SMOG readability formula

The SMOG readability formula calculates the U.S. grade level of a text sample based on sentence length and number of complex words (i.e., words that contain three or more syllables).

The McLaughlin SMOG is generally appropriate for secondary age (4th grade to college level) readers. SMOG tests for 100% comprehension, whereas most formulas test for around 50%-75% comprehension.

Note that numerals are fully syllabized (i.e., sounded out) for this test, so Readability Studio always overrides your numeral syllabication setting when calculating it.

The SMOG Formula

GL = ROUND(SQRT(W) )+ 3

Where:

GL	U.S. grade level
W	Number of complex words (3+ syllables)

This test requires a 30-sentence sample; however, note that Readability Studio always analyzes your entire document to guarantee the most accurate results and does not use subsamples for any of its test calculations. If a formula requires a subsample of a specific size, then normalization is used.

Note that this test is often referred to as an acronym for Simple Measure of Gobbledygook, although the origin of its name is actually a nod to Robert Gunning's Fog index.

Citations

McLaughlin, H. (1969). 'SMOG grading - a new readability formula', Journal of Reading, 22, 639-646.