SMOG, short for Simple Measure of Gobbledygook, estimates the school grade level needed to understand a text. It uses a single ingredient: polysyllabic words, words with three or more syllables. That focus makes it especially useful for spotting texts weighed down by long words.
Step 1: sample 30 sentences
Take 10 consecutive sentences from the start of the text, 10 from the middle, and 10 from the end, 30 in total. Skip headings, lists, and captions. If the text has fewer than 30 sentences, use all of them and adjust with the general formula below. Three spread-out samples stop an unusually dense paragraph from skewing the score.
Step 2: count polysyllables
Count every word with three or more syllables in those 30 sentences. Count repeated words each time they appear. Skip proper nouns in the original instructions, and count hyphenated words by their spoken syllables. Example words: “information” (4), “education” (4), “computer” (3), “beautiful” (3). Short words like “the” and “reading” (2) do not count.
Counting syllables takes practice. Clap or tap each vowel sound: “unbelievable” is un-be-liev-a-ble, five syllables. Silent final letters do not add syllables, and neither do most suffixes like -ed when they do not add a sound.
Step 3: apply the formula
The precise formula: SMOG grade = 1.0430 x square root of (polysyllables x 30 / sentences) + 3.1291. For a standard 30-sentence sample this simplifies to 1.0430 x square root of polysyllables + 3.1291.
Worked example: 42 polysyllables in 30 sentences. Square root of 42 is about 6.48. SMOG = 1.0430 x 6.48 + 3.1291 = 6.76 + 3.13 = 9.89, about grade 10. The quick mental version, square root rounded plus 3, gives 6 + 3 = 9, close enough for a rough check.
Reading and lowering the score
SMOG grades map to US school grades: grade 6-8 is comfortable general reading, grade 10 is fairly difficult, grade 12 and above is difficult, and grade 16-plus is graduate level. For web content aimed at the public, grade 8 or below is a good target.
To lower a SMOG score, replace polysyllabic words with shorter ones first, since SMOG counts only word length: “utilise” to “use”, “approximately” to “about”, “facilitate” to “help”. Shortening sentences alone does not move SMOG much, unlike Flesch, which is exactly why using both scores together gives a fuller picture.
SMOG versus Fry and Flesch: which to use
SMOG is the score most often required for health and education materials, because it is sensitive to long words, which are the main barrier in medical and instructional texts. If a guideline specifies a reading level, it often specifies SMOG by name.
Fry suits school texts and textbook passages, since it balances sentence length with syllable counts. Flesch suits web content and short articles, since it works on short samples and gives an ease score as well as a grade. Using the score your audience’s guidelines name matters more than picking the most flattering one.
A final counting tip: syllable counting is the main source of SMOG disagreement between two people scoring the same text. Agree the rules before scoring: count every word with three or more spoken syllables, count repeats, skip proper nouns, and treat hyphenated compounds by their spoken form. Two scorers using the same rules usually land within one grade of each other, which is close enough for editorial decisions.
Try it faster with the tool
You can follow every step above by hand, but the free SMOG Readability Score Calculator does the arithmetic for you. Enter your numbers and it gives the answer right away, so you can check your hand calculation or skip straight to the result.
Quick tips
- Sample 30 sentences: 10 start, 10 middle, 10 end.
- Count words with 3+ syllables, including repeats.
- SMOG grade = 1.043 x square root of count + 3.13.