Subject Line Tester
Test subject lines for deliverability and engagement before you send.
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Use tool →How to Write Better Email Subject Lines
Type your subject line above and hit Analyze. This subject line tester scores it across seven categories: length, power words, spam risk, personalization, readability, formatting, and engagement signals. Each category contributes to a score out of 100. You get a breakdown showing exactly where you're strong and where you can improve. Test as many variations as you want before you send.
The mobile preview shows how your subject line will look on a phone inbox. Most people check email on mobile first, so if your subject line gets cut off at 40 characters, your best selling point might never be seen. Front-load the important words.
What Makes a Good Subject Line?
The best subject lines are specific, concise, and give the reader a reason to open. Vague subjects like "Newsletter #47" or "Quick update" don't tell anyone why they should care. A subject line that says "Your trial expires Friday" or "3 mistakes killing your conversion rate" gives a clear reason to click.
Numbers, questions, and personalization tokens all correlate with higher open rates. But the biggest factor is relevance. No amount of power words will save a subject line that doesn't match what the reader signed up for. Write for your audience, not for a scoring tool.
About Spam Trigger Words
Modern spam filters use machine learning, not simple word lists. A single "free" in your subject line won't tank your deliverability. But stacking multiple trigger words — "FREE!!! ACT NOW!!! GUARANTEED WINNER!!!" — will. This tool flags patterns that increase spam filter risk, not words that will automatically block your email. Context matters more than individual words.