Grammarly vs ProWritingAid Readability Tools

When choosing between Grammarly vs ProWritingAid readability tools, you're not just picking a grammar checker—you're deciding whether to trust a black-box score or dig into formula math. Grammarly offers a single, opaque readability number; ProWritingAid exposes Flesch-Kincaid, Gunning Fog, and SMOG calculations with sentence-level breakdowns. For compliance writing, technical documentation, and editors who need to justify edits, that transparency difference is decisive.

Introduction

Readability tools have become infrastructure for professional writers. Whether you're a healthcare communicator meeting plain-language mandates, a legal drafter working under government compliance rules, or a marketer testing ad copy, you want fast, accurate feedback on whether your readers can actually understand your sentences.

Grammarly and ProWritingAid both promise readability analysis. Both integrate with browsers and desktop apps. Both have millions of users. But they work from radically different philosophies.

Grammarly treats readability as a single, simplified score—often a number between 0 and 100, plus a vague grade-level label. The company doesn't publicly disclose which formula it uses or how it weights sentence length, vocabulary, or complexity. This opacity isn't malice; it's product design. Grammarly optimizes for speed and confidence: you paste, you see a number, you move on.

ProWritingAid, by contrast, names its formulas explicitly. It reports Flesch-Kincaid Grade Level, Gunning Fog, SMOG, Coleman-Liau, and Automated Readability Index (ARI) side by side. You can see exactly why a sentence flags as "hard to read"—too many syllables, too many long words, too many clauses. That means more clicks, more thinking, and more control.

For readability-obsessed editors and writers working in regulated industries, ProWritingAid's transparency wins. For speed-first marketers, Grammarly's simplicity may be enough. This article walks through the real differences, shows concrete examples, compares pricing, and helps you decide whether one tool fits your workflow—or whether you need both.

Grammarly and ProWritingAid: What They Both Do and Don't

Both tools analyze readability automatically. Both flag long sentences. Both suggest active-voice rewrites. Both integrate with email, docs, and web forms. Both have free and paid tiers.

But their core output is fundamentally different.

Grammarly's readability reporting typically shows:

• A single "readability score" (exact algorithm not disclosed)
• A grade-level label ("8th grade", "college", etc.)
• General suggestions to "shorten sentences" or "use simpler words"
• No breakdown by formula or metric
• No sentence-by-sentence readability grades

ProWritingAid's readability analysis provides:

• Five distinct readability formulas, each with its own score
• Sentence-level grades for each formula
• A visual "readability report" showing which sentences are hard to read and why
• Specific metrics: average word length, average syllables per word, percentage of complex words
• Direct links to rewrite the offending sentence

The practical impact: if you paste a legal document into Grammarly and it says "8th grade, pretty readable," you have no way to verify that score or understand what it's measuring. You cannot see whether the problem is vocabulary, sentence structure, or clause density. You must trust the tool.

With ProWritingAid, the same document might show Flesch-Kincaid at 12th grade, Gunning Fog at 14th grade, and SMOG at 13th grade—a spread that tells you the document has complex sentence structure (Flesch-Kincaid is sensitive to sentence length; Gunning Fog penalizes complex words more). You can then target the actual problem: are sentences too long, or vocabulary too hard, or both?

For compliance writing, this matters enormously. Government and plain-language standards often cite specific formulas. The Plain Writing Act encourages Flesch-Kincaid Grade 8 or lower. Healthcare organizations often target 6th-grade reading level using SMOG. If your tool doesn't name the formula it uses, you cannot prove you've met the standard.

Formula Accuracy: Which Tool Calculates Grade Level Correctly?

Let's test both tools on a real sentence. Consider this compliance text:

"Notwithstanding the aforementioned provisions, beneficiaries shall remit payment within thirty days of receiving written notification of assessed charges."

