SMOG Index Calculator
A Readability Formula Built Around Complex Words
SMOG — Simple Measure of Gobbledygook — takes a different approach from most readability formulas by focusing almost entirely on polysyllabic words (three or more syllables) rather than balancing sentence length and syllable count together. It was originally developed for evaluating health and safety materials, where correctly identifying the required reading grade level genuinely matters.
The Formula
The result is expressed directly as a U.S. grade level (for example, a SMOG score of 9 suggests text is readable by someone with a 9th-grade reading level). The formula scales the polysyllable count to a standardized 30-sentence sample, which is why SMOG is described as most accurate on samples of 30 or more sentences.
Where This Matters
- Patient education and health materials — SMOG is widely used to verify that health information meets a target reading grade level, since it was designed specifically for this purpose.
- Public safety and legal notices — documents meant for a general audience benefit from a grade-level check before publication.
- Cross-checking another readability score — comparing SMOG against Flesch Reading Ease or Gunning Fog on the same text can highlight whether difficulty comes mainly from long words or long sentences.
Worked Examples
| Polysyllabic words | Sentences | SMOG grade level |
|---|---|---|
| 15 | 30 | 7.17 |
| 25 | 30 | 8.34 |
SMOG is most accurate on samples of 30 or more sentences; shorter samples are scaled up mathematically but may be less reliable.
How to Use This Calculator
- Paste or type your text into the text box (30 or more sentences is recommended for accuracy).
- Select Calculate to see the SMOG grade level along with sentence and polysyllabic word counts.
Related Calculations
Compare against the Gunning Fog Index Calculator and Flesch Reading Ease Calculator for two other widely used readability formulas.