The SMOG, or Simple Measure of Gobbledygook, grade estimates how hard a piece of English text is to read by counting its polysyllabic words. It is widely used in healthcare and public communication because it directly answers the question “what grade level can fully understand this on one reading?”
How it works
SMOG counts every word with three or more syllables — its polysyllables — across the text and relates that count to the number of sentences. The formula is:
SMOG Grade = 1.0430 × sqrt(polysyllables × (30 / sentences)) + 3.1291When the text has about thirty sentences this is the exact SMOG score. For shorter passages the tool normalises the polysyllable count to a thirty-sentence basis and marks the result as an estimate. Syllables are counted with a vowel- group heuristic, the same practical approach used for other English readability measures.
Why SMOG is preferred over Flesch-Kincaid for health content
Two formulas dominate readability measurement in English. Flesch-Kincaid blends average sentence length and average syllables per word, giving a score that responds to both how long your sentences run and how complex your vocabulary is. SMOG uses only polysyllabic word counts. The original research by McLaughlin (1969) showed that focusing purely on long words — the single strongest predictor of comprehension difficulty — made SMOG more accurate at estimating the grade level needed for full, first-reading comprehension of expository prose.
That is the key distinction: SMOG predicts complete comprehension on one reading, not just rough understanding. This makes it the standard readability measure in healthcare communication in the UK and the United States, where patient leaflets and consent forms need to be understood first time, in a clinical setting, often by anxious readers.
Example: before and after
High SMOG sentence: “The practitioner will evaluate your cardiovascular medication and provide recommendations appropriate to your individual circumstances.” (polysyllabic words: practitioner, evaluate, cardiovascular, medication, recommendations, appropriate, individual, circumstances — 8 in one sentence)
Rewritten for lower SMOG: “Your doctor will review your heart medicine and give you advice.” (polysyllabic words: none — 0 in one sentence)
The second sentence conveys the same meaning at a much lower reading level.
Targets and practical guidance
Many UK NHS patient information resources and US health literacy guidelines aim for a SMOG Grade of 6 to 8, which corresponds to approximately ages 11 to 14. Academic journal abstracts often land at grade 14 or above. Legal and insurance documents frequently score 16 or higher — which is one reason consumers struggle to engage with them.
To lower a SMOG Grade, focus on vocabulary first: swap utilise for use, assistance for help, demonstrate for show. Sentence splitting helps Flesch-Kincaid more than SMOG; for SMOG, vocabulary is the primary lever. After rewriting, paste into this tool to confirm the grade moved in the direction you expected.