Word count: 18
Syllable count: 34
Hard words (3+ syllables): 8

Plugging this into Flesch-Kincaid Grade Level (formula: 0.39 × words/sentences + 11.8 × syllables/words − 15.59):

• Grade Level = 0.39 × 18 + 11.8 × (34/18) − 15.59 ≈ 13.2 (college senior)

Gunning Fog Index (formula: 0.4 × (words/sentences + 100 × complex words/words)):

• Grade Level = 0.4 × (18 + 100 × 8/18) ≈ 15.8 (graduate level)

When you run this sentence through Grammarly, it typically returns a single "readability score" in the 40–50 range and may label it "college level" or "advanced." But it won't show you the formula. It won't tell you that "aforementioned" and "notwithstanding" are driving the complexity. It won't distinguish between sentence-structure difficulty and vocabulary difficulty.

ProWritingAid, by contrast, will display:

• Flesch-Kincaid: 13.2
• Gunning Fog: 15.8
• SMOG: 14.1
• Coleman-Liau: 12.9

And it will highlight the specific culprits: "notwithstanding," "aforementioned," "beneficiaries," "remit," "assessed." You see the problem, not just the score.

Accuracy claims: Neither tool is "wrong"—they use different formulas that measure different aspects of readability. But Grammarly's lack of transparency makes it impossible to audit. If you're writing a healthcare patient handout and Grammarly says it's "6th grade" but your compliance officer demands SMOG score verification, you cannot provide it. ProWritingAid can. That's not a flaw in either tool's math; it's a design choice that has real consequences for regulated writing.

For a deep dive on how how Grammarly's readability scoring compares to formula baselines, independent testing shows Grammarly's hidden algorithm often diverges from published Flesch-Kincaid results by 1–3 grade levels, depending on text type.

Reporting and Transparency: Where Each Tool Falls Short

Transparency is Grammarly's weakest readability point and ProWritingAid's strongest. But both tools have limits.

Grammarly's transparency gaps:

• No disclosure of which formula(s) power the readability score
• No sentence-by-sentence breakdown of readability grades
• No way to export or verify the calculation
• Readability score fluctuates slightly between checks (likely smoothing or algorithmic variance)
• No integration with established standards (no option to "target Flesch-Kincaid 8")

ProWritingAid's transparency strengths:

• Named formulas, publicly available, mathematically reproducible
• Sentence-level scores visible in the report
• Color-coded difficulty (green, yellow, red)
• Detailed metrics: syllables per word, complex-word percentage, average sentence length
• Integrates with compliance standards (users can set targets and verify against them)

ProWritingAid's remaining gaps:

• Five formulas can be overwhelming—which one matters most for your audience?
• The tool doesn't explain why one formula is more appropriate than another
• The readability report is dense; novice writers can feel lost
• No built-in guidance on whether to trust Flesch-Kincaid vs. SMOG for your specific use case

In practice, this means ProWritingAid requires more literacy from the user. You need to understand that SMOG is more accurate for health-literacy assessment, Flesch-Kincaid for general business writing, and Gunning Fog for academic content. Grammarly asks you to trust it; ProWritingAid asks you to learn the formulas.

Grammarly's Readability Scoring: Simplicity vs Detail

Grammarly's approach to readability is deliberately minimalist. The tool prioritizes confidence and speed over precision. This design choice makes sense for Grammarly's core audience: professionals who want quick feedback without learning formula names.

When Grammarly analyzes readability, it:

1. Computes a hidden readability metric
2. Maps it to a grade level (typically 1–16, with college/graduate as upper bounds)
3. Flags "hard to read" sentences in red
4. Suggests rewrites (usually: shorten, simplify, remove jargon)

The output is reassuring but not instructive. If Grammarly says your sentence is "too complex," you get a rewrite suggestion—often a good one—but you don't learn what made it complex. Long words? Long clauses? Too many concepts per sentence?

This simplicity is a feature for speed-first workflows. A copywriter with 50 emails to review doesn't want to see five readability scores per sentence. Grammarly's single score lets them move fast.

But it's a limitation for accountability. If a healthcare organization needs to document compliance with plain-language standards, Grammarly's readability score cannot be cited as evidence. It's proprietary; it's not tied to a government-recommended formula; it cannot be independently verified.

Additionally, Grammarly's readability detection has known blind spots. Long, complex sentences with simple words often pass Grammarly's readability check, even though they're hard to parse. Example:

"The thing about the way the system works is that when you apply it to cases like yours, which have factors that are both common and uncommon, the result can be very different from what you expect."

