I Tested the Free Email Subject Line Testers With a Fake Line

By Frederik Baun, conversion copywriter and founder of Words Sounds Visuals

I ran the same two test lines through three separate subject line testers on 1 August 2026 and built my own subject-line tool from what the gap between their scores revealed.

Test a Subject Line Right Now

If you came here to score a line, start with my subject line writer instead. One run is free without handing over an email, and it writes 17 lines against 17 formulas rather than grading the one line you already have against a word list.

That difference is what this whole post is about. Every tester below scores a line you wrote yourself, so I fed two lines through all of them to find out what those scores really measure.

The Two Lines

I aimed the real one at people who send cold email.

“Gmail’s spam update just tanked your reply rate”

It names a platform every reader knows and a consequence they care about, in eight words and 47 characters.

I built the fake one to hit scoring targets while meaning nothing at all.

“Are 7 proven secrets crushing your velvet horizon?”

It has a non-round number, two recognised power words, a question mark and exactly 50 characters. I picked every word in it for the scoring, not the meaning.

The Scores

SubjectLine.com rated my meaningless line 12 points higher than my real one. It told me the nonsense line had crushed it, while my real line was only on the right path.

Headline Studio split my two lines five points the other way, which is closer to sensible. But five points is still a rounding error between a line that could work and a line that could never work.

How the Scoring Works Underneath

SubjectLine.com shows its arithmetic, which I appreciated, because it makes the whole thing legible.

Here’s the breakdown it gave for each line.

Read that table again, because it explains the whole result.

My nonsense line picked up seven of its points from a keyword lookup. It contained “proven” and it contained “secrets”, so it scored, and the tool never asked what those words were doing there.

My real line lost six points for containing the word “update”. In the sentence I wrote, “update” refers to a specific change at Gmail. The reader has felt that change in their reply rate. The scorer sees a string on a list.

That’s the mechanism. Free subject line testers run a word lookup, add up the hits, and hand you a number. It’s a spell-checker for marketing vocabulary.

CoSchedule’s Headline Studio, in Detail

CoSchedule’s tester is the most detailed of the three, and it deserves its own look, because “free” tells you almost nothing about how it grades.

Paste a line, pick Email as the content type, and it hands you a score with a breakdown underneath.

It grades word count against a roughly nine-word target, and character count against a 40-to-50 range. It also scores:

• Word balance across common, uncommon, emotional and power words
• Sentiment
• Clarity
• Reading grade
• Skimmability

An analyser and a topic-to-headline generator both sit inside the same tool, now branded Headline Studio.

Run the same two lines through it and the gap narrows.

That’s five points, not 12, so it’s closer. But it’s still the same failure. The nonsense line hit every countable target. It landed within five points of a line that could work in a real inbox.

What it gets right is worth naming, because the mechanical checks are genuinely useful on their own. Character count is the standout, since anything past roughly fifty gets cut off on most devices.

The word balance breakdown caught something real too. “Uncommon: 0%” on my nonsense line was a fair hit, because a line built entirely from common words does read as forgettable.

Where it costs you is the free tier. Basic runs on credits, one spent per analysis.

On both of my test lines it told me exactly how many score-boosting suggestions existed, four on the first, two on the second. Every one of them sat behind the upgrade button. You get the count. The suggestions themselves stay locked.

Which Free Email Subject Line Testers Are Still Free

“Free” covers a range these days. Here is what each tool asked of me when I ran my two lines through it on 1 August 2026.

Of the three, SubjectLine.com scored my line without asking who I was. It offers an account for history and AI rewrites, and a paid tier for open-rate predictions. The core score sat in front of all of that.

SendCheckIt put a form in front of the result. It wanted a first name and an email address before it would show me a score. So budget an inbox for it.

On my first line Headline Studio reported four score-boosting suggestions and put all four behind the upgrade button. My second line got two, handled the same way. I saw the count and not the suggestions.

The Words That Cost You Points

Since the scoring is a lookup, it helps to know roughly what is on the lists.

Words that earned points in my test were “proven”, “secrets”, “you” and “your”, plus punctuation and a non-round number. Words on the penalty side included “update”.

The broader category of spam-adjacent language is well established, and worth avoiding on its own merits rather than because a scorer says so:

• Free
• Act now
• Urgent
• Limited time
• Risk free
• Guarantee
• Cash bonus
• Click here

Those spend your deliverability budget and read as desperate.

The trap is treating the list as the job. A line assembled from high-scoring words and cleared of low-scoring ones will pass a tester and still say nothing. My velvet horizon line is proof of exactly that.

What a Score Is Good For

These tools are still worth running. You just need to know what question they answer.

They are good at counting:

• Character length
• Word length
• Punctuation
• Obvious filter risks

A tester that tells you your line runs to 68 characters, before you send to 12,000 people, has earned its 30 seconds.

What decides the open is whether the line is anchored to something the reader already cares about. A scorer can look for a personalisation token, and it still cannot tell whether the thing you named matters to the person reading it.

Gmail is an anchor because your reader uses Gmail. Velvet horizon sits in the same grammatical slot and scores better.

So run the check, fix anything mechanical it surfaces, and then close the tab and ask the harder question yourself.

What I Do Instead

Full disclosure, the subject line writer I keep pointing at is mine, and it works the other way round.

Scoring assumes you already have a line worth checking. But my problem was sitting there with one mediocre line, a score of 61, and no idea what a better line would look like.

So it reads your finished email and returns 17 options, each built on a different formula. It gives you range rather than a verdict.

If you want both, the order matters. Generate a spread of options first, then run your favourite two through a free tester to catch the mechanical problems. Generation gives you something worth scoring, and scoring gives you a safety net.

Doing it the other way round means polishing the only line you thought of. That’s how you end up with a well-formed subject line about a velvet horizon.