How to Measure Content Readability: A Complete Guide to Flesch-Kincaid, Gunning Fog & More

Ever wondered why some articles feel effortless to read while others make you want to give up after the first paragraph?

It's not random. Readability, how easy your text is to understand, can actually be measured. And understanding these measurements can transform your writing from confusing to crystal clear.

Let's break down the science of readability, explore the most popular formulas, and show you how to use them in your own content.

What is Readability?

Readability measures how easy your text is to understand. It's not about vocabulary size or intelligence, it's about matching your writing to your audience's expectations and cognitive load.

A medical journal article should be different from a blog post. A children's book should be different from a legal contract. Readability formulas help you hit the right target.

The concept emerged in the 1920s when educators wanted to match textbooks to students' reading levels. Since then, dozens of formulas have been developed, each with different strengths and weaknesses.

Why Readability Matters

For Writers and Content Creators

Low readability kills engagement. Studies show that readers abandon difficult content within seconds. If your sentences are too long and your words too complex, people simply leave.

This matters for:

• Blog posts and articles - Readers scan before committing. Complex writing loses them at the scan.
• Marketing copy - Conversion rates drop when prospects struggle to understand your message.
• Documentation - Users who can't understand instructions can't use your product.
• Email campaigns - Unopened emails hurt, but unread opened emails hurt more.

For SEO

Google doesn't directly use readability scores in rankings, but the indirect effects are significant. Content that's hard to read leads to:

• Higher bounce rates (users leave quickly)
• Lower time on page
• Fewer shares and backlinks
• Poor user experience signals

All of these factors influence search rankings. Writing clearly isn't just good practice, it's good SEO strategy.

For Accessibility

Approximately 21% of adults in the United States read below a 5th-grade level. Worldwide, the numbers are even more striking. Clear, readable content isn't a luxury, it's essential for reaching your full potential audience.

The Most Popular Readability Formulas

Flesch Reading Ease

Developed by Rudolf Flesch in 1948, this is probably the most well-known readability formula. It produces a score from 0-100, where higher scores mean easier reading.

The Formula:

206.835 – 1.015 × (total words / total sentences) – 84.6 × (total syllables / total words)

Score Interpretation:

Best for: General audience writing. Most experts recommend aiming for 60-70 for web content.

Flesch-Kincaid Grade Level

Created by J. Peter Kincaid for the U.S. Navy in 1975, this formula converts readability into U.S. School grade levels. It's built into Microsoft Word and many other writing tools.

The Formula:

0.39 × (total words / total sentences) + 11.8 × (total syllables / total words) – 15.59

Interpretation: A score of 8.0 means the text is suitable for an 8th-grade reader (approximately 13-14 years old).

Best for: Educational content, compliance documents, and any situation where you need to target a specific reading level.

Pro tip: Most successful web content scores between 7th and 9th grade. This isn't dumbing down, it's respecting your readers' time and attention.

Gunning Fog Index

Robert Gunning developed this formula in 1952 specifically for business writing. It focuses heavily on complex words (three or more syllables).

The Formula:

0.4 × [(words / sentences) + 100 × (complex words / words)]

Interpretation: Like Flesch-Kincaid, the result represents a U.S. Grade level. A score of 12 means the text requires a 12th-grade reading level.

Best for: Business and professional writing. Gunning created it after observing that newspapers with Fog Index scores under 12 had higher readership.

Target: Aim for 12 or below for business communication, 8-10 for consumer-facing content.

SMOG Index

SMOG (Simple Measure of Gobbledygook) was developed by G. Harry McLaughlin in 1969. It's particularly popular in healthcare communications.

The Formula:

1.043 × √(number of polysyllables × (30 / total sentences)) + 3.1291

Key difference: SMOG only counts words with three or more syllables (polysyllables) and requires a 30-sentence sample for accuracy.

Best for: Healthcare materials. The CDC and many health organizations use SMOG because it correlates well with comprehension of health information.

Coleman-Liau Index

Unlike most formulas that count syllables, Coleman-Liau uses characters per word. This makes it easier to compute automatically.

The Formula:

0.0588 × L – 0.296 × S – 15.8

Where L = average number of letters per 100 words, and S = average number of sentences per 100 words.

Best for: Automated analysis systems and situations where syllable counting is impractical.

Automated Readability Index (ARI)

Another character-based formula, ARI was designed for real-time monitoring of readability (originally for teletypewriters).