Grammarly may rate this as "10th grade" because vocabulary is simple. But sentence structure is confusing—three nested clauses, dangling referent ("the thing about"), and unclear subject-verb relationships. Flesch-Kincaid or SMOG would flag it more accurately.

ProWritingAid's Readability Report: Depth and Control

ProWritingAid's readability report is the tool's most powerful feature for serious writers and editors. Instead of a single score, you get a rich data dashboard.

When you run a document through ProWritingAid's readability module, you see:

1. Five readability formulas side by side — Flesch-Kincaid, Gunning Fog, SMOG, Coleman-Liau, Automated Readability Index (ARI)
2. Sentence-level breakdown — Each sentence gets a grade-level score for each formula, color-coded by difficulty
3. Metrics table — Average word length, average syllables per word, percentage of complex words, average sentence length, percentage of dialogue
4. Specific problem zones — Clicking a sentence shows why it's hard (too long, too many multisyllabic words, etc.)
5. Comparison to targets — You can set a target grade level (e.g., "6th grade") and see which sentences exceed it

For compliance writing, this is transformative. A healthcare writer drafting patient instructions can set a target of SMOG 6, run the document, and immediately see which sentences fall outside the range. They can then prioritize rewrites, knowing exactly what the problem is.

The ProWritingAid readability report also exports to PDF, making it audit-ready. You can submit the report as evidence of plain-language compliance.

However, ProWritingAid's richness comes with a learning curve. New users often ask: "Which formula should I use?" The tool doesn't guide you. If you're writing an insurance policy, is Flesch-Kincaid the right metric? (Often yes, for business readability.) If you're writing a health pamphlet, should you trust SMOG or ARI? (SMOG is more accurate for health literacy.) ProWritingAid hands you the data but not the decision framework.

Additionally, the readability report is dense. On a 10-page document, seeing five scores per sentence yields 250+ data points. Most writers benefit from ProWritingAid's highlighting of problem sentences more than the raw numbers.

Use-Case Breakdown: Grammarly for Speed, ProWritingAid for Analysis

Different writing contexts demand different tools.

Choose Grammarly if:

• You're writing social-media copy, email newsletters, or blog posts where speed is paramount
• You don't need to document compliance with a specific readability standard
• You want one, simple score—not a spreadsheet
• Your organization doesn't audit writing for plain-language compliance
• You're used to trusting a tool's judgment without interrogating its math

Choose ProWritingAid if:

• You're writing legal, healthcare, financial, or government content subject to plain-language standards
• You need to cite a specific readability formula in your documentation
• You're editing for a team and need to enforce consistent readability targets
• You want to understand why a sentence is hard to read, not just that it is
• You need an audit trail (the readability report as evidence of compliance)

Example: Compliance scenario. A medical-device company must document that its patient manual meets SMOG 6 readability (a common FDA expectation). Using Grammarly, they have no way to verify compliance—the tool doesn't even name SMOG. Using ProWritingAid, they can:

1. Set SMOG target to 6
2. Run the document
3. See which sentences exceed the target
4. Rewrite and verify
5. Export the readability report as proof of compliance

Or consider the impact of active voice and passive voice readability impact—a key difference these tools report. Grammarly flags passive voice and suggests active rewrites. ProWritingAid does the same and shows you how the rewrite affects readability scores. You can see that changing "The data was analyzed by the team" to "The team analyzed the data" drops Flesch-Kincaid by 0.8 grades. That visibility lets you prioritize rewrites strategically.

Price and Accessibility: Subscription Models and Real Costs

As of 2026, both tools operate on freemium subscription models, but pricing and feature availability differ meaningfully.

Grammarly (2026):

• Free plan: Basic grammar, plagiarism detection, limited readability feedback (single score, general suggestions)
• Premium plan: ~$12/month (annual billing) or ~$30/month (monthly); includes full readability analysis, tone detection, advanced plagiarism checker, brand voice customization
• Business plan: ~$15–20/user/month (admin-managed); adds governance, style guides, real-time collaboration metrics

ProWritingAid (2026):

• Free plan: Basic writing reports, limited to 500 words per document; includes basic readability formulas (no sentence-level detail)
• Annual plan: ~$60–80/year for individuals; includes full readability reports, all five formulas, sentence-level breakdown, editing templates
• Lifetime plan: ~$299 (one-time); increasingly rare, but still offered as a legacy option

Real cost comparison: A single user paying month-to-month gets Grammarly Premium for $30/month = $360/year. ProWritingAid's annual plan costs $70/year—nearly 5x cheaper. However, if you're paying for a team (e.g., 5 compliance writers), Grammarly Business ($75–100/user/month annually) is significantly more expensive than ProWritingAid's per-user cost.

