Email Subject
Line Tester.
Score your email subject lines instantly. Checks for spam triggers, power words, ideal length, and capitalisation — then suggests AI-rewritten alternatives. Free, no signup.
What makes an email subject line work — and what kills it
An Email Subject Line Tester checks your line against the signals that spam filters and email clients use to decide where your message lands. Length, trigger words, punctuation patterns, and capitalisation all affect whether your email reaches the primary inbox or gets filtered before the reader ever sees it.
| Signal | What to do | Why it matters |
|---|---|---|
| Length | Keep under 50 characters | Gmail Mobile clips at ~40 chars |
| Spam words | Avoid 'FREE', 'Act Now', 'Guaranteed' | Trigger Bayesian filters on major ESPs |
| Capitalisation | Sentence case or Title Case | ALL CAPS is a spam signal and feels aggressive |
The elements this tester checks
This tool checks seven factors: character length, spam trigger word presence, use of power words (urgency, curiosity, value), capitalisation patterns, punctuation overuse, emoji count, and personalisation signals. Each factor is scored independently so you can see exactly what to fix — not just a single pass/fail number.
How Subject Line Scoring Works
7 factors that directly influence whether your email gets opened or ignored.
Ideal Length (20 pts)
40–60 characters is the sweet spot. Shorter loses context; longer clips on Gmail Mobile.
Spam Triggers (25 pts)
Words like 'guaranteed', 'act now' and 'click here' trigger spam filters and kill deliverability.
Power Words (15 pts)
Urgency, curiosity, FOMO and value words drive opens — each triggers a different emotional response.
Capitalisation (10 pts)
ALL CAPS screams spam. Sentence case or title case builds far more trust with both readers and filters.
Punctuation (10 pts)
One exclamation mark is fine. Two starts to look desperate. Three or more hurts open rates significantly.
Emoji (10 pts)
Zero or one emoji maximises impact. Multiple emojis reduce trust and can trigger mobile spam filters.
📝 Video Transcript
Frequently Asked Questions
What is the best email subject line tester?
ToolStack's free email subject line tester is the most complete option available. It scores your subject line across seven factors — length, spam triggers, power words, capitalisation, punctuation, emoji, and engagement signals — gives you a letter grade with a breakdown, shows inbox previews for Gmail and Outlook, and generates five AI-rewritten alternatives across urgency, curiosity, value, FOMO and personalisation angles. No signup required.
What is a good email subject line score?
A score of 80+ (grade A) is excellent and should deliver strong open rates. 65–79 (grade B) is good. Aim for at least 65 to maximise performance. Anything under 50 needs significant improvement before sending.
What are email spam trigger words?
Spam triggers are phrases that spam filters and email clients flag as suspicious, including 'guaranteed', 'act now', 'click here' and 'earn money'. Removing all spam triggers is the single highest-impact change you can make to improve deliverability.
What is the ideal email subject line length?
The ideal email subject line is 40–60 characters. This ensures full display in Gmail desktop (up to 70 chars) and Gmail mobile (up to 40 chars) without clipping. Subjects under 20 characters lack context; over 80 and most clients will truncate them.
How do I use A/B compare mode?
Click the 'A/B Compare' button above the input. Two subject line fields appear side by side. Enter your two versions and get instant scores for both. The winner is highlighted with a green badge — use it to pick the stronger subject line before you send.
What are email power words and how do they work?
Power words are emotionally charged words that drive action. Urgency words (now, tonight, expires, deadline) create time pressure. Curiosity words (secret, revealed, truth) make readers want to know more. FOMO words (exclusive, rare, limited edition) create fear of missing out. Value words (save, deal, proven, results) communicate clear benefit.
How do I improve my email open rate?
Keep your subject line between 40–60 characters, include 1–2 power words from the urgency or curiosity categories, remove all spam trigger words, add a specific number for credibility, limit punctuation to one mark, and use a maximum of one emoji. Use the AI alternatives feature to get 5 professionally rewritten options.
Email Subject Line Tester: Free Online Tool
Your email subject line is the gatekeeper to everything else — it determines whether your email gets opened or ignored, reported as spam, or deleted before being read. Most people write subject lines by feel. Our Email Subject Line Tester gives you data: a score based on length, word choice, emotional triggers, personalization signals, and known spam triggers.
Enter your subject line and the tester instantly analyzes it across 8 dimensions: length, personalization signals, urgency and scarcity triggers, power words, emotional resonance, spam word detection, emoji usage, and number/stats usage. Each dimension gets a sub-score and the overall subject line gets a 0-100 score.
Common uses include testing A/B subject lines before sending a marketing campaign, checking whether a subject line triggers spam filters before a cold email blast, optimizing newsletter subject lines for maximum open rate, and validating that a subject line looks good in the mobile email preview.
Most subject line testers are either too simplistic or require a paid subscription. Ours gives you multi-dimensional scoring — emotional triggers, spam word detection, mobile preview truncation — for free, without signup. Test unlimited subject lines and iterate until you hit your target score.
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