Email Subject Line Tester

The subject line decides whether an email is opened. You rarely get a second chance, so many people write several versions and pick one. This tester scores each line against seven visible rules, shows how it will look in an inbox that cuts it off, and lets you compare up to twenty lines in one go. It does not predict open rates; it catches avoidable mistakes.

Entering lines

Type or paste one subject line per line. The first 20 non-empty lines are tested. Press Sample for five examples. The text stays in your browser’s memory and is not saved. A switch at the top moves between “Scores” and “Inbox preview”.

The seven checks

Length gives 30 points for 25 to 50 characters, 20 for 15 to 24 or 51 to 60, and 8 otherwise. Trigger words give 25 for none, 12 for one and 0 for two or more phrases such as “free”, “act now” or “cash”. Capitals give 15 if there is no word in capitals of four or more letters. Punctuation gives 10 unless there are stacked marks such as “!!!”, “?!” or “$$$”, or more than one exclamation mark. Emoji give 5 for at most one. A specific hook gives 10 if the line contains a number, a question mark or the word “you” or “your”. Honesty gives 5 if there is no placeholder and no fake reply prefix. A line that begins with “Re:” or “Fwd:” on a new email also loses another 20 points, because it misleads the reader.

Reading the results

The scores table lists each line with its character and word counts, its score and a grade (Excellent at 85, Good at 70, Needs work at 50, Poor below), and what to fix, taken from the checks that failed. When you test more than one line, a message names the highest score. If several lines tie, it shows the first.

The inbox preview

Mail apps cut subject lines by screen width, not by a fixed number of characters, and the exact cut-off varies by app, device, font and settings. The preview lets you choose how many characters are shown: about 35 for a typical phone, about 50 for a large phone or narrow window, about 70 for a desktop mail app, or any number from 10 to 120. Each line is shown to that point, followed by an ellipsis, and the hidden part is shown below in grey. An optional preheader, the grey text that follows the subject, is shown after lines that fit. The width you choose is remembered; the text of your lines is not.

Worked scores

The five sample lines score as follows. “Quick question about your onboarding” (36 characters) scores 100. “Your quarterly report is ready to review now” (44 characters) scores 100. “3 ideas to cut your support queue by half” (41 characters) scores 100. “Re: your invoice” (16 characters) scores 65 because of the shorter length and the fake reply prefix. “FREE!!! Act now to WIN $$$ cash prizes” (38 characters) scores 40, with failures for trigger words (free, act now, cash), capitals, punctuation and a missing hook. In the preview at 35 characters, the quarterly report line reads “Your quarterly report is ready to r…” with “eview now” hidden.

Writing better lines

Put the important words first. Be specific rather than clever, and say what is inside. Use numbers and questions where they are honest. Avoid pretending to be a reply, and avoid urgency you cannot justify. Test the best candidates on your own list, since only your readers can tell you what works.

Limits

The rubric is this tool’s own, and the trigger list is a general heuristic. High scores do not guarantee opens, and spam filters look at much more than the subject. The cut-off preview is an approximation of how apps behave.

Related tools

• Cold Email Checker & Spam Word Highlighter grades the message body that the subject line introduces.
• Copywriting Framework Builder produces the longer copy behind the line.
• Email ROI Calculator puts a value on the opens and clicks that follow.
• Word Counter counts characters and words in any text you are drafting.