📖Readability Metrics Explained

Flesch Reading Ease, Gunning Fog Index, sentence analysis, and reading time — understand the science behind readability scoring.

Hugo Team·March 16, 2026

readabilityfleschgunning fogreading easesentence lengthpremium

Readability affects both user engagement and SEO.[3] Content that's too complex loses readers; content that's too simple may lack depth. Hugo's Readability module uses established linguistic formulas to score your content clarity. This Premium check adds 8% weight to your overall score.

Flesch Reading Ease

The Flesch Reading Ease score is the most widely used readability metric. Developed by Rudolf Flesch in 1948, it uses sentence length and syllable count to estimate reading difficulty on a 0–100 scale.[1]

text

Score = 206.835 − 1.015 × (words ÷ sentences) − 84.6 × (syllables ÷ words)

Flesch Reading Ease Scale

Very Hard

Difficult

Standard

Easy

Very Easy

0–2930–4950–6970–8990–100

Score	Difficulty	Audience
90–100	Very Easy	5th grade — easily understood by 11-year-olds
80–89	Easy	6th grade — conversational English
70–79	Fairly Easy	7th grade — suitable for most web content
60–69	Standard	8th–9th grade — ideal for general audiences
50–59	Fairly Difficult	10th–12th grade — college prep level
30–49	Difficult	College level — academic papers
0–29	Very Difficult	College graduate — professional/technical

Flesch Reading Ease (score)

Good

60 or higher

Warning

40–59

Poor

Below 40

For web content, aim for 60–70. Blog posts and marketing copy should target 70+. Technical documentation can be lower but should ideally stay above 40.[1]

Gunning Fog Index

The Gunning Fog Index estimates the years of formal education needed to understand your text on first reading. Created by Robert Gunning in 1952, it focuses on sentence length and complex words (3+ syllables).[2]

text

Fog Index = 0.4 × ((words ÷ sentences) + 100 × (complex words ÷ words))

Gunning Fog Index (grade level)

Good

8 or less (easy) / 9–12 (ideal for web)

Warning

13–15 (complex)

Poor

Above 15 (very complex)

A Fog Index of 12 means your text requires 12 years of education to understand. For web content, aim for 8–12.[2]

Sentence & Paragraph Analysis

Beyond the formulas, Hugo analyzes your writing at the structural level:

Very long sentences (40+ words) are flagged individually.[4]
If more than 30% of sentences exceed 25 words, you get a warning.[4]
Paragraphs over 150 words are flagged as too long for web reading.[4]
Complex word ratio (words with 3+ syllables) is calculated — over 25% triggers a warning.

Reading Time

Hugo estimates reading time based on an average reading speed of 230 words per minute.[5] This is displayed as an informational metric — there's no pass/fail threshold, but it helps you gauge whether your content length is appropriate for your audience.

💡Minimum Content

Readability analysis requires at least 50 words of visible text. Pages with fewer than 50 words receive a default score of 50.

References

[1]Flesch, R. (1948). "A new readability yardstick." Journal of Applied Psychology, 32(3), 221–233. — psycnet.apa.org
[2]Gunning, R. (1952). "The Technique of Clear Writing." McGraw-Hill.
[3]Google Search Central — Creating helpful, reliable, people-first content — developers.google.com
[4]Federal Plain Language Guidelines — Write short sentences — plainlanguage.gov
[5]Brysbaert, M. (2019). "How many words do we read per minute? A review and meta-analysis of reading rate." Journal of Memory and Language, 109, 104047. — doi.org