Readability
Readability refers to how easily a piece of text can be read and understood, typically measured by formulas based on sentence length and word complexity.
Definition¶
Readability is a measure of how easy a piece of text is to read and understand. In formal measurement, readability is calculated by formulas that use sentence length and word length (syllable count) as proxies for comprehension difficulty. In plain language, readable text uses clear vocabulary, appropriately varied sentence structure, and logical organization.
Common Readability Formulas¶
Flesch Reading Ease — the most widely used formula. Scores run from 0 to 100; higher scores are easier to read. A score of 60–70 is considered appropriate for general web content. The formula: 206.835 − (1.015 × average sentence length) − (84.6 × average syllables per word).
Flesch-Kincaid Grade Level — converts the same inputs into a US school grade level. A grade level of 8 means the text is readable by an average 8th grader.
Gunning Fog Index — uses a slightly different calculation; a fog index under 12 is generally recommended for web writing.
SMOG — commonly used for health communications; estimates the years of education needed to understand a text.
Readability and AI Writing¶
AI-generated text consistently scores lower on readability metrics than comparable human-written content on the same topic. The main drivers are uniform sentence length (which reduces burstiness and increases the Flesch penalty for consistent word counts) and nominalization — the conversion of verbs into nouns ("implementation" instead of "implement") which increases average syllable count.
Improving readability in AI text and improving its human-like quality are largely the same task: shorter sentences, more active verbs, varied structure.
Optimal Readability by Audience¶
| Audience | Target Flesch Score | Target Grade Level |
|---|---|---|
| General web readers | 60–70 | 7–9 |
| Business professionals | 50–65 | 9–12 |
| Academic / technical | 30–50 | College |
| Healthcare patients | 60–70+ | 6–8 |