SMOG Index Calculator
The SMOG Index estimates the US school grade a person needs to fully understand a piece of writing. It is often recommended for healthcare, patient information and public-health writing.
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What the SMOG Index measures
SMOG was published in 1969 by G. Harry McLaughlin, an associate professor of communications at Syracuse University (“SMOG Grading: A New Readability Formula”, Journal of Reading). He set it to complete comprehension. He built it on 390 passages from the McCall-Crabbs school reading lessons, not on medical text.
McLaughlin's method counts every word of three or more syllables in 30 sentences. This calculator scales the count to 30 sentences, so it also gives a score for shorter or longer text. It does not count a word whose third syllable comes only from -ing, -ed or -es.
McLaughlin wrote that he named SMOG "in tribute to Gunning's Fog Index". The name is usually expanded as Simple Measure of Gobbledygook.
How to interpret your SMOG score
| SMOG | Label on this calculator |
|---|---|
| Up to 5 | Elementary |
| 6–8 | Middle school |
| 9–12 | High school |
| 13–16 | College |
| 17+ | Graduate |
Reports in the medical literature attribute a 6th-grade target for patient materials to the American Medical Association and an 8th-grade target to the National Institutes of Health. No law sets a SMOG grade.
When to use the SMOG Index
- Healthcare communication — patient education, consent forms, medication leaflets.
- Public health messaging — vaccine guidance, emergency instructions, screening programs.
- Safety instructions — equipment manuals, warning labels.
How the SMOG Index is calculated
Count the complex words, multiply by 30 divided by the number of sentences, take the square root, and add 3.
McLaughlin's paper samples 10 sentences from the start, 10 from the middle and 10 from the end of a longer document. This calculator scores the whole passage instead.
Because the complex-word count sits under a square root, the effect of an edit depends on where you start. In 30 sentences, going from 30 complex words to 15 lowers SMOG from 8.5 to 6.9. Going from 10 to 5 lowers it from 6.2 to 5.2.
SMOG vs other readability formulas
SMOG and Gunning Fog both count words of three or more syllables. Fog adds average sentence length directly. SMOG counts complex words per sentence, so it also falls when you split long sentences.
Compared with Flesch-Kincaid, SMOG usually gives a higher grade on this calculator. SMOG is set to complete comprehension. The Navy set Flesch-Kincaid to a 35% cloze score, which Kincaid and colleagues say is comparable to about 75% on multiple-choice questions.
Coleman-Liau and ARI count characters instead of syllables. For health care, a 2013 review of readability formulas found SMOG best suited.
Frequently asked questions
What is the SMOG formula?
SMOG = 3 + the square root of (complex words × 30 ÷ sentences). A complex word has three or more syllables. The result is a US grade level, set to complete comprehension.
What is a good SMOG score?
For patient materials, the level most often cited is 6th grade, a target attributed to the American Medical Association. No law sets a SMOG score. For other writing, pick the grade your readers need.
Why does SMOG need at least 30 sentences?
McLaughlin found that 30-sentence samples gave far more accurate predictions than smaller ones. His method takes 10 sentences from the start, 10 from the middle and 10 from the end. With fewer sentences, a single long word swings the score.
Is SMOG more accurate than Flesch-Kincaid?
For healthcare and patient information, SMOG is the more cautious choice. It is set to complete comprehension, a stricter standard than Flesch-Kincaid uses, so it usually gives a higher grade. For general writing, either formula works. Use one consistently.
What does SMOG stand for?
SMOG is usually expanded as "Simple Measure of Gobbledygook." G. Harry McLaughlin introduced the formula in 1969 and wrote that he named it "in tribute to Gunning's Fog Index."
How do I lower my SMOG score?
Replace words of three or more syllables with shorter ones, and split long sentences. SMOG counts complex words per sentence, so both edits lower it.