// free tool
Subject Line Tester
Paste your subject line, get a spam score and open-rate prediction. Built on the frameworks from our email subject line formulas guide.
0 / 120 characters
// subject line score
// how to improve
// what gets emails opened
The Science Behind High-Open Subject Lines
Mobile-First Length
iPhone shows ~40 characters. Gmail on desktop shows ~60. Front-load your value — the most important words must appear in the first 30 characters or they get cut off.
Curiosity, Not Clickbait
"You won't believe what happened" gets clicks but kills trust. "The pricing mistake that cost us $12K" creates curiosity while promising value. Always deliver what the subject line implies.
Spam Filter Avoidance
ALL CAPS, multiple exclamation marks, and words like "FREE," "URGENT," or "ACT NOW" train spam filters to bury your email. One power word is persuasive. Three looks like a scam.