Writing toolRuns in your browserFree · No signup

Subject Line Tester for Cold Email

Type a subject line and watch it get scored on length, spam triggers, capitalization, personalization and clickbait, then see exactly how it looks in a phone inbox. Free, instant, and nothing leaves your browser until you ask for AI.

Your subject line

Score updates as you type. Nothing is sent until you ask for AI.

Subject line0 / 50 chars

What the email is about (optional)

Used as the preview text in the inbox mock and passed to the AI so variants stay on topic.

What a strong cold subject looks like

20 to 50 characters, 3 to 7 words, sentence case
One specific noun: a company, a role, a number, a trigger
No adjectives, no urgency, no fake "Re:"

Returns a critique plus rewritten variants. Uses one free AI run.

Subject line score

Type a subject line to score it

You get a 0 to 100 score, a 12-point checklist, and a mobile and desktop inbox preview.

Nothing leaves your browserNo account or email requiredUnlimited use

How it works

How to use the Subject Line Tester

01
Type your subject line
Paste the subject you plan to send and, optionally, the first line of the email so the preview text matches reality.
02
Read the score and checklist
Twelve checks run as you type. Red items cost the most points, amber items are worth fixing, and tips are optional lifts.
03
Preview, then improve with AI
Check the mobile and desktop inbox mockups for cutoff, then ask the AI for a critique and rewritten variants you can use with one click.

Why the subject line decides whether cold email gets read

A cold email is judged in about a second, and that second is spent on three things in order: who sent it, what the subject says, and what the first line of preview text shows. The recipient is asking three questions. Do I know this person? Is this about me? Will it cost me time? A subject line that answers the second question with a specific detail about the reader's company or role usually wins the open, even from a stranger.

The subject lines that work in cold outreach look nothing like marketing headlines. They are short, written in sentence case or lowercase, and contain a concrete noun: a company name, a job title, a number, a recent event. They read like something a colleague would type in a hurry. The moment a subject starts to look designed, with Title Case, adjectives, emoji or urgency, the reader files it under promotions and moves on.

Subject line length: what the inbox actually shows

Length matters because inboxes truncate. Most phone email apps show roughly 30 to 40 characters of a subject before cutting it off, and desktop clients show somewhere between 60 and 70 depending on window width. Since more than half of email is first opened on a phone, anything past the 35th character is a gamble. This tester scores 20 to 50 characters and 3 to 7 words as the safe zone, and the inbox preview shows you exactly which characters disappear on each screen.

Two practical rules follow from this. First, front-load the meaningful word. If the company name or the trigger event is at the end, it gets cut on mobile. Second, do not go too short either. One or two word subjects like "Question" or "Hello" look like phishing or a mass blast, and give the reader nothing to say yes to.

Mobile preview: about 35 characters of subject, then 45 characters of preview text on the next line.
Desktop preview: about 70 characters of subject, then preview text on the same line until the row runs out.
The preview text is the first line of your email body, so it should continue the thought, not repeat the subject.

How the score is calculated

The score starts at 100 and each check deducts points based on how strongly the pattern predicts a skipped or filtered email. Fake reply prefixes and clickbait cost the most because they break trust as soon as the email is opened. Length, spam triggers and capitalization come next. Personalization, questions and numbers are positive signals: their absence costs little or nothing, but their presence is called out so you can add them.

Length: 20 to 50 characters passes. Under 10 or over 70 loses 18 points, in between loses 8.
Word count: 3 to 7 words passes. A single word or more than 10 loses 10 points.
Spam triggers: one phrase from the trigger list loses 10, two or more lose 20. Subject hits matter more than body hits.
Capitalization: any word in all caps (acronyms of two letters excepted) loses 7, more than half the words in caps loses 15.
Exclamation marks lose 6 each up to 12. Emoji lose 4 for one and 10 for two or more.
Title Case On Every Word loses 5. Fake Re: or Fwd: loses 20. Clickbait patterns lose 15.
A merge token such as {{company}} passes; its absence loses 5. Questions and numbers are tips only.

Patterns that quietly kill replies, and what to write instead

The fake reply prefix is the worst offender. Adding "Re:" to a first email lifts opens for a day and then poisons the sender for good, because the reader feels tricked the moment they open it. It is also a deceptive subject line under CAN-SPAM, which prohibits subject lines that mislead about the content. Clickbait phrases like "you won't believe" and "the secret to" fail for a related reason: buyers have seen them thousands of times and associate them with content farms, not colleagues.

Generic openers such as "quick question" or "partnership opportunity" are not spam triggers, but they signal a template. Replace them with the thing you actually noticed: "{{company}} hiring 4 SDRs" or "your Q3 pricing change". Company names tend to outperform first names as merge tokens because they prove research; anyone can merge a first name. A short question and a specific number both help, since they give the reader something concrete to react to. When you are stuck, the AI improve button critiques the current line and returns variants written in the same voice, and ColdBox can rotate those variants across a sequence and keep the one that gets replies.

FAQ

Subject Line Tester questions

Straight answers, no fluff. Still stuck? Our deliverability team replies within a couple of hours.

Ask a human

Yes. The scoring, checklist and inbox previews run entirely in your browser with no account, no email gate and no usage cap. The optional AI improve feature calls our servers and is limited to a fair number of runs per day per person.

Aim for 20 to 50 characters and 3 to 7 words. Phone inboxes cut subjects off around 35 characters, so put the most specific word first. Going under 10 characters is worse than going slightly over 50, because very short subjects look automated and give the reader nothing to react to.

It rarely hurts, but the company name works harder. A first name can be merged by anyone, while a company name, a job title or a recent event shows you did research. If you use a merge token, make sure the fallback value reads naturally when the field is empty.

In B2B cold outreach, yes. Both are strongly associated with newsletters and promotions, which is exactly the category you are trying to avoid. A colleague emailing you about a real issue would not use either. The tester deducts points for each, and more for two or more.

It is the single worst pattern the tester checks. A fake reply prefix can lift opens briefly, but readers feel deceived on open, reply rates drop, and spam complaints rise. It also counts as a misleading subject line under CAN-SPAM. Use a real, specific subject instead.

No. The score removes the patterns that predictably lower opens and flag filters; it cannot measure relevance to a specific reader. A 95 with a generic topic will underperform a 75 that names the exact problem the reader has this quarter. Use the score to clean up, then test two variants against real recipients.

It does, though less than sender reputation and authentication. Filters weigh subject line hits more heavily than body hits because the subject is the first thing scanned, so all caps, trigger phrases and exclamation marks in the subject carry extra risk. Clean subject, authenticated domain and a warmed inbox together keep you in the primary tab.