The SMOG (Simple Measure of Gobbledygook) index is a readability formula developed by G. Harry McLaughlin in 1969. It estimates the years of education needed to understand a text by counting polysyllabic words (words with 3 or more syllables) and sentences. The result is a grade level from 4th grade through college.

The SMOG formula is: Grade = 1.0430 × √(polysyllables × 30 / sentences) + 3.1291. We count words with 3 or more syllables (polysyllables) and the number of sentences in your text, then apply this formula to produce a grade level. The formula works best with at least 30 sentences for accuracy.

SMOG is the preferred readability measure for consumer-oriented healthcare materials. A 2010 study recommended SMOG when evaluating patient-facing content. It correlates highly (0.985) with actual reader comprehension and is widely used by health literacy experts and healthcare organizations.

For general audiences, many organizations aim for 8th grade or lower. The CDC and NIH recommend 6th to 8th grade for patient education materials. However, your target depends on your specific audience—some materials may appropriately use higher grade levels for specialized populations.

SMOG focuses only on polysyllabic words (3+ syllables) and uses a different formula, while Flesch-Kincaid counts all syllables and considers sentence length. SMOG tends to produce slightly higher grade estimates and is preferred for healthcare content. Both are valid—choose based on your industry standards.