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Readability formulas: what a reading grade does and doesn’t tell you

Flesch Reading Ease, Flesch–Kincaid Grade Level and SMOG estimate how hard English text is to read from the length of its sentences and words. Here is how each works and where each falls short.

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In 30 seconds

  • Flesch–Kincaid and SMOG give a US school grade. Flesch Reading Ease gives a score from about 0 to 100; higher is easier.
  • SMOG tends to give a higher grade than Flesch–Kincaid, so name the formula with every grade.
  • Recommended targets for patient materials: AMA, grade 6 or lower; NIH, grade 8 or lower.
  • Formulas count; they don’t read. Pair one with a criteria-based assessment such as N-PLAT or PEMAT.

Flesch–Kincaid Grade Level

Grade = 0.39 × (words ÷ sentences) + 11.8 × (syllables ÷ words) − 15.59. The result is a US school grade.

Illustrative example

The same instruction, two ways. Each dot is one syllable.

Long words

Medication should be administered daily in conjunction with nourishment.

9 words in 1 sentence · 20 syllables → 0.39 × 9 + 11.8 × (20 ÷ 9) − 15.59 = grade 14.1

Plain words

Take one pill each morning with your breakfast.

8 words in 1 sentence · 10 syllables → 0.39 × 8 + 11.8 × (10 ÷ 8) − 15.59 = grade 2.3

Flesch–Kincaid Grade Level for each sentence, with the recommended targets shaded: AMA, grade 6 or lower; NIH, grade 8 or lower. The formula sees only these counts, not whether the words are familiar.

Flesch Reading Ease

Score = 206.835 − 1.015 × (words ÷ sentences) − 84.6 × (syllables ÷ words). The result runs from about 0 to 100; higher is easier.

Flesch Reading Ease, 0 to 100Higher is easier

The two synthetic sentences from the Flesch–Kincaid figure above, on this scale: long words score 9.7, plain words 93.0.

SMOG

Grade = 1.0430 × √(words of three or more syllables × 30 ÷ sentences) + 3.1291. SMOG was calibrated to full comprehension of the text, so it tends to give a higher grade than Flesch–Kincaid.

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Recommended targets

Health organizations recommend these reading levels for patient materials.

  • AMA: Grade 6 or lower
  • NIH: Grade 8 or lower

What a formula can’t tell you

Formulas count; they don’t read. Short words can still be unfamiliar, and a short sentence can still be unclear. Different formulas give different grades for the same text, so name the formula with the grade. Pair a formula with a criteria-based assessment such as N-PLAT or PEMAT.

How HealthLiteracyCopilot uses them

HealthLiteracyCopilot reports Flesch–Kincaid and SMOG, lets your organization choose the primary measure and target grade, and shows the grade of every version.

Your next step

Paste a public notice. See its reading level by three instruments, the gap to your target and up to three passages to fix first. No account.

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Sources

Questions

Which formula should we use?

Pick one, name it with every grade you report, and use it consistently. SMOG tends to give a higher grade than Flesch–Kincaid.

Is a low grade enough?

No. A formula measures length, not meaning or layout. Use it with a criteria-based assessment.