The Formula:

4.71 × (characters / words) + 0.5 × (words / sentences) – 21.43

Best for: Technical systems and real-time analysis. Often used alongside other metrics for a more complete picture.

Comparing the Formulas

Each formula emphasizes different aspects of text:

Practical recommendation: Use Flesch-Kincaid for general purposes, SMOG for health content, and Gunning Fog for business writing. When in doubt, run multiple formulas and look for patterns.

What These Formulas Don't Measure

Readability formulas are useful tools, but they have significant blind spots:

Content Quality

A text can score perfectly readable and still be:

• Factually incorrect
• Poorly organized
• Missing key information
• Boring

Readability measures surface-level characteristics, not content value.

Prior Knowledge

Technical writing for experts can legitimately use jargon. A cardiology journal article shouldn't avoid "myocardial infarction" just because it has many syllables, cardiologists expect precise terminology.

Formulas don't account for audience expertise.

Coherence and Flow

You could write a series of short, simple sentences that are completely disconnected. The readability score would be excellent, but comprehension would suffer.

Bad example (scores well, reads poorly):
"Dogs bark. Cats meow. The sky is blue. Markets fluctuate. Readability matters."

Cultural Context

These formulas were developed for English, primarily American English. They don't translate directly to other languages or account for cultural reading habits.

Visual Presentation

How text looks on the page, headings, bullet points, white space, font choice, significantly affects readability. Formulas ignore these factors entirely.

How to Improve Your Readability Scores

Shorten Your Sentences

Long sentences are the biggest readability killer. Aim for an average of 15-20 words per sentence. Break complex ideas into multiple sentences.

Before: "The comprehensive analysis of the quarterly financial reports, which included detailed breakdowns of revenue streams, operating expenses, and profit margins across all regional divisions, revealed significant variations in performance that warrant further investigation by the executive team."

After: "The quarterly financial analysis covered revenue, expenses, and profit margins across all regions. It revealed significant performance variations. The executive team should investigate further."

Use Simpler Words

When a shorter word works, use it.

Break Up Dense Paragraphs

Large blocks of text intimidate readers. Keep paragraphs to 3-4 sentences maximum for web content. Use:

• Bullet points for lists
• Numbered steps for processes
• Subheadings to break up sections
• White space generously

Write in Active Voice

Passive voice adds words and reduces clarity.

Passive: "The report was reviewed by the committee, and recommendations were made."

Active: "The committee reviewed the report and made recommendations."

Read Aloud

The best test for readability? Read your text out loud. If you stumble or run out of breath, your sentences are too long. If words feel awkward, simplify them.

Using Readability Metrics in Practice

Set Appropriate Targets

Different contexts require different targets:

Check While Writing, Not Just After

Many writing tools (including BotWash) can show readability scores in real-time. Use this feedback during drafting, not just during editing.

Don't Optimize Blindly

A perfect readability score shouldn't come at the cost of accuracy, nuance, or your authentic voice. Use scores as a guide, not a goal.

If your sentence is long but clear and necessary, keep it. If technical terminology serves your audience, use it. The goal is communication, not a number.

Compare Before and After

The most useful application of readability metrics is measuring improvement. Run your first draft through a readability analyzer, then revise and compare.

Readability Tools and Resources

Built-in Tools

• Microsoft Word: Shows Flesch-Kincaid scores in the Spelling & Grammar checker
• Google Docs: Use add-ons like "Readable" for analysis
• Hemingway Editor: Free web-based tool focusing on readability

Online Analyzers

• WebFX Readability Test: Tests multiple formulas simultaneously
• Readable.com: Comprehensive analysis with suggestions

BotWash Metrics

BotWash calculates readability metrics for every transformation, showing you exactly how your text changed. The Basic metrics tier (available free) includes:

• Word count, sentence count, paragraph count
• Average sentence and word length
• Flesch-Kincaid grade level and readability level

Pro and Team tiers add AI detection analysis (perplexity, burstiness, and pattern detection) and voice profile analysis.

The Bottom Line

Readability formulas are imperfect but useful. They won't tell you if your content is good, but they'll tell you if it's accessible.

The key insights:

1. Match your audience - Know your readers and target appropriately
2. Shorter is usually better - Both sentences and words
3. Test multiple formulas - No single score tells the whole story
4. Use scores as guides - Not rigid goals
5. Read it out loud - The ultimate readability test

Your readers don't care about your readability score. They care about whether they can understand and use your content. The formulas are just tools to help you get there.

Try a transformation to see your readability metrics in action, or explore all formulas to discover text transformations for any use case.