SMOG Index Explained: How It Works and When to Use It
What the SMOG formula counts, where it came from, what its grade means, and why health writers use it. Taken from the 1969 paper and later research.
SMOG is one of the simplest readability formulas, and one of the strictest. This post explains how it works, using the original 1969 paper and later research.
Where it came from
G. Harry McLaughlin published SMOG in the Journal of Reading in 1969. He was an associate professor of communications at Syracuse University. The paper says he named it "in tribute to Gunning's Fog Index."
He built the formula from 390 passages in the McCall-Crabbs Standard Test Lessons in Reading, a general school reading test. The paper is not about medicine. Health researchers adopted SMOG later.
How to calculate it
McLaughlin's method:
- Take 30 sentences: 10 near the start, 10 in the middle and 10 near the end.
- Count every word of three or more syllables. Count repeats too.
- Take the square root of that count and add 3.
That is the simple version. The full version from his regression is:
Grade = 1.0430 × √(polysyllables × 30 ÷ sentences) + 3.1291
A quick example. A 30-sentence sample with 27 long words scores 1.0430 × √27 + 3.1291 = 8.5. Double the long words to 54 and the score is 10.8. Because of the square root, each added long word raises the score a little less than the one before.
What the grade means
McLaughlin defined the SMOG grade as "the reading grade that a person must have reached if he is to understand fully the text assessed." Other formulas of the time used a lower bar, such as answering half or three quarters of test questions correctly.
That is why SMOG runs higher than other formulas on the same text. It is not harsher on long words. It is aiming at a stricter target.
It is also an estimate. McLaughlin reported that the formula predicts the grade "within one and a half grades in 68 percent of cases."
SMOG compared with Flesch-Kincaid
| SMOG | Flesch-Kincaid | |
|---|---|---|
| Published | 1969 | 1975 |
| Counts | Words of 3+ syllables | Every syllable |
| Sentence length | Through a fixed 30-sentence sample | Words per sentence |
| Built on | School reading-test passages | US Navy training manuals |
| Target | Full comprehension | A lower comprehension bar |
Both formulas respond to long words and long sentences. In SMOG, a six-syllable word counts once, the same as a three-syllable word. In Flesch-Kincaid, every syllable counts.
A study of 148 patient handouts shows the usual gap. SMOG averaged grade 9.6. Flesch-Kincaid averaged 6.5 on the same documents.
Why health writers use it
Two studies recommend it. A 2013 review of formulas on health information concluded that SMOG "appears best suited for health care applications." A 2010 study of online Parkinson's disease information also recommended SMOG for consumer health material.
The reasoning is practical. If a patient misreads a dose, the cost is high, so a formula set to full comprehension is the cautious choice.
No regulator requires SMOG. Our post on healthcare readability rules lists what the regulations and FDA guidance say.
Limits to know
- It needs 30 sentences. McLaughlin found that 30-sentence samples "gave vastly more accurate predictions than smaller samples." A score for one paragraph is a rough guide.
- Tools scale short text. This site's SMOG calculator uses the simple "3 + square root" version and scales the count up when you have fewer than 30 sentences. Treat short-text results as estimates.
- Splitting sentences lowers the score. More sentences in the same text means fewer long words per sentence.
- It only sees word length. It cannot tell a familiar long word from a rare one.
How to use it
- Score at least 30 sentences if you can.
- Compare the result with the target your organisation has set.
- If SMOG is far above Flesch-Kincaid, look at your long words first. Our table of shorter substitutes can help.
- Test the text with real readers. A formula predicts. It does not confirm.
For help choosing between formulas, see which readability formula to use.
How we checked
The history, method, formula and quotes come from McLaughlin's 1969 paper, which we read in full. An earlier version of this post described SMOG as built for medical text and gave example scores that did not reproduce. Both have been corrected.
Frequently asked questions
What does the SMOG index measure?
SMOG counts the words of three or more syllables in a sample of 30 sentences. It takes the square root of that count and adds 3. The result is the US school grade a reader needs to understand the text fully.
Why is SMOG usually higher than Flesch-Kincaid?
SMOG is set to full comprehension, a stricter standard than other formulas use. In one study of patient handouts, SMOG averaged grade 9.6 and Flesch-Kincaid averaged 6.5 on the same documents.
Is SMOG required for health materials?
No. No US federal rule names a readability formula. Two health-literacy studies recommend SMOG for patient materials, so many health writers choose it.