
What we don't score, on purpose
- Lowercase vs Title Case. One large study found capitalized words got more replies; another found lowercase got more opens. When the data disagrees, we don't take points off.
- A question mark. Studies point both ways, so a question is fine.
- Plain numbers. "6 sdrs" is specific, which is good. The % and $ signs are another story.
- A single emoji. It's flagged as rare in B2B, not scored as a mistake.
- A follow-up's Re:. Mark the lines as follow-ups in an existing thread, and a Re: is honest.
Where the rules come from
Length and wording rules come from published cold email studies that measure replies, from about 9,000 campaigns to 31 million emails. Inbox cut-offs come from device tests. Filter and legal rules come from public sources:
How long should a cold email subject line be?
3 to 4 words, and under 33 characters if you can. In a benchmark of about 9,000 cold email campaigns, reply rates peak at 3 and 4 words and fall by about half at 9 to 11. A study of 712K cold emails sees replies drop from 5 words up, and one of 31M puts the peak a little later, at 5 to 6 words, so a 6-word line is fine.
Length matters more in cold email than anywhere else, because the inbox cuts it, and a phone shows about half of what a desktop does.
| Inbox | Shows about | Range in tests |
|---|---|---|
| Gmail app | 36 characters | 33 to 40 |
| iPhone Mail | 44 characters | 41 to 48 |
| Outlook app | 38 characters | 36 to 40 |
| Gmail on desktop | 70 characters | 60 to 88 |
Inboxes cut by width, not by a character count, so "WWW" takes more room than "iii". Front-load the part about them: anything past 33 characters may not show on a phone. The Gmail, iPhone Mail and desktop figures are from the device tests linked here; the Outlook app's are from other published tests. emailtooltester device tests
Cold email subject lines that get replies
Good cold and sales email subject lines are short, specific to them, and read like a note from a colleague. Here are common ones next to better versions, graded by the tester above.
- Instead of: Grade CQuick questionTry: Grade Aquestion about sdr ramp
- Instead of: Grade CPartnership OpportunityTry: Grade Abrightline + northwind
- Instead of: Grade D{{first_name}}, idea for {{company}}Try: Grade Aidea for brightline's outbound
- Instead of: Grade DRe: our callTry: Grade Anotes from tuesday
- Instead of: Grade DTransform Your Sales Process With AI-Powered AutomationTry: Grade Aramp for the austin sdr class
- Instead of: Grade FFREE demo - save 50% today!!!Try: Grade Acutting ae ramp time
"notes from tuesday" only works if there was a Tuesday. Never write a subject the email can't back up.
Patterns that work
- A trigger event
- new vp of sales at acme
- Their own words
- your point on detention fees
- A referral
- Dana suggested I reach out
- Their goal
- q3 pipeline for emea
Follow-up email subject lines
The best follow-up subject is usually the one you already sent: reply in the same thread, so the "Re:" is real and they see the context.
Starting a new thread? Give it a new angle instead of "Following up" or "Checking in", which say nothing about why you're writing again.
- Grade Athe ramp numbers I mentioned
- Grade Aanother idea for the austin team
- Grade Ashould I close the loop?
Subject line spam words to avoid
Filters read the subject line harder than the body, so the tester checks it against 240 trigger phrases, weighted 1.5x as the spam checker weighs them. No single word sends you to spam, but stacking urgency, money and hype in a subject is how promos read.
See the full spam word listCheck the whole email with the spam checker
Subject line tester FAQ
What is a good subject line for a cold email?
Short, specific and about them: 3 to 4 words that name their company, role, a recent event or their goal, like "new sdr class at brightline". No fake Re:, no shouting, no promo words, and nothing the email doesn't back up.
What's the character limit for an email subject line?
Email allows far longer subjects than anyone reads; the limit that matters is the inbox. About 36 characters show in the Gmail app on a phone, 44 in iPhone Mail, 38 in the Outlook app, and 60 to 88 in Gmail on a desktop, depending on window width. Inboxes cut by width, so wide letters show fewer. The tester shows each cut.
Should cold email subject lines be lowercase?
The data doesn't settle it. A study of 712K cold emails found all-lowercase lines got about 11% fewer replies than average, while another found lowercase got the most opens. That's why the tester doesn't score case. Pick the style that reads like a note from a colleague to your buyer.
Do questions work in cold email subject lines?
They can. Studies disagree, so the tester doesn't penalize a question mark. A specific question ("still hiring in denver?") does fine; a generic one ("Do you have 15 minutes?") reads as a template.
Does "quick question" still work?
Less than it used to. It's the most-used cold subject line, and across 31M cold emails it got 2.5% replies against 2.9% for other subjects. Say what the question is about instead.
Is it OK to put "Re:" in a cold email subject line?
Only on a real reply. A fake "Re:" or "Fwd:" claims a thread that doesn't exist; spam filters score it (SpamAssassin's GB_FAKE_RF rule), and the FTC's CAN-SPAM guide says a subject line must accurately reflect the content of the message, B2B email included.
Should you put their name in a cold email subject line?
A first name alone doesn't help: a study of 712K cold emails saw about 6.6% fewer replies with one merge field in the subject. Naming their company, role or a recent event does: 7% replies against 3% in a 5.5M-email B2B study. Whatever you merge in, check it's filled, because an empty {{first_name}} is the worst of both.
Do emojis help cold email subject lines?
Rarely in B2B. Reply data on a single emoji is mixed, so the tester only notes it. 2 or more reads as a newsletter, and 5 or more reads as a promotion.
How is this different from other subject line testers?
Most subject line testers and checkers grade newsletter subject lines for open rates, and some reward emoji and exclamation marks. This one grades cold outreach on reply data and spam signals, catches unfilled merge tags and fake replies, compares up to 10 versions at once, and asks for no email address.
Are my subject lines saved anywhere?
No. The tester runs in your browser, and nothing you type is sent, stored or logged. Our analytics record only grades, score ranges and which checks fired, never your text.
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