Most readability tools surface a single grade-level number — usually Flesch–Kincaid. The honest answer is that no single formula is definitive, and the differences between formulas are often more informative than the average.
What "grade level" means
A grade-level score predicts the US school grade at which a typical reader could understand the passage on first read. A score of 8.0 means an eighth-grader (~age 13) should follow it without difficulty. The number is a ceiling, not a floor — a passage written at grade 8 is also accessible to grade 12 readers.
The six common readability formulas
Flesch–Kincaid Grade Level
The most-cited grade-level formula. Same inputs as Flesch Reading Ease but expressed as a US grade.
0.39 × (words/sentences) + 11.8 × (syllables/words) − 15.59Gunning Fog Index
Targets complex words — three or more syllables. Designed by Robert Gunning for business writing.
0.4 × (words/sentences + 100 × complex_words/words)SMOG Index
"Simple Measure of Gobbledygook" — counts polysyllabic words across 30 sentences. Designed for healthcare and patient-education writing where misunderstanding has consequences.
1.0430 × √(complex_words × 30/sentences) + 3.1291Automated Readability Index (ARI)
Uses character count instead of syllable count.
4.71 × (chars/words) + 0.5 × (words/sentences) − 21.43Coleman–Liau Index
Also character-based but with different coefficients.
0.0588 × (chars/words × 100) − 0.296 × (sentences/words × 100) − 15.8Dale–Chall
Compares your text to a fixed list of 3,000 words a 4th-grader knows. Words outside the list are "hard". Most reading-research scholars consider this the gold standard for fiction and educational materials.
Why formulas disagree — and what to do about it
Run the same passage through all six and you'll see a 2–3 grade range. The disagreement is informative: when ARI and Coleman–Liau disagree with the others, your text has unusually long or short words. When Gunning Fog and SMOG outscore the rest, polysyllabic vocabulary is the bottleneck.
Try it
The Word Counter shows all six scores at once. Expand "All readability scores" below the editor.
