SMOG Index Calculator

How it is calculated

The tool counts sentences and words with three or more syllables (polysyllables), then applies grade = 1.0430 × √(polysyllables × 30 ÷ sentences) + 3.1291. The factor 30 ÷ sentences scales the count up or down to a 30-sentence sample, which is what the original SMOG method assumes.

Syllables are estimated from the spelling: each run of the vowels a, e, i, o, u, y counts as one, minus one for a final silent e. Only the letters a to z are looked at, so the tool is meant for English text; accented letters and digits are ignored. A sentence is counted for every run of ., ! or ?, so abbreviations such as Dr. and decimals such as 3.5 each add a sentence. The estimate often overcounts words like every or minutes, and short samples give rough results.

Common mistakes

• Pasting a very short passage and treating the grade as exact. SMOG is built for about 30 sentences, and a few sentences make the result jump around.
• Pasting text in a language other than English. Only the letters a to z are examined, so the syllable count is meaningless for Turkish, German and other languages.
• Leaving in headings, bullet labels, dates or abbreviations such as Dr. and e.g. Each full stop is counted as a sentence end, which lowers the grade.
• Reading 15.58 as an age. It is a school grade level, not an age.
• Rewriting purely to lower the number, for instance by chopping sentences. Splitting sentences lowers the score without necessarily making the text clearer.