SMOG — "Simple Measure of Gobbledygook" — is a grade-level readability formula introduced by G. Harry McLaughlin in 1969. It estimates the years of education needed to understand a text based purely on the count of polysyllabic words (three or more syllables) across a sample of 30 sentences. The full formula is:
grade = 1.043 × √(polysyllabic words × 30 / sentences) + 3.1291
The classic hand method is even simpler: take 10 sentences from the start, 10 from the middle, and 10 from the end (30 total), count every word of three or more syllables, take the square root of that count, and add 3. Worked example: if a 30-sentence sample contains 36 polysyllabic words, √36 = 6, so the SMOG grade is about 6 + 3 = 9.
SMOG is the standard in healthcare and patient-education writing precisely because it is conservative — for the same text it typically returns a grade one to two levels higher than Flesch–Kincaid. That built-in caution is a feature when the cost of a reader misunderstanding is high, such as medication instructions or consent forms, where guidance often calls for a grade 6 target. A key nuance: SMOG was calibrated for 100% comprehension, whereas Flesch–Kincaid targets around 50–75%, which explains the gap — they are not "wrong" relative to each other, just calibrated to different bars. Because it depends only on long-word density, very short texts (fewer than 30 sentences) give unstable results; use a full sample or repeat and average. Compare it against Gunning Fog, which also keys on polysyllabic words, for a cross-check.