SMOG Readability Calculator (SMOG Index Grade)

SMOG readability calculator

SMOG predicts the school grade needed to fully understand a text. Because it targets complete comprehension, it usually returns a grade or two higher than Flesch-Kincaid for the same passage, and it is widely recommended for patient information and public-health materials.

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SMOG Index

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Readability scores for your text
Formula	Score	Reads as
Flesch Reading Ease	–
Flesch-Kincaid Grade	–	–
Gunning Fog	–	–
SMOG	–	– *
Coleman-Liau	–	–
Automated Readability	–	–
Dale-Chall	–

Formula

Score

Reads as

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Coleman-Liau

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Automated Readability

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Dale-Chall

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The SMOG Index formula

SMOG = 1.0430 × √(polysyllables × 30 ÷ sentences) + 3.1291

Words of three or more syllables, scaled to a 30-sentence sample

What it counts

Only polysyllables: words with three or more syllables, including repeats. McLaughlin’s hand method counts them in 30 sentences (10 from the start, middle and end). We use the regression form, which scales your count to 30 sentences.

What score to aim for

Health-literacy guidance commonly asks for SMOG grade 8 or below for patient materials.

Limits to keep in mind

SMOG was designed for at least 30 sentences. On a short paragraph the scaling exaggerates each long word, so treat results on fewer than 30 sentences as rough.

Where it comes from

G. Harry McLaughlin published SMOG (“Simple Measure of Gobbledygook”) in 1969. It was built to predict 100% comprehension, stricter than other formulas that target 50–75%.

Source: McLaughlin, G. H. (1969). “SMOG grading: a new readability formula.” Journal of Reading, 12(8), 639–646.

How we count syllables, sentences and words, and how our numbers compare on test passages, is documented in Readability formulas explained.

SMOG Index: common questions

How many sentences does SMOG need?

The formula was built on 30-sentence samples. It still returns a number for shorter text by scaling, but the estimate becomes unstable below 30 sentences.

Why is SMOG used for health information?

It predicts the grade for full comprehension rather than partial understanding, and it is quick to calculate by hand. Many public-health and patient-education guides recommend it.

What is the simple SMOG formula?

McLaughlin’s shortcut: count polysyllables in 30 sentences, take the square root of the nearest perfect square, and add 3. The regression form used here (1.0430 × √(count × 30/sentences) + 3.1291) is the precise version.

Why is my SMOG grade higher than Flesch-Kincaid?

SMOG aims for 100% comprehension and weighs long words heavily, so it is usually one to two grades higher. That is expected.