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SMOG Index

The readability standard for health and safety information.

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Published by G. Harry McLaughlin in 1969 and validated against full comprehension, which is why healthcare bodies specify it. It needs at least thirty sentences.

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SMOG grade for full comprehension

Your longest sentences

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Quick Tool Overview

The SMOG Index is the readability formula healthcare organisations actually use, because it was validated against full comprehension rather than partial. That is why it returns a grade one to two higher than Flesch-Kincaid on the same text, and why health literacy guidance specifies it.

What it does
Calculates the SMOG readability grade
You give it
At least thirty sentences
You get back
A grade for full comprehension, plus the long words
Best for
Patient information, safety notices and public health material
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About the SMOG Index

SMOG — Simple Measure of Gobbledygook — was published by G. Harry McLaughlin in 1969 and is the formula healthcare organisations actually use. It was validated against 100 per cent comprehension, meaning a reader at the indicated grade should understand everything, where other formulas target 50 to 75 per cent.

That stricter target is why SMOG typically returns a grade level one to two higher than Flesch-Kincaid on the same text, and why health literacy guidance specifies it. The formula counts polysyllabic words — three or more syllables — across thirty sentences, takes the square root and adds three. It needs at least thirty sentences to be used as designed, and this tool says so plainly when your text is shorter rather than returning a number that looks authoritative and is not.

Key features of the SMOG Index

Full-comprehension standard

Validated at 100 per cent understanding, where other formulas target 50 to 75 per cent.

Sample size enforced

Below thirty sentences the tool warns you rather than returning an authoritative-looking number.

Polysyllabic words listed

They are the entire input to the formula, so seeing them is seeing the score.

Health guidance built in

The grade 6 to 8 target that patient-facing material is expected to meet.

Explains the gap

Why SMOG reads higher than Flesch-Kincaid, so a difference is not mistaken for an error.

How to use the SMOG Index

  1. 1

    Paste your text

    Thirty sentences or more, which is what the formula requires.

  2. 2

    Read the SMOG grade

    The grade level at which a reader should understand everything, not just most of it.

  3. 3

    Check the sentence count

    Below thirty sentences the tool warns you, because the result is not reliable there.

  4. 4

    Review the polysyllabic words

    Every word of three or more syllables is listed, since those are the entire input.

Frequently asked questions

Because it targets full comprehension rather than partial. Flesch-Kincaid and its relatives were validated at 50 to 75 per cent comprehension; SMOG was validated at 100 per cent, so it sets a higher bar for the same text.

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