Free subject line tester for cold email
Type a subject line and your first line to get a score out of 100, a phone and desktop inbox preview, and the fixes.
- Type a subject line and the first line of your email. The score updates as you type.
Inbox preview
Type a subject to see where a phone cuts it.
How the subject line tester scores your subject
A subject line tester is a tool that scores an email subject line before you send it and shows what to change. This one is built for cold email. It checks length, where a phone cuts the subject off, spam words, capitals, punctuation, merge tags and the preview text that sits next to the subject in the inbox.
Every subject starts at 100. Each rule below takes points off when it finds a problem, and what is left is the score. The rules are plain JavaScript running on this page. Nothing you type is uploaded, stored or sent to an AI model.
It grades the subject line and the first line only. To check the whole email body for trigger words, links and length, paste it into the free email spam checker for cold email copy.
| Check | What it flags | Points off |
|---|---|---|
| Length | ||
| Word count | 6 to 9 words, or 10 and more | 5, or 12 |
| Phone cut-off | A subject wider than the phone preview row, so the app cuts it with "..." | 8 |
| Capitals and punctuation | ||
| All capitals | The whole subject in capitals | 20 |
| Shouted words | Words of 4+ letters in capitals, like FREE | 6 each, up to 12 |
| Title Case | Every Word Starting With A Capital | 6 |
| Exclamation marks | One, or two and more (also "!?") | 10, or 16 |
| Other symbols | An emoji (8), runs like $$$ or *** (6), more than one question mark (5), a [Tag] at the start (5) | 5 to 8 |
| Spam filter patterns | ||
| Fake reply | Re: or Fwd: on a first email | 15 |
| Opening | Starts with a dollar amount (10), with "Buy" (8) or with an "ADV" tag (10) | 8 to 10 |
| Split letters | F.R.E.E, F-R-E-E and similar | 10 |
| Words | ||
| Spam and promo words | Six groups: money, urgency, overpromise, pushy calls to action, scam patterns, newsletter words | 8 per hit in the subject, 4 in the preview, up to 24 per group |
| Merge tags | {first_name}, [Company Name], %FIRSTNAME% and similar, left in the text | 10 each, up to 20 |
| Empty nudge | "Following up", "checking in", "touching base" and similar | 6 |
| Preview text | ||
| Missing | No first line entered | 6 |
| Greeting or intro | Opens with "I hope this finds you well", "My name is" or only "Hi Ana," | 6 |
| Repeat | The preview starts by repeating the subject | 4 |
Score = 100 minus the points, never below 0.
Where the rules come from
Some rules mirror what filters and mailbox providers publish. SpamAssassin, the open source spam filter, has named rules for a subject in all capitals (SUBJ_ALL_CAPS), one that starts with a dollar amount (SUBJ_DOLLARS) or with "Buy" (SUBJ_BUY), letters split up with symbols (GAPPY_SUBJECT), an "ADV" label (ENGLISH_UCE_SUBJECT) and a missing subject (MISSING_SUBJECT). Google's sender guidelines ask senders not to start a subject with Re: or Fwd: unless the message really is a reply or a forward.
The rest are our rules for cold email, and so are the point values. No filter publishes a fixed price per word, so treat the score as a list of what to fix first.
How the phone cut-off is measured
The tester measures your subject in a phone font against a row about 290 pixels wide, roughly what a 390-pixel phone screen leaves for the subject after the sender's picture and the margins. If it does not fit, the preview shows where the app adds "...". Apps and fonts differ, so the exact character count can move by a few either way.
How to read your subject line score
Ready to send
Short and about the reader. Nothing in it looks like a promotion.
Minor fixes
One or two things, usually length, a greeting in the preview or a single loaded word.
Risky
It reads like a campaign. Cut the promo words and the punctuation before it goes to a list.
Rewrite
Start again from one fact about the reader in two to five words.
The fixes under the score are sorted by points, so the top line moves the number most. Fix it, watch the score change, then work down the list. The inbox preview updates at the same time, which is the quickest way to see what a prospect sees before they decide to open.
A high score means nothing in the subject works against you. It cannot promise an open. That depends on who you picked, why you wrote to them now and whether the sender is a name they trust.
Worked example: from 27 to 100
An agency wants to offer a pipeline audit to a B2B SaaS team that is hiring sales reps. Here is the first draft, run through the tester. It is the sample loaded above, so you can edit it and watch the score move.
FREE Pipeline Audit For Your Team - Act Now!!
Hi {first_name}, I hope this email finds you well.
SDR hiring at Northwind
Saw Northwind is hiring two SDRs. That usually means the pipeline target just went up.
| What the tester found | Points | The fix in the rewrite |
|---|---|---|
| 2 exclamation marks | 16 | None |
| Merge tag {first_name} in the preview | 10 | Preview starts with the reason for writing |
| Cut off on a phone | 8 | 23 characters, fits with room to spare |
| Money word "free" | 8 | No offer in the subject |
| Urgency "act now" | 8 | No deadline |
| FREE in capitals | 6 | Normal case |
| Title Case | 6 | Sentence case |
| Preview opens with "I hope this email finds you well" | 6 | Opens with their hiring news |
| 8 words | 5 | 4 words |
73 points off, so the first draft scores 27. The rewrite trips no rule.
