What Is Flesch–Kincaid Readability?

Flesch–Kincaid readability refers to two related formulas that estimate how hard an English text is to read. The Flesch Reading Ease score gives a number usually between 0 and 100, where higher is easier. The Flesch–Kincaid Grade Level converts the same inputs into a US school grade. Both rely only on average sentence length and average syllables per word.

Origin

Rudolf Flesch, an Austrian-born writing consultant in the United States, published the Reading Ease formula in 1948 in the Journal of Applied Psychology as part of his campaign for plain English. In 1975, J. Peter Kincaid and colleagues recalculated it as a grade-level formula for the US Navy, testing it on enlisted personnel reading technical training material. The grade-level version became a US military standard and later a default in word processors.

The formulas

Flesch–Kincaid Grade Level:

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

Flesch Reading Ease:

206.835 − 1.015 × (words / sentences) − 84.6 × (syllables / words)

Longer sentences and longer words both push the grade up and the ease score down. Word length counts for much more per unit: one extra syllable per word on average adds 11.8 grade levels, while one extra word per sentence adds only 0.39.

How to read the scores

A worked example

You can score your own text with the readability checker.

Why it matters for vocabulary learners

Readability scores are a quick way to choose reading material. For learners, the ideal text is one where you already know almost all the words, so you can understand it and pick up the few new ones from context. A text that scores far above your level will usually contain too many unknown words for that.

The formulas also show why vocabulary matters in writing: much of what makes text "hard" is long, Latin-derived words. Learning advanced words is valuable for reading; choosing simpler ones is often better for writing. See how to improve vocabulary for writing.

Limits and common misconceptions

• Syllables are a proxy, not a measure of familiarity. Ennui and terse are short; basketball and everybody are long. Frequency-based measures such as the Lexile measure handle this somewhat better.
• Meaning and structure are ignored. Jumbled sentences get the same score as clear ones.
• Grade levels are US-specific and designed for native readers, not for second-language learners at CEFR levels.
• Writing to the formula can backfire. Chopping sentences and swapping precise terms for vague ones raises the score without making text clearer.
• Different tools give different numbers, mostly because they count syllables and sentences differently.

Sources: Flesch, R. (1948). A new readability yardstick. Journal of Applied Psychology. Kincaid, J. P., Fishburne, R. P., Rogers, R. L., & Chissom, B. S. (1975). Derivation of new readability formulas for Navy enlisted personnel. Naval Technical Training Command.