Email Subject Line Tester
Test your subject lines for open rates, spam risk, and emotional impact. Get AI-powered suggestions to boost engagement.
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Why Testing Your Subject Line Matters
More Than You Think
47%
of consumers decide to open an email based solely on the subject line.
69%
of users mark emails as spam just because of the subject line, making spam-risk analysis essential before hitting send.
50%
or more emails are opened on phones, which makes mobile-first subject line formatting absolutely essential.
20–25%
Too many punctuation marks increase spam risk by
Perfect Every Subject Line - Before it Reaches The Inbox
With Our Subject Line Tester
All in one quick scan. No overthinking. No complicated analysis
Analyze
Get an instant score out of 100 + insights on clarity, urgency, engagement, spam risk, sentiment, reading level, and overall structure— so you know exactly what to fix without spending time trying to fix it.
Text Length
Shows whether your subject line is too short, too long, or just right.
Clarity Score
Checks if your message is instantly understandable.
Urgency Score
Measures how strongly it motivates action.
Engagement Score
Predicts how likely it is to grab attention.
Spam-Risk Score
Flags triggers that could send you to junk.
Word Balance
Evaluates power words, emotional words & filler words.
Reading Grade Level
Ensures your subject line is easy for all audiences.
Sentiment
Detects emotional tone (positive, neutral, negative).
Skimmability
Shows how quickly readers can grasp the message.
Better Alternatives
Auto-suggests improved versions in one click.
Comparison View
Lets you compare multiple subject lines side-by-side.
Compare Subject lines
Test variations side-by-side to identify which one performs better, and confidently choose the strongest version for your campaign.
See how it looks on Mobile
Over 50% of emails are opened on phones. Preview your subject line on mobile and desktop to ensure nothing gets cut off and your key message stays visible. Example: Desktop shows ~60–70 characters. Mobile shows ~32–40 characters. If your core message or offer appears later in the subject line, your audience may never see it. If your subject line doesn’t work on mobile, it doesn’t work.
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FAQs
Yes! The tool is completely free to use, with no login, credit card, or signup required.
Type or paste your subject line into the input box and click Generate. The tool instantly reviews length, clarity, mobile-fit, and overall impact and then gives you improvement suggestions.
It’s a tool that evaluates your subject line before you send your email. It checks for issues like length, formatting, and effectiveness so you can optimize for higher open rates.
The tester analyzes your subject line using inbox best practices, readability checks, mobile preview considerations, and performance benchmarks. It highlights what needs fixing and suggests stronger alternatives.
AI suggestions are generated based on proven email copywriting patterns, engagement data, and subject line performance principles. The tool rewrites your input to create clearer, more compelling variations.
There’s no limit. You can analyze and test as many subject lines as you want.
No. Your subject lines are processed in real time and are not stored, shared, or used for training.
Review the analyzer’s insights, apply the suggested improvements, and finalize the version that best fits your campaign. Then send your email with confidence knowing your subject line is optimized.
Interested In Collaborating?
Get in touch today!
Enter your information below to request a free, no-obligation consultation.
Our company needed to revamp our entire email system for our main brand of bicycles, Sixthreezero. We hired InboxArmy to build out new automated flows, post purchase email flows, marketing campaigns, and anything else we could come up with to stimulate our sales. I worked with the director and his designers to develop the emails that would connect with our target market. InboxArmy managed me, I literally had to keep up with how fast they work.
Immediate increase in revenues, our return customer conversion rate increased by roughly 50%, and our open to click rates all increased. The ROI was about 26 to 1. Zero complaints, our resources were not wasted hiring InboxArmy.
Andrew Bowman
E-Commerce B2C Director
