SMOG Index calculator

Paste your text to get its SMOG grade — the school grade a reader needs to fully understand it — with the polysyllable count and working shown. SMOG is the go-to formula for health and patient information. Free, and your text stays in your browser.

Your text

Checked in your browser — your text isn’t sent anywhere or saved.

Paste or type some text to see its readability scores.

The SMOG Index formula

SMOG grade = 1.043 × √(polysyllables × 30 ÷ sentences) + 3.1291

Polysyllables — every word of three or more syllables — no exceptions for names or suffixes.
× 30 ÷ sentences — scales the count to a 30-sentence sample, the size the formula was built for.
The result — a US school grade. Because of the + 3.1291, SMOG never scores below about grade 3.

Worked example

Here is the SMOG Index worked through for our sample bedtime story — press Try a sample story above to load it. It’s a short tale about an owl who can’t sleep, and it counts up like this:

Sentences: 18
Words of 3+ syllables: 1

1.043 × √(1 × 30 ÷ 18) + 3.1291 = 4.5

So the story scores 4.5 — Grade 5 · ages 10–11.

What your score means

Score	Level	Who reads it easily
1 or below	Kindergarten–1st grade	Ages 5–7 · picture books, first readers
2–3	2nd–3rd grade	Ages 7–9 · early chapter books
4–5	4th–5th grade	Ages 9–11 · confident young readers
6–8	Middle school	Ages 11–14 · most adults read this comfortably
9–12	High school	Ages 14–18 · news features, business writing
13–16	College	Academic and specialist writing
17+	Graduate	Dense professional or scientific prose

What is a good SMOG score?

For patient leaflets, public-health notices, and consent forms, health-literacy guidance commonly recommends a 6th–8th grade reading level — so a SMOG grade of 8 or below.

Expect SMOG to read a grade or two higher than Flesch-Kincaid for the same text. That’s by design: it predicts the level needed for complete understanding rather than partial understanding, which is why health writers like its caution.

Where it comes from

G. Harry McLaughlin published SMOG in 1969 in the Journal of Reading as a quicker, more accurate alternative to existing formulas. The name — Simple Measure of Gobbledygook — is a nod to Gunning’s Fog Index.

The original hand method is famously quick: take ten sentences from the start, ten from the middle, and ten from the end; count the words of three or more syllables; take the square root of the nearest perfect square; and add three.

Limitations

SMOG needs a decent sample. It was calibrated on 30 sentences, and shorter texts are scaled up to match, so a single paragraph can swing by several grades.

It also bottoms out around grade 3, which makes it unhelpful for picture books and early readers — use the Flesch-Kincaid Grade Level or the reading level checker for those.

How to calculate SMOG with the formula

Count the sentences in your text and the polysyllables — words of three or more syllables, including names and repeated words. Multiply the polysyllables by 30 and divide by the number of sentences, which scales your count to a 30-sentence sample.

Take the square root of that, multiply by 1.043, and add 3.1291. With no long words at all you get 3.1; with one long word in every sentence you get about 8.8, or grade 9.

Why SMOG scores higher than Flesch-Kincaid

SMOG looks only at long words per sentence — short words don’t count at all, and the square root means the first few long words move it the most. Flesch-Kincaid averages syllables across every word, so a handful of long words barely registers.

Our sample bedtime story shows the effect: it has one three-syllable word in 18 sentences. Scaled to 30 sentences and rooted, that gives a SMOG grade of 4.5 — against 1.1 on Flesch-Kincaid for the same text.

Frequently asked questions

What does SMOG stand for?

Simple Measure of Gobbledygook. G. Harry McLaughlin chose the name in 1969 as a nod to Robert Gunning’s earlier Fog Index.

Why is SMOG used for health information?

It predicts the grade needed for full comprehension, so it errs on the side of caution, and studies have found it consistent when checking healthcare materials. Many health-literacy guides recommend it for that reason.

How many sentences does SMOG need?

The formula was designed for a 30-sentence sample. This calculator scales shorter texts to match, but results for fewer than 30 sentences are less reliable.

Why can’t my SMOG score go below 3?

The formula adds 3.1291 to every result. Even text with no three-syllable words at all scores about 3.1, so SMOG can’t distinguish between very easy texts.

Is SMOG better than Flesch-Kincaid?

Not better, just stricter. SMOG aims at full comprehension and suits health and safety information where misunderstanding matters. Flesch-Kincaid is the more common general-purpose grade and works better for very simple text.