Enter the number of complex words and total sentences into the calculator to determine the Smog Readability Index. This calculator can also evaluate any of the variables given the others are known.

Smog Readability Formula

The following formula is used to calculate the Smog Readability Index.

SRI = 1.0430 * sqrt((30 * (complex words/total sentences)) + 3.1291


  • SRI is the Smog Readability Index Complex words are words with three or more syllables Total sentences is the total number of sentences in the text

To calculate the Smog Readability Index, first calculate the ratio of complex words to total sentences. Multiply this ratio by 30. Then, take the square root of the result. Multiply this by 1.0430 and add 3.1291 to get the final Smog Readability Index.

What is a Smog Readability?

Smog Readability is a formula that estimates the years of education a person needs to understand a piece of written text. It was developed by G. Harry McLaughlin in 1969 and is based on the number of polysyllabic words (words with three or more syllables) in a text. The higher the Smog Index, the more complex the text is, and the more education a person would need to understand it. It is commonly used to assess the readability of English texts.