No invented statistics. Every number is sourced or labelled as an example. Our standards

Calculator

Cold Email Subject Line Checker

Type or paste a subject line and this tool will tell you how long it is, whether it is likely to be cut off in a mobile inbox preview, and whether it contains the kind of words and punctuation that make a cold email read like a mailshot. It is a set of plain rules of thumb, not a deliverability test — and nothing you type leaves your browser.

Type a subject line to see what stands out about it.

How this works

The checker trims any repeated spaces, then runs six simple tests and reports what it finds.

Character length. Mobile inbox previews cut subject lines off, and where they cut depends on the app, the device, the font size and whether the phone is held in portrait. Rather than pretend there is one number, the tool flags anything over roughly 45 characters as at risk of truncation on a phone, and anything over 60 as likely to be shortened on desktop too. The practical test is not "is it under the limit" but "do the first 45 characters still make sense on their own".

Word count. Anything over nine words is flagged as long. Two words or fewer is flagged too, because very short lines can read as vague or automated unless they are genuinely specific.

Capitals. The tool counts words of three or more letters that are entirely in upper case, and separately flags a line that is all capitals. Genuine acronyms will trip this — the tool cannot tell VAT from SHOUTING, so use your judgement.

Exclamation marks. Any exclamation mark is flagged, and more than one is flagged more firmly.

Promotional trigger words. A short list of words that filters and readers both tend to react badly to in cold outreach: free, risk-free, guarantee, act now, limited time, cash, winner, no obligation, urgent, click here, buy now, exclusive offer, discount, cheap and a few others. Matching is on whole words, so "freedom" will not flag as "free".

Unmerged personalisation tags. If the line still contains something like a double-brace placeholder or a square-bracket token, the tool says so, because an unresolved merge tag arriving in someone's inbox is worse than no personalisation at all.

A worked example

Take the subject line FREE trial — act now, limited time only!!

That is 41 characters and 8 words, so length is not the problem. But the tool will report one word in all capitals, two exclamation marks, and three promotional trigger words (free, act now, limited time). Four separate flags on a line that is only eight words long.

Compare it with Quick question about your Q4 pipeline: 37 characters, 6 words, no capitals beyond a normal abbreviation, no exclamation marks, no trigger words. The tool has nothing to flag. That does not make it a good subject line — it is deliberately bland, and whether it earns an open depends entirely on whether the reader recognises the problem you are about to raise. It just means nothing about the line is actively working against you.

That distinction matters. If you want the thinking behind what actually makes someone open and reply, this guide on writing cold emails that get replies covers the part a checker cannot.

What this doesn't account for

This is a rule-of-thumb check, not a deliverability guarantee. Whether your email reaches an inbox is decided overwhelmingly by things this tool cannot see: your domain's sending reputation, your SPF, DKIM and DMARC records, how many people mark your mail as spam, how fast you are sending, whether the list is any good, and what is in the body of the email. A perfectly clean subject line sent from a domain with a poor reputation will still land in junk.

The checker also has no idea whether your subject line is any good. It cannot tell you whether it is relevant to the person receiving it, whether it is honest about what is inside, or whether it sounds like the twelfth identical email they have had this week. Those are the questions that actually decide whether you get a reply.

The trigger-word list is deliberately short and general. Filters do not work from a fixed word list — modern ones score the whole message and the sender behind it. Treat a flagged word as a prompt to reread the line, not as a verdict. And if you are sending cold email to UK contacts, the rules on what you can and cannot do in cold outreach matter considerably more than your punctuation.

Spotted a mistake or a number we can't back up? Tell us and we'll fix it.