Subject Line Tester and Checker
Paste a subject line. Get an AI score out of 100, plus concrete feedback and rewrites to lift your open rate.
Instant score
Rated 0–100 on clarity, curiosity, urgency, and open-worthiness.
Actionable feedback
Specific improvements you can act on today.
Three rewrites
Fresh alternatives you can copy straight into your ESP.
What the AI actually checks
Clarity
Does the reader know what's inside before they open it? "Quick question" tells them nothing. "Your invoice is 3 days overdue" tells them exactly what's waiting. Vague subject lines get left for later. Later usually means never, and never means an unopened email dragging down your sender reputation.
Curiosity
A subject line should raise a question the reader wants answered, not one they can already answer. "Newsletter #47" answers itself: skip. "The mistake that's costing you opens" doesn't. Curiosity earns the open. It just can't be a lie the email doesn't pay off.
Relevance
Does it match what this specific reader actually cares about, or could it go out to anyone on any list? "Big news" fits every inbox and belongs to none. "Your cart is about to expire" only makes sense to someone who has a cart. Relevant beats clever.
Urgency
Real urgency comes from a real deadline. "Don't miss out" is filler because nothing is actually running out. "Sale ends at midnight" is a fact the reader can act on. Manufactured urgency gets ignored the second time it shows up.
Personalization
A first name in the subject line isn't personalization, it's a mail-merge trick readers see through instantly. Real personalization is content that could only be relevant to that segment: their order, their industry, their last action. That's the part readers actually respond to, and response is what your sender reputation tracks over time.
Length and deliverability
Aim for 30 to 50 characters so the line survives on mobile, where most opens happen. Skip the words that trip spam filters: "free," "guarantee," all-caps urgency, stacked exclamation points. A subject line that never reaches the inbox scores zero. Get the other five right, and the sixth, whether people open it, tends to follow.
FAQ
How long should a subject line be?
Aim for 30 to 50 characters. Most opens happen on mobile now, and mobile inboxes cut subject lines off well before desktop does, so anything longer just disappears.
Does personalization actually improve open rates?
Not the first-name kind, most readers have seen that trick enough times to ignore it. What moves the needle is content that's genuinely specific to that reader: their order, their industry, their last click. Generic personalization is just a longer generic subject line.
What words trigger spam filters?
Words like "free," "guarantee," and "act now" in all caps, plus excessive exclamation points and dollar signs. None of them will get you blocked on their own, but stack enough of them and filters start scoring the whole email as risk.
What's a good subject line score?
The higher the number, the more of these six things you're getting right: clarity, curiosity, relevance, urgency, personalization, and length. A low score is usually one specific miss, not six things all going wrong at once.
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