Free Email Subject Line Tester
Score your email subject lines from 0 to 100. Detect spam trigger words, check character length and personalisation, preview how your email looks in the inbox, and get actionable suggestions to boost open rates.
Instant Score
Get a 0-100 subject line score with colour-coded grading in seconds
Spam Check
Detect 40+ spam trigger words that can land your email in the junk folder
Actionable Tips
Get specific suggestions to improve open rates and inbox placement
Type or paste your email subject line below to get an instant score and spam check.
Subject Lines That Score Well
Click any example to see how it scores.
How to Write Email Subject Lines That Get Opened
Use these evidence-based techniques to craft subject lines that land in the primary inbox and compel recipients to open.
1. Keep It Short and Scannable
The ideal subject line is 30-50 characters and 4-8 words. Most mobile email clients display only 30-40 characters, so front-load your most important words. Subject lines that fit on a single line without truncation consistently outperform longer alternatives.
2. Personalise With Merge Fields
Including the recipient's first name or company name in the subject line can lift open rates by up to 26%. Tokens like [First Name] or {company} signal that the email was written specifically for them rather than blasted to a list. Always test your merge fields before sending to avoid broken tokens.
3. Avoid Spam Trigger Words
Words like “FREE”, “BUY NOW”, “URGENT”, and “ACT NOW” are red flags for spam filters. Even if your email makes it through, these words erode trust. Use our subject line checker above to identify and replace any spam triggers before you hit send.
4. Create Curiosity Without Clickbait
Questions, numbers, and mild urgency cues create curiosity gaps that compel opens. “Quick question about your Q2 pipeline” is more intriguing than “Our services can help you”. But avoid misleading subject lines — high open rates mean nothing if recipients immediately bounce.
5. Use Sentence Case, Not ALL CAPS
ALL CAPS subject lines trigger spam filters and feel aggressive. Sentence case (capitalising only the first word) feels more personal and conversational, mimicking how a colleague would write. Title Case is acceptable for more formal communications but still outperforms ALL CAPS.
6. Test, Measure, and Iterate
A/B test two to three subject line variations with 10-15% of your list, then send the winner to the rest. Track open rate, reply rate, and meeting-booked rate — not just opens. Use this email subject line tester to score each variation before you send, and keep a swipe file of your highest-performing lines.
Frequently Asked Questions
Everything you need to know about writing and testing email subject lines
How does the email subject line tester work?
Our email subject line tester evaluates your subject line across multiple criteria including character length (ideal 30-50), word count (ideal 4-8), spam trigger word detection (40+ known spam words), personalisation tokens, emoji usage, question format, number inclusion, urgency words, and capitalisation. Each factor is weighted and combined into an overall score from 0 to 100, with specific suggestions for improvement.
What is the ideal email subject line length?
The ideal email subject line is 30-50 characters and 4-8 words. This range displays fully on both desktop and mobile email clients without truncation. Lines under 30 characters may lack context to compel opens, while lines over 50 characters get cut off on mobile devices where over 60% of emails are read.
What are spam trigger words in email subject lines?
Spam trigger words are terms that email spam filters flag as suspicious. Common examples include “FREE”, “BUY NOW”, “URGENT”, “ACT NOW”, “CLICK HERE”, “LIMITED TIME”, “GUARANTEED”, and “NO OBLIGATION”. Our outsourced SDR guide covers email deliverability best practices in more detail.
Does personalisation improve email open rates?
Yes. Emails with personalised subject lines (e.g. including the recipient's first name or company name) see up to 26% higher open rates. Personalisation tokens like [First Name] or {company} signal relevance and help your email stand out. However, incorrect merge fields that display the raw token instead of a name can seriously damage trust.
Should I use emojis in email subject lines?
A single, relevant emoji can increase open rates by up to 56% by helping your email stand out visually in a crowded inbox. However, using three or more emojis can look unprofessional and trigger spam filters. Stick to one emoji that reinforces your message, and always test performance with your specific audience.
Why does ALL CAPS hurt email deliverability?
Writing subject lines in ALL CAPS triggers spam filters because it mimics the style of spam and phishing emails. It also feels like shouting, which reduces trust and open rates. Use sentence case or title case instead. Even partial ALL CAPS (e.g. “FREE BONUS Inside”) can increase your spam score.
Do question subject lines get more opens?
Question subject lines can significantly boost open rates because they create a curiosity gap — the recipient opens the email to find the answer. Questions that are relevant to the recipient's pain points work best. For example, “Are you still losing deals to competitor X?” is more compelling than a generic question like “Want to learn more?”.
How can I A/B test my email subject lines?
Use this subject line tester to score multiple variations, then send the top 2-3 to small segments of your list (10-15% each). After 2-4 hours, send the winning subject line to the remaining recipients. Test one variable at a time — length, personalisation, emoji, or question format — so you can attribute the open rate difference to a specific change.
Related Resources
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