SMOG Index
Score readability from polysyllable words and sentence count.
About SMOG Index
SMOG counts words of three or more syllables and related them to sentence count, leaning heavily on difficult vocabulary. It is often the recommended index for health and safety text because of its scoring at higher grade levels. SMOG tends to grade a little more strictly than other readability measures, making it a cautious choice wherever clarity is a legal or safety concern.
Formula
SMOG Index
SMOG = 1.043 x sqrt(polysyllables x 30/sentences) + 3.1291
- polysyllable words
- Words with three or more syllables
Worked example
With the default values below, SMOG Index returns SMOG Grade of 13.0. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Sentencesminimum 1
- 10
- Polysyllable wordsminimum 0
- 30
Results
- SMOG Grade
- 13.0
- SMOG Band
- Demanding text
- Words per Sentence
- 30.0
Step by step
- 1Values used
- 2SMOG Index
- 3SMOG Grade= 13.0
- 4SMOG Band= Demanding text
- 5Words per Sentence= 30.0
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is SMOG Index calculated?
SMOG = 1.043 x sqrt(polysyllables x 30/sentences) + 3.1291 SMOG counts words of three or more syllables and related them to sentence count, leaning heavily on difficult vocabulary. It is often the recommended index for health and safety text because of its scoring at higher grade levels.
Why does SMOG Index matter?
SMOG tends to grade a little more strictly than other readability measures, making it a cautious choice wherever clarity is a legal or safety concern.
What values do I need to enter?
This calculator takes 2 inputs: Sentences, Polysyllable words. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.