SMOG estimates the years of education needed to understand text. Widely used in healthcare communication.
Built and maintained by Paul Clark, Redmoon Software · Last verified · Sources
Use SMOG whenever you are writing for health and medical audiences. The National Institutes of Health, the CDC, and most patient-education guidelines recommend SMOG specifically because it correlates with 100% comprehension rather than approximate understanding — critical when readers must follow instructions exactly.
SMOG is the strictest of the polysyllable-based formulas; expect SMOG to land 1–2 grade levels above Flesch–Kincaid on the same text. Compared to Dale–Chall, SMOG ignores familiarity (treats "everyone" and "epistemology" similarly) but is far simpler to compute.
SMOG (Simple Measure of Gobbledygook) was created by G. Harry McLaughlin in 1969 and is the most widely used readability formula for health communication.
The score is the years of education needed to comprehend the text. SMOG 7 means a typical 7th grader should understand it.
Unlike Flesch-Kincaid, SMOG is calibrated for 100% comprehension rather than approximate understanding.
SMOG correlates strongly with comprehension among adults reading medical material. It is the recommended formula by the National Institutes of Health (NIH) for evaluating patient education materials.
The original formula expects exactly 30 sentences. This tool scales the polysyllable count proportionally when your text is longer or shorter.
For consumer health communication, aim for SMOG 6–8.
Input
Patients experiencing persistent symptoms should consult their physician immediately for evaluation.
Output
SMOG Index: 14.2 — College level.
Five polysyllabic words ("experiencing", "persistent", "symptoms", "physician", "immediately", "evaluation") drive the SMOG estimate. SMOG uses √polysyllables, so a few extra long words have a noticeable but bounded effect.
This is worth stating plainly before anything else. Published SMOG is defined over a sample of exactly 30 sentences: ten from the start of a document, ten from the middle, ten from the end. You count the polysyllabic words in that sample, take the square root, and add a constant.
This tool does not ask you to sample. It counts polysyllables across whatever text you paste and normalises to a 30-sentence equivalent — the polysyllable count is scaled by 30 ÷ sentence count before the square root, then run through the same 1.043 and 3.1291 constants. On a document of reasonable length the result tracks the sampled figure closely, which is the point of the normalisation.
On very short text it does not, and cannot. Scaling a handful of sentences up to a 30-sentence equivalent multiplies whatever the polysyllable rate happens to be in those few sentences, so a paragraph with one unusually long word can read several grades harder than the document it came from. If you need a figure defensible against the published method, give it at least 30 sentences.
SMOG is the standard in health literacy, and the reason is a calibration difference rather than a formula difference. Most readability formulas are fitted to roughly 50–75% comprehension of the passage. SMOG was fitted to 100% comprehension — full understanding, not the gist.
That makes SMOG scores run about one to two grades higher than Flesch–Kincaid on the same text, which looks like pessimism and is actually a different question being answered. When the cost of partial comprehension is someone taking a medication wrongly, the stricter target is the right one.
It also means you should not mix the two in a single report without saying which is which. A document at "grade 8 by Flesch–Kincaid" and "grade 10 by SMOG" has not got worse between two lines of a table.
The method behind this tool, so you can check it rather than take it on trust.
McLaughlin, G.H. (1969). “SMOG Grading: A New Readability Formula.” Journal of Reading, 12(8), 639–646.
Designed around a 30-sentence sample and polysyllable counting. It is the formula most often recommended for health material.
Classic 0–100 readability score.
Readability scoresU.S. grade level for your text.
Readability scoresPenalises complex (3+ syllable) words.
Readability scoresNo syllables needed — just characters.