Starter Plan · 1 token

Subject Line Checker

Score any subject line 0-100 against the rules that actually move open rates

Paste a subject line and get a score 0-100 against 40+ deliverability and engagement rules — length, spam triggers, urgency, personalization, emoji use, capitalization, and more. The score breakdown tells you exactly what works and what hurts. Then FUEL generates 5 alternatives, each pre-scored, so you can pick the one most likely to land in the inbox.

How It Works

Paste your subject line into the checker

See a 0-100 score against 40+ deliverability and engagement rules

Read the breakdown — what works, what hurts, and why

Get 5 AI-generated alternatives, each pre-scored

Copy the winner and ship it

Features

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40+ rules covering length, spam triggers, urgency, emoji use, capitalization

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0-100 score with line-by-line explanation

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Brand voice match — alternatives match your tone, not generic copywriter speak

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5 pre-scored alternatives so you skip the rewrite cycle

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Works for cold email, newsletters, drip sequences, and broadcast

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No token cost on re-checks of the same subject line

Example Output

**Your subject:** "ACT NOW: Save 50% on our biggest sale ever!!!"
**Score:** 34/100

**What hurts:**
- ALL CAPS in "ACT NOW" — penalized by every major spam filter
- Three exclamation points — high spam-trigger weight
- "biggest sale ever" — vague, unverifiable, low engagement signal
- 51 chars — fine on length, but everything else cancels it out

**Top alternative (87/100):** "Your 50% off ends Friday — here's what to grab first"

Frequently Asked Questions

Does this work for cold email or just newsletters?

Both. The 40+ rules cover deliverability (which matters for cold) and engagement (which matters for newsletters and drips). The score breakdown highlights which rules each line passes or fails.

How are the alternatives generated?

They use your brand voice (from Foundation) and your customer phrases (from Customer Voice Mining if you have it). Generic AI subject lines test poorly — these are tuned to your audience.

Will it match my industry?

Yes. The scoring weights some rules differently for SaaS, ecommerce, B2B services, and creator-style sends, based on what your Foundation data says about your business.

Ready to use Subject Line Checker?

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