SMOG readability calculator
For patient and public information, aim for grade 6–8. The formula was designed for samples of at least 30 sentences, so short texts give rougher estimates.
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SMOG Index
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Formula: 1.043 × √(polysyllables × 30 ÷ sentences) + 3.1291
- Sentences
- 1
- Words with 3+ syllables
- 0
How to read the SMOG score
| Score | Meaning |
|---|---|
| 5 or below | Very easy: understood by most 10–11 year olds |
| 6–8 | Plain English: the target for email, web copy and public information |
| 9–10 | Fairly difficult: fine for engaged professional readers |
| 11–12 | Difficult: typical of specialist reports |
| 13–16 | College level: academic and technical writing |
| 17+ | Graduate level: legal and scientific text |
Lower scores mean easier text.
Where the SMOG Index comes from
Published by G. Harry McLaughlin in 1969 (“SMOG Grading: a New Readability Formula”). It predicts the grade needed for full comprehension, and is widely recommended for health materials, for example by the UK’s Chartered Institute of Editing and Proofreading and many public health agencies.
Other readability 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)
- Gunning Fog Index: 0.4 × [(words ÷ sentences) + 100 × (complex words ÷ words)]
- Coleman–Liau Index: 0.0588 × L − 0.296 × S − 15.8 (L = letters per 100 words, S = sentences per 100 words)
- Automated Readability Index: 4.71 × (characters ÷ words) + 0.5 × (words ÷ sentences) − 21.43
Questions people ask
Why is SMOG used for health information?
It predicts the level needed for full comprehension rather than partial understanding, and it correlates well with comprehension tests. Health agencies prefer the stricter standard for instructions people must follow correctly.
How many sentences do I need for SMOG?
McLaughlin’s method uses 30 sentences (10 from the start, middle and end). The formula scales shorter samples to 30, but under about 10 sentences the result is only a rough guide.