SMOG Index calculator
SMOG counts only one thing: words of three or more syllables, scaled to a 30-sentence sample. The result is the US school grade needed to fully understand the text.
Paste text to see all readability scores update live.
The SMOG Index formula
1.043 × √(polysyllables × 30 ÷ sentences) + 3.1291
Because it targets 100% comprehension rather than the 50–75% used by other formulas, SMOG usually gives a grade one or two levels higher. That is why many health-literacy guides recommend it.
G. Harry McLaughlin published SMOG (“Simple Measure of Gobbledygook”) in 1969. It is often preferred for patient and health materials because it predicts full comprehension.
What your score means
| Score | Reading level |
|---|---|
| ≤6 | Suitable for most patient leaflets |
| 7–8 | General public information |
| 9–12 | High-school level |
| 13–16 | College level |
| 17+ | Graduate level |
Drag the inputs, watch the score
The formula only “sees” these numbers. Change them and notice which one moves the score most.
Grade level (higher = harder)
6.7
US grade 7
Only polysyllables matter here – sentence length affects SMOG only through how many sentences the words are spread over.
Questions people ask
How many sentences does SMOG need?
The original method takes 10 sentences from the start, 10 from the middle and 10 from the end of a text. The calculator scales whatever you paste to a 30-sentence equivalent, but results under about 10 sentences are unreliable.
Why is SMOG used for health information?
It was validated against 100% comprehension and correlates well with tested reading comprehension, so it errs on the side of caution for readers who must follow instructions correctly.
Does sentence length affect SMOG?
Only indirectly. The formula counts polysyllabic words per sentence, so the same words spread over more sentences lower the score.