Free email subject line tester
Craft subject lines with confidence! Our free tester checks your subject line against best practices to spot weak spots and help you create emails that get opened.
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What data did we consider when building our
subject line tester
Here's a look at the best practices we considered when building our subject line tester, as well as the data that backs it up. Our subject line tester assesses your subject line based on:
Length
Mobile devices show the first 30 characters of a subject line. You can go longer if necessary, but try to keep the main message in the first 30 characters so everyone can see it.
Reading level
The Flesch-Kincaid readability test measures how easy text is to read. Since people often scan their inbox, subject lines should be easy to read — ideally at a 6th-grade level or below.
Helpful words
Our subject line research identified keywords that correlate with higher opens. Words like "exclusive", "discover", "tips", and "new" have been shown to boost engagement.
Negative words
Our research has identified words that commonly trigger spam filters or reduce open rates. These can be pushy or misleading. We'll flag those words so you can avoid them.
Capitalization
All-caps subject lines are a big no-no since they may be flagged by spam filters. Some audiences may also react better to either sentence case or mixed case subject lines. Test yours to find out.
Numbers
Our research shows that using numbers in subject lines can improve open rates. Specific figures like "5 tips" or "30% off" help recipients gauge the value immediately.
Emojis
Our research shows that using emojis in subject lines improves open rates in some industries by up to 6.4%. One well-chosen emoji can help your email stand out in a crowded inbox.
Questions
Research shows that subject lines with question marks get higher than average opens. Questions spark curiosity and create a psychological need to find the answer.
Personalization
Using personalization in a subject line can help it stand out in the inbox. Merge tags like {first_name} and language like "you/your" increase open rates by up to 26%.
How our email subject line checker works
Enter your subject line
Type or paste the subject line you're planning to use. You'll see a live inbox preview as you type.
Get instant analysis
Our algorithm checks 9 research-backed criteria in milliseconds — length, readability, spam triggers, and more.
Optimise & send
Review the detailed breakdown, apply the suggestions, and optionally get AI-powered rewrite ideas.
FAQs about email subject line optimization
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