Paste your draft subject line
Type or paste the exact subject line you're planning to send into the input box above. The checker analyzes it as you type, with no need to click a "submit" button.
Free subject line checkerMake the first line
Make the first line
earn the open.
Paste a subject line for instant, rules-based feedback on length, personalization, spam language, questions, and urgency. Nothing is sent to a server.
Score your subject line before you send
Run a free, rules-based email deliverability test on length, spam language, personalization, and urgency.
How to use the subject line checker
No signup, no install — just paste and edit.
Read each rule's feedback
The score breaks down into five checks — length, spam language, personalization, question usage, and urgency — each with a short explanation of why it helped or hurt the score.
Edit and re-check
Adjust the line based on the feedback and watch the score update live. Aim for a strong starting point, then trust your judgment on the specific context of the email.
What the checker scores and why it matters
Every subject line starts at 100 points. The score subtracts points for patterns that are statistically more likely to hurt opens or trip spam filters.
Length (25–50 characters)
Most inbox previews show roughly 40–60 characters before truncating, and mobile screens cut off even sooner. Lines between 25 and 50 characters keep the full idea visible. Under 15 characters costs the most points because it's often too vague to justify an open; over 60 characters is penalized for likely truncation.
Spam-trigger language
The checker scans for a short list of phrases long associated with promotional or low-quality email — things like "100% free," "act now," "guaranteed," and "winner." Each match costs points, up to a capped penalty, because stacking several of these phrases reads as a mass blast rather than a real message from a person.
Personalization
The checker looks for a supported merge tag — {{firstName}}, {{company}}, or {{title}} (or the bracketed equivalents) — anywhere in the line. Personalized subject lines signal a real, specific message written for one person. This one is optional and only costs a small deduction when missing, since not every strong subject line needs a name in it.
Question usage
Exactly one question mark can make a subject line feel conversational and prompt a reply-style reaction. Zero questions is fine too — a direct, specific statement works just as well. More than one question mark loses points because stacked questions tend to read as pressure rather than curiosity.
Urgency language
Words like "urgent," "asap," "deadline," "expires," and "last chance" are checked separately from the spam list because they're common in legitimate emails too. A single urgency word costs a small deduction as a nudge to confirm the deadline is real; two or more compounds the penalty since stacked urgency reads as manufactured pressure.
Best practices with concrete examples
The rules above map directly to real edits. Here's what a stronger rewrite typically looks like.
Subject lines that tend to score well
Specific, personalized, and honest about the ask — these examples stay in the 25–50 character range with at most one question mark and no manufactured urgency.
- "Quick question about {{company}}'s onboarding flow" — specific, personalized, one clear topic
- "{{firstName}}, worth 10 minutes next week?" — direct ask, real personalization, single question
- "Notes from your {{company}} demo" — concrete and relevant, no pressure language
- "Following up on outreach automation" — plain statement, no filler words
Vague, pressured, or promotional phrasing
"ACT NOW: 100% free consultation, limited time!!" — stacks two spam-trigger phrases and forced urgency, and the exclamation marks compound the promotional feel.
"Hi" — under 15 characters with no context. Technically opens the message, but gives the recipient no reason to click before deleting.
"Are you free? Can we talk? Is now good??" — three question marks reads as pressure, and the score reflects it.
Subject line checker questions
Does this tool send my subject line to a server?+
No. The subject line checker and the spam word checker both run entirely in your browser using JavaScript. Nothing you type is transmitted, logged, or stored anywhere — refresh the page and it's gone.
Does a high score guarantee my email will land in the inbox?+
No. These are rules-based editing checks; actual inbox placement still depends on sender reputation, mailbox warm-up, recipient engagement, and other factors no static checklist can fully predict.
Why did a normal-sounding word get flagged as a spam trigger?+
The checker matches a fixed list of commonly abused promotional phrases, like "act now" or "risk free." A flagged word can still be the right choice — judge it in context. "Free" inside "free trial" reads very differently than "free money."
What subject line length works best?+
The checker rewards 25–50 characters as a concise, scannable range. Under 15 characters often reads as too vague to earn an open, and past 60 characters the line is likely to truncate on phones and some inbox previews.
Do merge tags like {{firstName}} or {{company}} actually help?+
Personalization tends to earn attention because it signals a real, specific message. The checker looks for {{firstName}}, {{company}}, or {{title}} tags — the same tags InboxJoy uses when AI drafts a sequence for you.
How is the subject line score calculated?+
It starts at 100 and subtracts points for risk factors: length outside the ideal range, spam-trigger phrases, missing personalization, multiple question marks, and forced urgency language. It's a transparent rules engine, with every deduction traceable to one of the checks above.
How often are the trigger word lists updated?+
The lists cover well-known, persistently abused phrases and are reviewed periodically. They're intentionally conservative — a short, high-confidence list beats a bloated one that flags ordinary business language.
Can I use these tools if I'm not an InboxJoy customer?+
Yes. Both tools are free, need no signup, and work for any email you're drafting — cold outreach, newsletters, or internal copy.
Once my subject line is ready, how do I actually send the email?+
Connect Gmail or Outlook to InboxJoy in one click, import leads by CSV, and let AI draft the full sequence around your reviewed subject line and merge tags. Sending windows, per-mailbox caps, and a warm-up ramp starting at 8 sends a day control the pace, and replies automatically stop further follow-ups.
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