Why does SMOG need thirty sentences?

Because the index was validated on samples of thirty sentences and its grade table assumes that size. With fewer, each extra polysyllabic word moves the result sharply, so the tool warns and reports the sentence count rather than hiding the uncertainty.

Does SMOG use sentence length?

No. The formula counts only polysyllables and sentences; it never measures how long the sentences are. That is why a text of very long one-syllable sentences can score easier here than it actually reads.

Why does SMOG often score higher than other formulas?

Because it responds only to words of three or more syllables and treats every one as difficulty, with no exclusion for familiar vocabulary or proper nouns. A passage with ordinary sentence lengths but some specialist terms will score higher on SMOG than on Flesch.

What is SMOG used for?

Health-literacy and public-information work, where a reader may encounter a document once and needs to understand it at first reading. It is also used in research on informed consent and patient materials, which is where its validation sample comes from.

Can I use SMOG on a short text?

You can, and the tool will compute it, but the result is indicative only. Below thirty sentences each additional polysyllabic word shifts the index substantially, and the warning on the page says so rather than presenting the number as reliable.

SMOG Index Calculator

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SMOG index

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SMOG to grade

SMOG	Reading grade
3 - 5	4th grade
6	5th grade
7	6th grade
8	7th grade
9	8th grade
10	9th grade
11	10th grade
12	11th grade
13	12th grade
14 - 18	College

The SMOG Index Calculator computes the Simple Measure of Gobbledygook from your polysyllable count, shows the formula's arithmetic step by step, and warns you when the sample is smaller than the thirty sentences it was validated on.

What is SMOG Index Calculator?

SMOG stands for Simple Measure of Gobbledygook, and it earns its place because it needs almost nothing: a count of words with three or more syllables and a count of sentences. The SMOG Index Calculator applies it, prints the arithmetic, and is unusually careful about the sample size, because SMOG has the strictest validation requirement of any common readability formula.

The formula is 1.043 times the square root of the polysyllable count multiplied by thirty divided by the sentence count, plus 3.1291. The thirty is not incidental: the index was validated on samples of thirty sentences, and the published grade table assumes that sampling. The calculator therefore uses the whole text when you paste thirty sentences or more and warns when you have fewer, because with five sentences each additional polysyllabic word swings the result by roughly half a grade.

SMOG counts only polysyllables. It ignores sentence length altogether, which is both its strength and its blind spot. The strength is that it does not need a word list or a syllable dictionary beyond a simple three-syllable test, which is why health-literacy research adopted it for patient materials. The blind spot is that a text of long, single-syllable sentences will score easier here than it reads, since nothing in the formula notices how far the reader travels before a full stop.

The grade conversion is usually presented as a table rather than a formula, and this tool shows the published mapping from a SMOG value to a reading grade, running from early primary school up to college. The bands are displayed beside the score so the index can be read without a lookup.

Like every formula here, SMOG inherits the error of a heuristic syllable count, which the page documents: vowel groups counted, silent final e dropped, -ed and -es discounted unless needed, and a short list of exceptions for hiatus words. A word the heuristic misjudges by a syllable can move a short sample noticeably, which is another reason the sample-size warning is prominent.

The tool attaches no target. Health and government plain-language work often aims for a SMOG value near 8 or below, and a clinical guideline for specialists will legitimately sit much higher. Read the index, read the grade, check the sentence count, and decide against the audience rather than against a number you were told to reach.

SMOG readability is popular outside linguistics for a practical reason: it needs only a count of polysyllabic words and a sentence count, with no word list, no dictionary and no special software. That makes it easy to apply consistently across a large set of documents, which is how health agencies use it to check that patient information sits at a readable level. The trade is precision, because it ignores sentence length and treats every polysyllabic word as equally hard. As a result it can judge a passage of simple vocabulary in sprawling sentences as easier than it is. A sample drawn per the thirty-sentence convention will be more stable than the whole text, and the counts on the page let you check which was used. Read the index as one measure among several.

How to use SMOG Index Calculator

Paste your text, ideally thirty sentences or more so the sample matches the published validation.
Read the SMOG index and the grade label, then the polysyllable count and share beside them.
Check the arithmetic line, which shows the exact numbers substituted into the formula.
Read the sample-size warning if it appears, and weight the index accordingly.
Use the SMOG-to-grade table to place the score in a reading grade before deciding whether to revise.

When to use SMOG Index Calculator vs related tools

Use SMOG when you want a vocabulary-only difficulty estimate and the sample is long enough to trust it, which makes it the usual choice for patient and public information. When sentence length should also count, the Gunning Fog Index Calculator uses a related complex-word rule but includes sentence length.

To see SMOG alongside Flesch, Fog and the letter-based formulas for the same passage, the Readability Grade Scorer puts every index on one page, backed by the same syllable counts this calculator uses.

Privacy & Security

This tool runs entirely in your browser — no data ever leaves your device. There is no server round-trip, no upload, no logging, and no account required. Your input is processed locally using client-side JavaScript and is never stored, transmitted, or accessible to anyone else. When you close the tab, everything disappears.