Email Subject Line Tester — Score Length, Spam Words & Preview
Score a subject line for length, spam trigger words, capitals and punctuation, and preview how it truncates on iPhone and in Gmail.
- 31 characters: fits most inboxes without truncation
- 5 words: in the range that reads at a glance
- No common spam trigger phrases
- Addresses the reader (“you”/merge tag) (+4)
Heuristics, not a deliverability test. Filters weigh sender reputation, list hygiene and engagement far above wording; the subject is what you control in the next five minutes.
Runs entirely in your browser. Nothing you enter is sent to us or stored. Subject lines are one variable; the campaign, the list and the offer are the rest. Talk to us about email marketing.
Found a problem, or something that could be better? We read every message and we fix things quickly.
This free email subject line tester scores a subject line on the factors that measurably affect open rates and spam placement: length and where it gets cut off on a phone, word count, known spam trigger phrases, capital letters, punctuation and emoji. It shows the line as it will appear in iPhone Mail and Gmail, with the preheader beside it, because that pairing is what recipients actually see.
It is a heuristic, and the page says so. Inbox placement depends far more on sender reputation, list hygiene and how many people engage with your mail than on wording. But the subject line is the one variable you control in the five minutes before sending, and the common mistakes, a 90-character subject, three exclamation marks, a fake “Re:”, are cheap to catch.
How to use
- Type the subject line. The score and findings update as you type.
- Add the preheader (preview text) and from name to see the complete inbox row; a good preheader continues the subject rather than repeating it.
- Fix anything flagged: shorten to fit 40 characters on mobile, remove spam phrases and shouting capitals, keep to one exclamation mark at most.
- Compare two or three variants and pick the strongest, then A/B test them in your email platform if it supports it.
Frequently asked questions
How long should an email subject line be?
Under about 40 characters shows in full on most phones, where the majority of email is opened first; up to about 70 shows in desktop Gmail. Front-load the important words so a truncated line still makes sense. Very short lines (one to three words) can work for known senders but say too little from a stranger.
Which words trigger spam filters?
Classic promotional language: free, act now, limited time, guarantee, no obligation, click here, winner, $$$, 100%, and fake Re:/Fwd: prefixes. No single word gets a message filtered; filters score many signals together and weigh reputation highest. Several triggers in a subject plus a poor sending reputation is the combination that lands in spam.
Do emoji help or hurt?
One relevant emoji can lift opens a little for consumer lists and looks out of place in most B2B mail. Two or more read as spam. Some corporate mail clients render them as boxes, so never let an emoji carry the meaning.
What is a preheader?
The preview text shown after or under the subject in the inbox, taken from the first text in the email unless you set it explicitly. It is the second-most-read line in the message. Use it to add the detail that did not fit the subject, not to repeat it and not to leave “View this email in your browser” as the first thing people see.
Can this predict my open rate?
No. Open rates depend on who you are sending to, whether they asked for the mail, when it arrives and what they think of the sender, and Apple Mail Privacy Protection inflates the number anyway. Treat the score as a checklist of avoidable problems, and measure real results by clicks and replies.
Subject lines that work, and why
Specific beats clever: “Your invoice for September” outperforms “Big news inside”. Numbers make a line concrete (“3 changes to the pricing page”). Questions open a gap the reader wants closed. A person's name or company in the line signals relevance if it is genuinely personal and reads as spam if it is merge-field theatre. Urgency works when it is true and once; a weekly “last chance” trains people to ignore you.
Before you send
Send a test to yourself and read it on a phone. Check the from name, because it is read before the subject. Make sure the preheader is deliberate. And check the link tracking: a subject line only earns the open; the email has to earn the click, which is where a UTM link builder and a clear call to action come in.
Related free tools
- Word Counter — Count words, characters, sentences and reading time, and see keyword density and your most-used phrases as you type.
- UTM Link Builder — Build clean, consistent UTM tracking links so you know exactly which campaigns bring you customers.
- SERP Snippet Preview — Preview how your page title and meta description will look in Google — with live character-count checks.