SMOG Index Calculator

The SMOG Index estimates the years of education a person needs to fully understand a piece of writing on the first read. It is the readability formula most often used in healthcare, patient information, and public-health communications, where comprehension matters more than approximation.

What the SMOG Index measures

SMOG was published in 1969 by Irish-born linguist G. Harry McLaughlin, who wanted a readability formula that predicted full comprehension rather than partial comprehension. Most formulas in use at the time — Flesch, Dale-Chall, Gunning Fog — calibrated against readers who answered 50% to 75% of comprehension questions correctly. McLaughlin argued that for safety-critical writing (medication instructions, equipment manuals, legal warnings) the bar should be 100%.

SMOG counts complex words (three or more syllables) across a passage of at least 30 sentences and produces a US grade level. The formula is deliberately conservative — it tends to score 1 to 2 grades higher than Flesch-Kincaid on the same text, which is why it's preferred when the cost of misunderstanding is high.

Despite the technical pedigree, McLaughlin chose the name as a wink: SMOG officially stands for Simple Measure of Gobbledygook. He published the formula in the Journal of Reading and the name stuck.

How to interpret your SMOG score

For healthcare and patient information, the American Medical Association recommends targeting a SMOG of 6 or below. The National Institutes of Health uses 6 to 8 depending on audience. For general-audience web writing, 7 to 8 is comfortable; above 12, comprehension drops measurably.

When to use the SMOG Index

• Healthcare communication — patient education, consent forms, medication leaflets, hospital signage.
• Public health messaging — vaccine guidance, emergency instructions, screening programs.
• Plain-language compliance — government and regulated industries where comprehension is legally significant.
• Insurance and benefits explanations — Summary of Benefits and Coverage documents.
• Safety instructions — equipment manuals, warning labels, hazard communication.
• Translation source review — checking whether a source document is simple enough to translate accurately.

How the SMOG Index is calculated

Count the complex words (3+ syllables) in your text, multiply by 30 divided by the total sentence count, take the square root, and add 3. The result lands directly on the US grade-level scale.

The original 1969 specification recommends sampling 10 sentences from the start, 10 from the middle, and 10 from the end of a longer document and applying the formula to that 30-sentence sample. Modern calculators (this one included) score the entire passage instead — accurate enough for routine editing, but for audit-grade scoring of long documents the original sampling protocol is still the gold standard.

Because the complex-word count drives the formula linearly under a square root, halving the number of 3+ syllable words drops the SMOG by roughly 1.4 grades. Vocabulary substitution is by far the strongest lever.

SMOG vs other readability formulas

SMOG and Gunning Fog both use complex-word counts but weigh them differently — Fog also factors sentence length, while SMOG isolates vocabulary. As a result, Fog moves more on sentence-length edits; SMOG moves more on word choice.

Compared to Flesch-Kincaid, SMOG typically reads 1 to 2 grades harder on the same passage. This isn't a calibration error — it's the design choice. SMOG predicts 100% comprehension; Flesch-Kincaid predicts the more lenient 50% to 75% threshold the US Navy used. If you're writing medication instructions, prefer SMOG; if you're writing a blog post, Flesch-Kincaid is fine.

Coleman-Liau and ARI both swap syllable counting for character counting. They're more robust on technical text where syllable counters get confused — but they're not the right choice for healthcare, where the SMOG-AMA convention has decades of evidence behind it.

Frequently asked questions