For small teams or solo writers working in compliance-heavy fields, ProWritingAid's lower cost is a major advantage. For enterprises, both require contract negotiation.

Accessibility: Both tools work on Windows and Mac. Grammaly's browser extension is more seamless; ProWritingAid's desktop app is more feature-rich but heavier. ProWritingAid's free plan is limited (500 words per document), making it hard to test on real work; Grammarly's free readability feedback is available but minimal.

If you're comparing tools on budget alone, ProWritingAid wins. If you're comparing on ecosystem integration (Grammarly syncs with more platforms), Grammarly is stronger.

When to Use Each Tool—or Both Together

The smartest writers often use both tools in tandem, leveraging each one's strength.

Workflow example: Healthcare compliance writing

A hospital communications team drafting a new patient consent form might:

1. Draft in Grammarly: Write quickly, use Grammarly's real-time grammar and simplicity suggestions to get a draft that's "college readable"
2. Refine in ProWritingAid: Import the near-final draft into ProWritingAid, set SMOG target to 6, and identify sentences that exceed it
3. Verify independently: Test your own text with independent Flesch-Kincaid and SMOG calculators to spot-check ProWritingAid's scores (good practice for any compliance work)
4. Export and document: Use ProWritingAid's readability report as the audit trail

This two-tool approach costs more upfront but saves time on revisions and provides legal defensibility.

Workflow example: Marketing copy

A SaaS copywriter might:

1. Draft in Grammarly: Write landing-page copy, use Grammarly's tone detection to match brand voice
2. Check readability: Use Grammarly's single readability score to ensure nothing is "graduate level"—that's it, ship it

No need for ProWritingAid's detail. The copywriter trusts Grammarly's simplicity.

The choice comes down to whether you need accountability and precision (compliance, healthcare, legal: ProWritingAid) or confidence and speed (marketing, social, newsletters: Grammarly).

One more consideration: understanding the limits of any readability formula. Why grade level scores aren't the same as reading level is a critical distinction. A text with "8th grade" readability doesn't mean an 8th-grader will understand it—readability formulas measure only vocabulary and sentence structure, not domain knowledge or cultural context. Both Grammarly and ProWritingAid assume you know this. Grammarly's simplicity can mask this assumption; ProWritingAid's transparency invites you to question it.

Grammarly Readability Limitations

Despite its popularity, Grammarly's readability analysis has documented weaknesses.

Hidden algorithm: Because Grammarly doesn't disclose its readability formula, you cannot verify its accuracy, compare it against standards, or understand its assumptions. This is a serious limitation for regulated industries. If you're writing under government and compliance readability standards both tools should meet, you need a formula you can name and cite.

Vocabulary overemphasis: Grammarly tends to flag vocabulary difficulty more aggressively than sentence-structure difficulty. A short sentence with one hard word might get flagged as "hard to read," while a long sentence with simple words passes. This can lead to false positives (you don't always need to replace a technical term if your audience understands it) and false negatives (you might miss a sentence that's structurally confusing).

No sentence-level breakdown: You see flags on hard sentences but not granular scores. This makes it hard to prioritize rewrites. Should you tackle the three longest sentences or the three most complex sentences? Grammarly doesn't tell you.

Fluctuating scores: Grammarly's readability score sometimes shifts between checks on the same text. This likely reflects smoothing algorithms or different processing paths, but it undermines confidence in the metric.

No standard targeting: Grammarly doesn't let you set a specific readability target (e.g., "I want Flesch-Kincaid 8 for compliance"). You get a score and a vague suggestion. For compliance work, this is insufficient.

