What makes a good email subject line?

30–60 chars, no spam words, numbers and questions, power words, no ALL CAPS.

Yes, 100% free with no account required.

📊 20+ signals analyzed instantly

Write subject lines that actually get opened

Paste any subject line and get a real-time score, spam check, and specific tips to lift your open rate — in seconds.

✓ No sign-up

✓ Free forever

✓ Instant results

Subject Line Analyzer

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Your subject line

0 chars

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Start typing to see your score

What we check

Every signal that affects your open rate

SubjectBot runs your subject through the same filters used by spam engines and real human psychology research.

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Spam Trigger Detection

We check against 40+ words that trigger spam filters — "free", "click here", "act now", "guaranteed". These silently kill deliverability.

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Length Optimization

The sweet spot is 30–60 characters. Too short = no context. Too long = mobile truncates. We score your length and tell you how to fix it.

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Power Word Analysis

Words like "secret", "discover", "proven", and "results" statistically increase open rates. We identify them and reward natural usage.

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Numbers & Questions

"7 ways to..." and "Are you making this mistake?" consistently outperform flat statements. We check for both and give credit for each.

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Caps & Punctuation

ALL CAPS looks like shouting. Excessive !!! looks desperate. Both get flagged by spam filters. We catch both and explain the damage.

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Actionable Score

0–100 score with a letter grade, plus per-check cards with a ✅/⚠️/❌ and specific copy telling you what's wrong and what to do instead.

How it works

From subject line to score in seconds

No account. No setup. Just type and see your score update live.

Type your subject

Paste or type any subject line. Scoring happens as you type — no button, no delay.

See your score

Get an instant 0–100 score with a grade and a visual breakdown of every check that passed or failed.

Read specific tips

Each failed check explains exactly what to change and why. No vague advice — specific, actionable guidance.

Iterate to 80+

Edit your subject and watch the score update. Keep going until you hit Excellent (80+) before you hit send.

FAQ

Common questions

Everything you need to know about SubjectBot.

Great subjects are 30–60 characters, free of spam trigger words, contain a number or question when natural, create curiosity without being clickbait, and use proven power words. SubjectBot checks every one of these automatically.

We start at 100 and apply deductions for spam words (−15 to −35), bad length (−8 to −25), ALL CAPS (−20), and excessive punctuation (−15). We add bonuses for numbers (+5), questions (+5), and power words (+8). Score capped at 0–100.

Yes — 100% free with no account, no credit card, and no usage limits. SubjectBot is part of the Zenbot suite of free tools for creators and marketers.

A high score means you've avoided the common, preventable mistakes. But open rates also depend on list quality, sender reputation, send time, and audience fit. SubjectBot removes the own-goals — you still need to nail the message for your specific audience.

Target 75 or above before sending. 80–100 is Excellent — best practice subject lines. 60–79 is Good but worth a quick edit. Below 60 means there are specific issues that will hurt performance and are worth fixing.