The rewrite drops nothing the prospect needed. The audit offer moves into the body, where there is room to explain it. The subject only has to tell Northwind's head of sales that this email is about them.
What makes a cold email subject line work
Most subject line testers were built for newsletters and store promotions. Omnisend's tester FAQ says an emoji can help a subject stand out in a busy inbox. SubjectLine.com lists emotion and urgency among the signals it scores, and Smartlead's analyser rates emotional impact. That makes sense for a sale email to people who signed up for it.
A cold email lands with someone who has never heard of you. It has to look like one person wrote it to another, so the habits a newsletter grader rewards tend to hurt here. This tester scores the other way round.
How long should a cold email subject line be?
Aim for 2 to 5 words, and make sure the whole thing fits on a phone. The tester takes a few points off from 6 words and more from 10, and shows exactly where a phone cuts the line. If you need a longer subject, put the word that matters at the start.
Do spam words in subject lines matter?
Yes, more than in the body, because the subject is the first thing a filter and a person read. One "free" in a normal sentence is a small risk. "FREE" plus "act now" plus two exclamation marks is the pattern of a blast, and that is why those words cost double in the subject here.
Plain beats clever
Write in sentence case or all lowercase, and leave out exclamation marks and emoji. A subject like "SDR hiring at Northwind" or "question about your Q1 hiring" reads like a colleague's note. Lowercase is never penalized here.
Make it about them
The best cold subjects name something true about the reader: their company, a role they are hiring, a tool they use, a launch. That one detail does the work emotion and urgency do in a newsletter, and it tells them why the email exists.
Use the preview text
The first line of the email shows next to the subject, so it is part of the pitch. "Hi Ana, I hope this finds you well" spends that space on nothing. Open with the reason you wrote, and keep greetings for the line after.
Never fake a reply
"Re:" on a first email may earn an open, but it misleads the reader and Google's sender guidelines ask senders not to do it. The same goes for "following up" on an email they never got. If you are writing a real follow-up, give it news; our cold email follow up templates and timing post has 8 to start from, and the guide to building a cold email sequence covers what each touch should say.
Better subject lines bring more replies. Then they need answers
A subject that gets opened is the start of the job. The replies that follow need a fast, right answer, and that is where Revbound comes in. It pulls every reply from Instantly, Smartlead, Email Bison, PlusVibe, SendKit and ArcSend into one inbox and sorts them with AI: interested, meeting requested, out of office, referral, not interested.
Revbound drafts or sends the answer, offers times that are actually free on your Cal.com or Google Calendar, and books the meeting when the lead says yes. Out-of-office replies get a note when the person is back, and quiet leads get followed up for about two months. Want to write a single answer by hand first? The free email response generator for sales replies drafts one for each reply type.
Plans start at $49 a month with the AI included, and every plan on the Revbound pricing page comes with a 3-day free trial of every feature.
Subject line tester questions
How do I test an email subject line?
Type the subject and the first line of the email into a subject line tester, such as the free Revbound subject line tester. It scores the subject out of 100, shows a phone and desktop inbox preview, and lists what to fix in order of points. Fix the top item, watch the score change, then send the winning version to a small batch before the full list.
How long should a cold email subject line be?
Aim for 2 to 5 words, short enough that the whole line shows on a phone. Long subjects get cut off with "..." in a phone inbox, so the most important word should come first. The Revbound subject line tester shows exactly where a phone cuts your subject.
Do spam words in subject lines matter?
Yes. The subject is the first thing a spam filter and a reader see, so words like "free", "act now" and "guaranteed" carry more risk there than in the body. A single loaded word is a small risk; several plus capitals and exclamation marks look like a blast. The Revbound tester counts a spam word in the subject at double the weight of one in the preview.
What is a good subject line score?
On the Revbound subject line tester, 90 to 100 is ready to send and 70 to 89 needs a minor fix. A cold email subject under 70 should be rewritten before it goes to a list. Other testers use their own scales and often reward urgency and emoji, so scores do not carry over between tools.
Should cold email subject lines be lowercase?
Lowercase or sentence case both work, because both look like a person typed them. What hurts is Title Case on every word, words in capitals and exclamation marks, which read like a newsletter. The Revbound subject line tester never penalizes lowercase.
Can a subject line tester predict my open rate?
No tester can see your list, your sender reputation or your timing, so no score is a reliable open rate forecast. The Revbound subject line tester does not estimate open rates. It flags what is likely to hurt a cold subject line and leaves the proof to a real send.
What is the difference between a subject line tester and a spam checker?
A subject line tester grades the subject and preview text: length, phone cut-off, capitals, punctuation and loaded words. A spam checker grades the whole email, including the body, its links and its length. Revbound offers both free, and both run in the browser.
Is the Revbound subject line tester free and private?
Yes. The Revbound subject line tester is free with no sign-up, and it runs entirely in your browser. The subject, preview text and name you type are never uploaded, stored or sent to an AI model.
Gmail, SpamAssassin, Omnisend, SubjectLine.com and Smartlead are trademarks of their owners, and Revbound is not affiliated with them. Read the subject line rules in Google's email sender guidelines.
Your subject earns the open. Revbound books the meeting
When the replies come in, Revbound answers interested leads with times that are free on your calendar and books the call when they say yes. Every feature, free for 3 days.