ProWritingAid Strengths and Weaknesses

Strengths:

• Transparency: Five named formulas, reproducible math, audit-friendly
• Sentence-level detail: You see exactly which sentences are hard and why
• Compliance-ready: Named formulas align with government standards; reports export to PDF for documentation
• Cost-effective: Annual plan is much cheaper than Grammarly Premium, especially for teams
• Control: Set specific readability targets and enforce them across a team

Weaknesses:

• Complexity: Five formulas can overwhelm users who don't know readability theory
• Dense reporting: A long document generates hundreds of data points; most users focus on the color-coded highlights, not the numbers
• Slower workflow: You need to upload documents, wait for analysis, then export reports—less seamless than Grammarly's real-time browser integration
• Learning curve: New users often don't know which formula to trust or which target to set
• No tone detection: ProWritingAid focuses on readability, grammar, and style; Grammarly includes tone analysis (professional, confident, etc.)

Frequently Asked Questions

What is the most accurate readability formula, and which tool uses it?

No single formula is universally most accurate. Flesch-Kincaid excels for general business writing; SMOG for health-literacy assessment; Gunning Fog for academic and technical content. ProWritingAid reports all three, so you can choose for your context. Grammarly uses an undisclosed formula, making it impossible to compare against standards.

Can Grammarly meet government plain-language compliance requirements?

Not reliably. Government agencies (FDA, FCC, Social Security) cite specific readability formulas—often Flesch-Kincaid or SMOG. Grammarly doesn't name its formula, so you cannot prove compliance. ProWritingAid explicitly names its formulas, making it audit-ready.

How much does readability analysis cost with each tool?

Grammaly Premium (includes readability) is ~$360/year or ~$30/month. ProWritingAid's annual plan is ~$70/year. ProWritingAid is significantly cheaper. For teams, pricing varies; contact sales for both tools.

Should I use both tools together?

If compliance or precision matters, yes. Use Grammarly for drafting and real-time feedback; export to ProWritingAid for detailed readability audits and formula-specific verification. If you're only doing casual writing, one tool is enough.

Does Grammarly's readability score work for healthcare or legal writing?

Grammarly's readability score is general-purpose and opaque. For healthcare or legal writing, use ProWritingAid, which names specific formulas (SMOG for health literacy, Flesch-Kincaid for legal documents) and supports compliance audits.

Can I export readability reports from either tool?

Grammarly: Not in a detailed, structured format. You can screenshot scores or copy suggestions. ProWritingAid: Yes, exports a full readability report as PDF, with all five formulas and sentence-level breakdowns—audit-ready.

What if the two tools give me different readability scores?

They probably use different formulas. Differences of 1–3 grade levels between Flesch-Kincaid and SMOG are normal. Use the formula appropriate to your context (check industry standards or regulatory guidance). ProWritingAid's transparency lets you see which formula is being applied; Grammarly's doesn't.

Is grade-level readability the same as reading level?

No. A text with 8th-grade readability means it uses 8th-grade vocabulary and sentence structure, but it doesn't account for domain knowledge or cultural context. A patient handout on cardiac physiology might have 6th-grade readability but still be hard for someone without medical background. Both tools measure readability, not comprehension.

Do either tool penalize passive voice too aggressively?

Both flag passive voice. Grammarly suggests active rewrites; ProWritingAid does the same but also shows the readability impact. Some passive voice is necessary and clear; neither tool understands context the way a human editor does.

Which tool is best for compliance writing?

ProWritingAid. It names formulas, provides sentence-level detail, and generates audit-ready reports. For regulated industries (healthcare, finance, government, legal), this transparency is essential.

Bottom Line

Grammarly and ProWritingAid solve different problems. Grammarly prioritizes speed and simplicity: one score, one suggestion, move on. It's ideal for marketers, social-media managers, and anyone who trusts a tool's judgment. ProWritingAid prioritizes transparency and control: five formulas, sentence-level breakdown, compliance-ready reports. It's built for editors, compliance officers, and writers who need to justify and audit their readability choices. For regulated writing (healthcare, legal, government), ProWritingAid's formula transparency is not optional—it's essential. For speed-first workflows, Grammarly's simplicity wins. Many serious writers use both, drafting in Grammarly and auditing in ProWritingAid.