SMOG Index calculator
Calculate the SMOG readability grade of any text. Paste text below; your SMOG Index score is highlighted alongside the other formulas.
Your scores will appear here. Paste some text above, or use the sample. Everything is calculated in your browser.
The SMOG Index formula
SMOG grade = 1.043 × √( polysyllables × 30 ÷ sentences ) + 3.1291
It combines:
- Number of polysyllabic words (three or more syllables)
- Number of sentences, scaled to a 30-sentence sample
Where it came from
G. Harry McLaughlin published SMOG (Simple Measure of Gobbledygook) in 1969. His original hand method: take 10 sentences from the start, middle and end of a text, count words of 3+ syllables, take the nearest square root and add 3.
When to use it
Health and patient information. Because SMOG was calibrated to predict 100% comprehension rather than partial understanding, it tends to give higher grades than other formulas, and many health bodies recommend it for that reason.
Watch out for
SMOG was designed for samples of 30 sentences. On very short texts the result is less stable, so paste a longer passage when you can.
Reading the grade
| Score | Reader |
|---|---|
| 1–5 | Primary school, age 6–11 |
| 6–8 | Middle school, age 11–14: general public |
| 9–12 | High school, age 14–18 |
| 13–16 | College |
| 17+ | Graduate / professional |
Other formulas
Flesch Reading Ease & Flesch-Kincaid · Gunning Fog Index · Coleman–Liau Index · Automated Readability Index
Questions people ask
What does SMOG stand for?
“Simple Measure of Gobbledygook”, a name chosen by its creator, G. Harry McLaughlin, in 1969.
Why is my SMOG score higher than my Flesch-Kincaid grade?
SMOG predicts the grade needed for complete comprehension, while Flesch-Kincaid was calibrated to a lower comprehension threshold. SMOG results are usually one to two grades higher.
What SMOG grade should patient leaflets have?
Many health organisations recommend grade 6–8 or below, so that most adults, including those with limited literacy, can understand the material.