Readability Scores Explained: Flesch, Fog & More

You've finished writing an article, a landing page, or an email. It sounds good to you — but will your audience actually understand it? Readability scores give you an objective, formula-based answer. This guide covers the five most widely used readability formulas, how they work, and how to use them to improve your writing.

What Are Readability Scores?

Readability scores are numerical measurements that estimate how easy or difficult a piece of text is to read. They analyze surface-level features — sentence length, word length, syllable count, or character count — and produce a score or grade level.

These scores matter because readers who struggle with your text will leave. Research consistently shows that content matched to its audience's reading level gets higher engagement, lower bounce rates, and better conversion. The average American adult reads at a 7th–8th grade level, and most successful online content is written at or below that threshold.

Readability scores are used by:

• Content marketers — to ensure blog posts and landing pages are accessible
• Technical writers — to calibrate documentation for the target audience
• Educators — to match materials to student reading levels
• Healthcare professionals — to verify patient-facing documents are understandable
• Legal teams — to simplify contracts and disclosures for consumers

Flesch Reading Ease

Developed by Rudolf Flesch in 1948, this is the most widely recognized readability formula. It produces a score from 0 to 100, where higher scores mean easier reading.

Formula: 206.835 − 1.015 × (total words / total sentences) − 84.6 × (total syllables / total words)

The formula penalizes long sentences and multi-syllable words. Here's how to interpret the score:

For example, Harry Potter scores around 80 (easy), The New York Times around 50–55 (fairly difficult), and a typical insurance policy around 20–30 (very confusing).

Flesch-Kincaid Grade Level

Created by J. Peter Kincaid for the U.S. Navy, this formula uses the same inputs as Flesch Reading Ease but outputs a U.S. grade level instead of a 0–100 score.

Formula: 0.39 × (total words / total sentences) + 11.8 × (total syllables / total words) − 15.59

A result of 8.2 means an 8th grader should be able to understand the text. This is the formula used by Microsoft Word's built-in readability checker and is widely adopted in government and military communications. Most federal agencies require public-facing documents to score at or below an 8th-grade level. For web content, aim for grade 6–8.

Gunning Fog Index

Robert Gunning developed this index in 1952 to help newspapers and business writers identify overly complex prose. It focuses on "complex words" — words with three or more syllables.

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

The definition of "complex words" excludes proper nouns, familiar compound words (like "newspaper"), and common suffixes (-es, -ed, -ing) that push words to three syllables. The result approximates the years of formal education needed. A Fog Index of 12 means a high school senior should understand the text. Ideal scores for general writing are 7–8. Scores above 12 suggest the text is too complex for a general audience.

Coleman-Liau Index

Unlike the previous formulas, the Coleman-Liau Index counts characters instead of syllables. This makes it easier to compute programmatically, since counting characters is trivial while counting syllables requires language-specific rules.

Formula: 0.0588 × L − 0.296 × S − 15.8
where L = average number of letters per 100 words, S = average number of sentences per 100 words

The result is a U.S. grade level. The Coleman-Liau Index is popular in software implementations because it avoids the notoriously difficult problem of syllable counting in English (is "fire" one syllable or two? Is "caramel" two or three?). It correlates well with syllable-based formulas for most text types but can diverge for text with many short, technical abbreviations.

Automated Readability Index (ARI)

The ARI is another character-based formula, designed in 1967 for real-time readability monitoring on typewriters. It uses characters per word and words per sentence.

Formula: 4.71 × (characters / words) + 0.5 × (words / sentences) − 21.43

Like Flesch-Kincaid and Coleman-Liau, the result is a grade level. ARI tends to produce slightly higher grade levels than other formulas for the same text. Because it relies only on character and word counts, it's extremely fast to compute and is still used in military and industrial readability assessments.

Which Score Should You Use?

Each formula has strengths and trade-offs. Here's a side-by-side comparison:

Our recommendation: Use at least two formulas and average them. If you're writing web content, Flesch Reading Ease gives the most intuitive score. If you need a grade level for compliance or education, use Flesch-Kincaid. For programmatic analysis at scale, Coleman-Liau or ARI are easiest to implement accurately.

How to Improve Your Readability Score

Readability is a skill you can practice. Here are ten actionable techniques:

1. Shorten your sentences. Aim for 15–20 words per sentence on average. Break long sentences at natural pauses.
2. Replace multi-syllable words. Use "help" instead of "facilitate", "use" instead of "utilize", "start" instead of "commence".
3. Use active voice. "The team completed the project" is clearer than "The project was completed by the team".
4. Cut unnecessary adverbs. "Very important" can just be "critical" or "essential".
5. Use bullet points and lists. They break complex information into scannable chunks.
6. Add subheadings. Readers scan before they read. Descriptive subheadings let them find what they need.
7. Explain jargon. If you must use a technical term, define it immediately after.
8. Write one idea per paragraph. Short paragraphs (2–4 sentences) are easier to process than dense blocks.
9. Read your text aloud. If you stumble or run out of breath, the sentence is too long.
10. Use a readability tool. Test your writing, revise, and test again. Iteration is the fastest path to clear prose.

Common Readability Myths

• "Lower grade level is always better." Not true. If your audience is a group of physicians, writing at a 5th-grade level may lack the precision they expect. Match the score to your reader.
• "Readability = dumbing down." Simplifying language doesn't mean simplifying ideas. You can explain quantum mechanics in short sentences — you just need more of them.
• "Readability scores measure quality." They measure surface complexity only. A readability score can't tell you if your argument is logical, your facts are correct, or your writing is engaging.
• "One formula is enough." Different formulas weight different factors. Using two or three formulas gives a more balanced picture.
• "Short words are always clearer." "Use" is clearer than "utilize", but "photosynthesis" is clearer than any attempt to avoid it in a biology text. Context matters more than word length.