The SMOG index: the readability formula health writers actually trust

If you write a discharge leaflet, a medication label, or a consent form, "most people will probably get the gist" is not good enough. A patient who half-understands when to take a pill is a patient at risk. That is the exact problem the SMOG index was built to measure — and it is why, of all the readability formulas, SMOG is the one healthcare communicators reach for first.

G. Harry McLaughlin published SMOG — Simple Measure of Gobbledygook — in 1969, after concluding that the popular formulas of the day were too lenient for materials where misunderstanding has consequences. The SMOG index calculator returns a US school-grade estimate, just like Gunning Fog or Flesch–Kincaid, but it is built and calibrated differently. This piece explains how the score is produced, what makes it stricter, and how to read it correctly.

The one thing SMOG measures

Most readability formulas combine two signals: sentence length and word difficulty. SMOG is unusual in leaning almost entirely on a single variable — the number of polysyllabic words, meaning words of three or more syllables. McLaughlin's research found that polysyllable count alone predicted comprehension difficulty remarkably well, so SMOG concentrates its firepower there.

That focus matters when you read your score. A SMOG result is essentially telling you: your long words are too dense for this audience. It will not reward you for short sentences the way Gunning Fog does — sentence length enters the formula only indirectly, through how the polysyllable count is normalised. If you want to lower a SMOG grade, the lever is almost always the same one: replace heavy words with plain ones.

How the score is calculated

The formula this calculator uses is McLaughlin's regression equation:

1.043 × √( polysyllables × 30 / sentences ) + 3.1291

In words: count every word of three or more syllables, scale that count to a 30-sentence equivalent, take the square root, multiply by 1.043, and add 3.1291. The square root is what gives SMOG its characteristic shape — difficulty climbs fast at first and then more slowly, so piling on a few extra long words hurts a short, simple text more than it hurts an already-dense one.

One honest detail about this implementation. The textbook SMOG procedure asks you to pull a sample of exactly 30 sentences — ten from the start, ten from the middle, ten from the end — and count polysyllables in that fixed sample. This calculator does not make you do that. It counts polysyllables across your whole text and uses the 30 / sentences factor to scale the result to the 30-sentence basis the formula expects. The practical upshot: you can paste any length of text and get a sensible number, but the score is most reliable on passages of roughly 30 sentences or more, because that is the sample size McLaughlin validated against. On a two-sentence snippet, the scaling has very little to work with and the grade should be treated as a rough hint, not gospel.

What counts as a "polysyllable" here — and what doesn't

This is the most important difference between SMOG and the Gunning Fog Index, and it is easy to miss. Fog's "complex word" rule is forgiving: it throws out words that only hit three syllables through a common -es, -ed, or -ing ending, and it ignores hyphenated compounds. SMOG, as implemented here, does none of that. Every word of three or more syllables counts, full stop — "deciding", "processes", and "well-meaning" all tally toward your polysyllable total even though Gunning Fog would wave them through.

That is faithful to McLaughlin's original method, which counted polysyllables raw, and it is part of why SMOG runs stricter. It also means the two formulas will disagree on the same text, and the disagreement is informative: if your SMOG grade is well above your Fog grade, your difficulty is coming from ordinary long words and inflected verbs, not from rare jargon. Knowing that tells you what to cut.

Why SMOG reads a grade or two higher

Run the same paragraph through SMOG and through Flesch–Kincaid and SMOG will usually come back one to two grades higher. This is not a bug; it is the whole point. SMOG is calibrated to 100% comprehension — the grade at which a typical reader understands the text completely — whereas most formulas are tuned to around 50–75% comprehension, the level at which a reader gets the general sense. For a magazine article, "gets the general sense" is fine. For dosage instructions, it is not.

So when you compare formulas, do not assume the lower number is the "right" one. They are answering different questions. If your goal is that nearly everyone in the audience fully understands every sentence — the standard for medical, legal-rights, and safety material — SMOG's stricter grade is the one to honour.

Reading and acting on your score

The calculator maps the raw number into the same grade bands as the rest of the readability suite: grade 6 and under is plain and easy, grade 6–9 is the sweet spot for a general audience, and anything drifting into the teens is high-school-to-college register. Public-health guidance commonly targets grade 6 to 8 for patient-facing material, precisely because SMOG's full-comprehension calibration means hitting grade 8 here is a genuinely accessible document.

To bring a high SMOG grade down, work on the polysyllables directly. "Administer" becomes "give"; "discontinue" becomes "stop"; "physician" becomes "doctor"; "approximately" becomes "about". Each swap removes a three-syllable word from the count, and because of the square root, the early swaps move the needle most. Splitting sentences helps too — more sentences lowers the polysyllables / sentences ratio — but the durable win is plainer vocabulary.

Where SMOG sits in the toolkit

SMOG is the right primary measure for health, medical, safety, and patient-consent writing, where full comprehension is the bar. For business prose and journalism, the Gunning Fog Index — which specifically targets jargon density — is often the better fit, and for a quick, intuitive single number on general copy, Flesch Reading Ease remains the familiar choice. Because each formula weights things differently, the safest read on any important document is to look at several at once: a readability consensus averages SMOG alongside the others so one formula's quirk does not mislead you. (SMOG is a measure of text difficulty only; it is general information about readability, not medical, legal, or professional advice.)

When you need to know whether your writing is genuinely safe for the people who have to act on it, paste it into the SMOG index calculator. It will count your polysyllables, return the grade, and tell you in plain terms how close you are to a level your whole audience can fully understand — which, for the documents SMOG was built for, is the only standard that counts.