Will your subject line survive the inbox?
One line, a sender name, and about a second of somebody's attention.
The email is written and this is what gets read
You spend an afternoon on the email and forty seconds on the line that decides whether anyone opens it. It is also the part with the most ways to go quietly wrong.
It gets cut off mid-thought
Inboxes show about sixty characters, and a phone about half that. Anything after the cut is written for nobody, including whatever made the email worth opening.
The merge tag goes out unfilled
"Hi {{first_name}}" is the most recognisable sign of a broken send, and it does not arrive at one person. It arrives at the whole list at once.
The free second line goes to waste
Every inbox shows a preview after the subject. Set nothing and it takes whatever the email starts with, which is usually "View this email in your browser".
Two lines in, an answer out
Paste the subject
First line. Exactly as you would send it, merge tags and all.
Then the preview text
On the line below. Leave it out if you have not written one: that is an answer too.
See where it stands
Where it cuts off, what is unfilled, and what reads like the emails people clear unread.
What it checks
Where it gets cut off
Measured against both the desktop and the phone cut, and reported either way, so you know which half of your list sees the whole thing.
Merge tags still in the text
Every common format, in the subject and in the preview. Reported above everything else, because this is the one that cannot be taken back.
Whether the preview earns its place
Whether it is set at all, and whether it says something the subject did not. Two lines saying one thing is a line of space wasted.
Shouting and punctuation
Capitals and repeated exclamation marks are what the emails people delete unread have in common. Acronyms are left alone: an email about VAT is not shouting.
How many emoji
One can earn attention. Three is decoration that renders differently on every client, and some corporate filters strip them and leave gaps in the sentence.
Fake reply prefixes
Re: on something that is not a reply gets opened once and remembered. In some markets it counts as a deceptive header outright.
Straight talk about what it cannot see
It measures what can be measured about a line of text. Most advice about subject lines is folklore, and this deliberately reports none of it.
It cannot tell you if the line is interesting
That depends on who is reading, what you sent last time, and whether they wanted to hear from you. No count reaches any of it.
The cut-off is pixels, not characters
Every client differs, and a line of narrow letters survives longer than a wide one. The counts here are a reliable guide, not the exact truth for one inbox.
None of this affects spam filtering
That is decided by your sending reputation and your SPF, DKIM and DMARC records. Words like "free" change what a reader expects, not where the email lands.
Your copy never leaves your machine
The whole check runs in your browser. Nothing is uploaded, which is also why there is no limit on it.
Check the line before the send button
It takes ten seconds, and the merge tag it catches would have gone to everyone.