Readability scores appear in everything from word processors to SEO tools, but most people never see what's actually being calculated. The formulas are older than you might expect — developed in the mid-20th century — and simpler than the term "formula" suggests. Here's what's actually happening behind the number.
Nearly all readability formulas, including Flesch, boil a text down to two measurements: average sentence length (words per sentence) and average word complexity (usually approximated by syllables per word). The theory is straightforward — longer sentences and longer words both take more effort to process, so texts with more of both score as harder to read.
This score runs from 0 to 100, where higher means easier to read. The formula is:
The constants (206.835, 1.015, 84.6) were derived by testing the formula against real texts with known comprehension difficulty, then adjusting the weights until the formula's output matched human-judged difficulty ratings reasonably well.
| Score | Reading level |
|---|---|
| 90-100 | Very easy — 5th grade |
| 70-89 | Easy — 6th-7th grade |
| 60-69 | Standard — 8th-9th grade |
| 50-59 | Fairly difficult — 10th-12th grade |
| 30-49 | Difficult — college level |
| 0-29 | Very difficult — college graduate |
This is a rescaled version of the same underlying measurements, designed to output a U.S. school grade level instead of a 0-100 score:
A result of 8.0 means the text is roughly readable by an eighth grader. This version is used more often in professional, government, and technical writing contexts, since "8th grade level" is a more intuitive target than "Flesch score of 64" for most writers.
These formulas are useful precisely because they're simple and consistent — but that simplicity means they measure surface-level complexity, not actual comprehension difficulty. A formula can't tell whether your vocabulary is jargon-heavy, whether your logic is hard to follow, or whether your sentence structure is confusing despite being technically short. A short, choppy sentence with unclear pronoun references can score "easy" while genuinely confusing a reader.
General audience web content typically aims for a Flesch score in the 60-70 range (roughly 8th-9th grade). Academic or technical writing for an expert audience can reasonably run lower without being a problem — the formula doesn't know your readers already understand the jargon. The right target depends entirely on who's reading, not on chasing the highest possible score.