The Flesch-Kincaid formulas
The Flesch-Kincaid family is two related formulas built from three counts: total words, total sentences, and total syllables.
| Metric | Formula | Output |
|---|---|---|
| Flesch Reading Ease | 206.835 − 1.015 × (W/S) − 84.6 × (Syl/W) | 0–100 (higher = easier) |
| Flesch-Kincaid Grade | 0.39 × (W/S) + 11.8 × (Syl/W) − 15.59 | U.S. school grade |
W = words, S = sentences, Syl = syllables. Both reward two things: shorter sentences (a smaller words-per-sentence ratio) and simpler words (fewer syllables per word). That is the whole game — everything you do to improve readability moves one of those two ratios.
Reading Ease bands
| Score | Difficulty | Audience |
|---|---|---|
| 90–100 | Very easy | 5th grade |
| 80–90 | Easy | 6th grade |
| 70–80 | Fairly easy | 7th grade |
| 60–70 | Plain English | 8th–9th grade |
| 50–60 | Fairly difficult | 10th–12th grade |
| 30–50 | Difficult | College |
| 0–30 | Very difficult | College graduate |
Bands follow Flesch's original interpretation. Source: Flesch-Kincaid readability tests.
A worked example
Take "The cat sat on the mat. The dog ran fast." That is 10 words, 2 sentences, 10 syllables — so W/S = 5 and Syl/W = 1. Reading Ease = 206.835 − 1.015 × 5 − 84.6 × 1 = 117.2 (capped at 100-ish — very easy). The grade comes out negative, which we report as 0: a young child could read it. Now stack three subordinate clauses into one 40-word sentence with words like "notwithstanding" and watch both numbers collapse.
Frequently asked questions
What is the Flesch-Kincaid formula?
Flesch Reading Ease = 206.835 − 1.015 × (words ÷ sentences) − 84.6 × (syllables ÷ words). Flesch-Kincaid Grade Level = 0.39 × (words ÷ sentences) + 11.8 × (syllables ÷ words) − 15.59.
What is a good Flesch Reading Ease score?
90–100 is very easy (5th grade), 60–70 is plain English (8th–9th grade), 30–50 is difficult (college), below 30 is very difficult. For most web and business writing, target 60 or above.
Why do different calculators give slightly different scores?
The formulas are fixed, but syllable counting and sentence splitting are not — English has no perfect syllable algorithm. Small counting differences produce small score differences. ClearPen uses a documented heuristic and shows several formulas so you can read the consensus.