SMOG Readability Calculator

SMOG index: 1.043·√(30·polysyllables/sentences) + 3.1291

G. Harry McLaughlin introduced the SMOG (Simple Measure of Gobbledygook) in 1969, pitching it as a steadier readability metric than Flesch for healthcare and legal writing. What it counts is polysyllables, meaning words with 3 or more syllables, across a sample of 30 sentences (10 pulled from the start, 10 from the middle, 10 from the end). The formula is grade = 1.043·√(30·polysyllables/sentences) + 3.1291, and the number it spits out is the US school grade needed to fully understand the text. Say you find 20 polysyllables across 30 sentences: 1.043·√20 + 3.1291 ≈ 7.8, roughly 8th grade. The healthcare rule of thumb is that patient-facing material should land at grade 6 or below.

Applications

It shows up when teams need to simplify medical language in patient leaflets, informed-consent forms and drug labels, where the FDA/NIH ask for SMOG ≤ 6. You'll also see it in public health messaging, in plain-language audits of insurance contracts, and in checks on teaching materials for adult literacy programs.

FAQ

Why exactly 30 sentences? That's the sample size McLaughlin validated against, and it keeps the polysyllable count from bouncing around. Anything shorter pushes the variance up, so for fewer than 30 sentences you're better off with the conversion table.

What counts as a polysyllable? Any word carrying 3 or more syllables, inflected forms included, so "running" is 2 and "completing" is 3. Proper nouns and numbers written as digits are usually left out.

SMOG vs Flesch-Kincaid? The two set different bars: SMOG targets full comprehension, while Flesch-Kincaid settles for 50–75%. On the same text SMOG usually comes out about 2 grades higher, which is part of why healthcare tends to favor it.

Does it work in Portuguese? Not really. SMOG was calibrated for English, so for Portuguese you want adapted indices like Flesch BR or the Índice Gulpease adaptado. Run raw SMOG on PT-BR and it will read the text as easier than it is, since Portuguese tends to use more polysyllables.