Email subject line tester

What this tool measures, and what it deliberately does not

Three things are counted. Characters, in Unicode code points rather than UTF-16 units, so an emoji counts as one character and not two. Words, split on whitespace, which is the figure worth watching when you are trying to make a line scannable rather than short. And the capital-letter ratio, which is uppercase letters as a share of all the letters present, ignoring digits, spaces and punctuation so that an acronym in a short line does not read as shouting.

There is no score. A single number out of a hundred would be the most shareable thing this page could produce and it would be invented: nobody outside the large mailbox providers can measure how a subject line performs, the providers do not publish a scoring function, and what actually decides whether your mail is opened is who you are to the person receiving it. A stranger reading the third email of a course they asked for behaves nothing like a stranger reading a promotion, and no arrangement of characters closes that gap.

So the output is a set of observations, each of which you can check yourself, and each of which you are free to overrule. The flags are the useful part precisely because they are boring: they catch the things people do by accident at eleven at night, not the things they do on purpose.

Where each inbox cuts a subject line off, and why the numbers are approximate

An inbox list does not truncate at a character count. It truncates at a pixel width, in a proportional typeface, at whatever size the reader has chosen, in a column whose width depends on the window, the device, the reading pane and in several clients on whether a preview pane is open at all. "Forty characters" is a translation of a pixel measurement, and the translation moves: a line of narrow letters fits a good deal more than a line of capitals and ms.

The three widths this tool uses are approximations, stated as such. iPhone Mail held upright shows roughly 30 to 40 characters of subject before the line runs out, and this tool marks the cut at 38, the wide end of that range, so a line this tool shows as clipped is clipped on every phone rather than only on the narrowest one. Outlook on a desktop, at a typical window width with the reading pane to the right, shows roughly 60. Gmail in a desktop browser, with a wide window, shows roughly 70. Treat the shortest of the three as your working budget, because it is the one your reader is most likely holding.

The preheader is the text the inbox shows after the subject when there is room, taken from the top of the message body. It is counted separately here because it is a separate opportunity, not an extension of the subject: on a phone it is often the line that decides, and left unset it fills with whatever your email happens to begin with, which is frequently a logo alt text or the word "View". Writing it is close to free and almost nobody does it.

The five flags, one at a time, and why each is on the list

Capital letters are flagged when more than half the letters in the line are uppercase and there are at least four letters to judge. Shouting reads as bulk mail to a person before it reads as anything to a filter, and the filters do weigh it: it is one of the oldest features in every content classifier ever shipped.

A second exclamation mark is flagged. One is a choice. Two is a tone, and the tone is a sales letter. The same goes for a second emoji: one emoji in a course subject line can work as a marker — a reader who sees the same small glyph five mornings running knows what the message is without reading it — and two is decoration competing with the words.

A leading "Re:" or "Fw:" is flagged hardest of the five, because it is a lie with a mechanism: it borrows the appearance of a thread the reader was part of. It works once, it is the single fastest way to teach somebody to distrust your name, and it is explicitly the kind of deceptive header that anti-spam law in most jurisdictions treats as an offence rather than a tactic.

The last flag is a word list — free, urgent, act now, guaranteed, limited time, click here and their neighbours. Be careful how much weight you give it. A modern filter decides mostly on your sending reputation, your authentication and whether people who receive your mail engage with it; a word in a subject line is a small signal that matters mostly when everything else about you is already unknown. The list is here because those words tend to appear in lines that are trying too hard, and that is a copy problem you can fix.

The "Day N of M" prefix, and what it costs

A course is a sequence, and a reader who has agreed to five mornings of something benefits from being told which morning this is. The prefix does two jobs: it tells someone who opens their mail on Thursday that they have two left, and it makes the fourth message recognisable in a list at a glance without being read.

It is not free. "Day 2 of 5: " is twelve characters, which is roughly a third of what iPhone Mail will show you, so the tool renders the prefixed line through the same three widths and lets you see what fell off the end. If the answer is the only interesting word in the line, move that word to the front or drop the prefix and put the day number in the preheader instead. What you should not do is keep the prefix and let it eat the subject, which is the outcome this checkbox exists to make visible.