Analyze Text Readability — Flesch-Kincaid and More

Readability Analysis

Readability formulas estimate how easy or difficult a text is to read. They use measurable text features — word length, sentence length, syllable count — as proxies for complexity.

Flesch Reading Ease

The most widely used formula, scoring 0-100 (higher = easier):

Score = 206.835 - (1.015 × ASL) - (84.6 × ASW)

ASL = Average Sentence Length (words per sentence)
ASW = Average Syllables per Word

Score interpretation:

Score	Level	Audience
90-100	Very Easy	5th grade
80-89	Easy	6th grade
70-79	Fairly Easy	7th grade
60-69	Standard	8th-9th grade
50-59	Fairly Difficult	10th-12th grade
30-49	Difficult	College
0-29	Very Difficult	College graduate

Flesch-Kincaid Grade Level

Converts the Flesch score to a U.S. grade level:

Grade = (0.39 × ASL) + (11.8 × ASW) - 15.59

A grade level of 8.0 means the text is readable by an average 8th grader.

Gunning Fog Index

Emphasizes complex (3+ syllable) words:

Fog = 0.4 × (ASL + percentage of complex words)

Implementation

function syllableCount(word) {
  word = word.toLowerCase().replace(/[^a-z]/g, "");
  if (word.length <= 3) return 1;
  word = word.replace(/(?:[^laeiouy]es|ed|[^laeiouy]e)$/, "");
  word = word.replace(/^y/, "");
  const matches = word.match(/[aeiouy]{1,2}/g);
  return matches ? matches.length : 1;
}

function fleschReadingEase(text) {
  const sentences = text.split(/[.!?]+/).filter(s => s.trim());
  const words = text.trim().split(/\s+/);
  const totalSyllables = words.reduce((sum, w) => sum + syllableCount(w), 0);

  const asl = words.length / sentences.length;
  const asw = totalSyllables / words.length;

  return 206.835 - (1.015 * asl) - (84.6 * asw);
}

Improving Readability

1. Shorten sentences — aim for 15-20 words per sentence
2. Use simpler words — "use" instead of "utilize", "help" instead of "facilitate"
3. Use active voice — "The team built the app" vs. "The app was built by the team"
4. Break up long paragraphs — 2-4 sentences per paragraph for web content
5. Avoid jargon — or define technical terms when first used

Limitations

Readability formulas have significant limitations. They cannot assess logical coherence, cultural context, or domain knowledge requirements. Technical documentation may score as "difficult" but be perfectly clear to its target audience. These scores are best used as guidelines, not rigid rules.