SMOG Readability Formula: How to Calculate and Interpret the Score

The SMOG readability formula estimates the education level a reader may need to understand a piece of writing. It is especially useful for patient education, public information, safety instructions, and other content where misunderstanding can create real risk.

The short version: SMOG looks at polysyllabic words—words with three or more syllables—and turns that count into an estimated grade level. If a draft uses many long, technical, or unfamiliar terms, SMOG will usually be stricter than some other readability formulas.

If you just need the result for a draft, paste your text into the SMOG Index Calculator. If you want to compare SMOG with Flesch Reading Ease and Flesch-Kincaid Grade Level, use the main Flesch-Kincaid calculator or the Readability Score Checker.

#What the SMOG formula measures

SMOG stands for Simple Measure of Gobbledygook. The name is playful, but the use case is practical: it helps writers spot text that may be too demanding for readers because it contains too many long words.

SMOG focuses on:

• the number of sentences in the sample
• the number of polysyllabic words in those sentences
• the grade level implied by that density of long words

A polysyllabic word is usually a word with three or more syllables. Examples include “medication,” “authorization,” “communication,” “emergency,” and “implementation.” These words are not automatically bad. Many are necessary. SMOG simply asks whether the draft is asking readers to process too many of them at once.

#The SMOG readability formula

A common version of the SMOG formula is:

 1SMOG grade = 1.043 × √(polysyllable count × 30 ÷ sentence count) + 3.1291

Where:

• polysyllable count = number of words with three or more syllables
• sentence count = number of sentences in the sample
• 30 standardizes the result to a 30-sentence sample

The original SMOG method is often described with a 30-sentence sample: 10 sentences from the beginning, 10 from the middle, and 10 from the end of the text. For shorter web copy, tools usually apply the formula to the available text, but very short samples can be unstable.

#A worked SMOG example

Use this short sample:

Patients should call the clinic if symptoms continue. A nurse can review medication instructions and explain emergency warning signs.

First, count the sentences:

• Sentence count: 2

Now count the polysyllabic words. Depending on syllable rules, likely candidates include:

• patients
• symptoms
• continue
• medication
• instructions
• emergency

That gives a polysyllable count of 6.

Put those values into the formula:

 1SMOG grade = 1.043 × √(6 × 30 ÷ 2) + 3.1291
 2SMOG grade = 1.043 × √90 + 3.1291
 3SMOG grade = 1.043 × 9.49 + 3.1291
 4SMOG grade ≈ 13.0

That result may look high for two plain-looking sentences. This is why SMOG is useful: it notices that a short healthcare sample can still contain several words with three or more syllables. It is also why you should avoid overreacting to a tiny sample. A longer passage gives the formula more context.

#How to interpret a SMOG score

SMOG returns an estimated grade level. Lower usually means easier. Higher usually means more demanding.

These ranges are guides, not guarantees. A reader’s background, stress level, motivation, health literacy, and familiarity with the topic all matter.

For patient-facing or high-stakes public content, many teams try to keep material around grade 6-8 when accuracy allows. Technical, legal, or specialist content may reasonably score higher, but even expert readers benefit from clear structure.

#Why SMOG can score higher than Flesch-Kincaid

SMOG often feels stricter than Flesch-Kincaid because it reacts strongly to clusters of long words.

Flesch-Kincaid Grade Level uses sentence length and syllables per word. SMOG is built around polysyllable count. That difference matters when a draft has controlled sentence length but still contains dense vocabulary.

Compare these two sentences:

Your medication authorization requires documentation before administration.

We need paperwork before we can give the medicine.

The first sentence is short, but it packs in several long words. SMOG will notice that burden. The second sentence is not childish. It simply uses words more readers can process quickly.

That does not mean SMOG is always “better.” It means SMOG is useful when vocabulary burden is the risk you care about most. For broader writing checks, compare SMOG with Flesch-Kincaid Grade Level, Flesch Reading Ease, and other formulas.

#When to use SMOG

Use SMOG when the reader needs to understand the content correctly the first time.

Good use cases include:

• patient education handouts
• discharge instructions
• public health pages
• medication or safety guidance
• government service instructions
• nonprofit and community education materials
• school or training materials where grade level matters

SMOG is especially helpful when a draft contains necessary technical terms. It pushes you to decide which terms must stay, which terms can be replaced, and which terms need a quick explanation.

#When SMOG is less useful

SMOG is not a full writing-quality score. It can overstate difficulty when long words are familiar to the audience, and it can miss other problems that are not about word length.

Do not rely on SMOG alone when:

• the sample is very short
• the audience already knows the technical vocabulary
• layout, visuals, or step order are the main source of confusion
• the text uses many short but unfamiliar terms
• accuracy requires specific terminology

A low SMOG score also does not prove the writing is good. The content can still be vague, incomplete, poorly organized, or inaccurate.

#How to improve a high SMOG score without losing meaning

Start with vocabulary, but do not delete precision.

#Replace inflated words first

Some long words are just formal padding.

Use the simpler word when it keeps the meaning. Keep the technical word when it is the accurate term readers need.

#Define necessary terms near the first mention

Bad:

Complete the preauthorization protocol before administration.

Clearer:

Complete the preauthorization steps before giving the medicine. Preauthorization means the insurer approves the treatment before it starts.

The clearer version still contains a long word, but it helps the reader immediately.

#Break clusters of long words

A sentence with one hard term may be fine. A sentence with five hard terms can overwhelm readers.

Before:

The organization implemented comprehensive communication procedures to improve medication reconciliation documentation.

After:

The team changed how staff explain medicine changes. The new process also improves the records used to compare a patient’s medicines.

The revised version keeps the idea but reduces the pile-up of long abstract words.

#Use examples when the term must stay

Sometimes the best fix is not replacing the word. It is adding a concrete example.

Watch for symptoms of infection, such as fever, swelling, redness, or pain that gets worse.

“Infection” is necessary. The examples make the warning easier to act on.

#A practical SMOG editing workflow

Use this process when clarity matters:

1. Run the draft through the SMOG Index Calculator.
2. Find sections with many three-syllable words close together.
3. Mark which long words are necessary terms.
4. Replace inflated words that do not add precision.
5. Define unavoidable terms in plain language.
6. Split long or crowded sentences.
7. Recheck the score and read the passage out loud.

If the SMOG grade drops and the text still says the right thing, the edit helped. If the score drops because you removed important detail, revise again.

#Bottom line

The SMOG formula is a strong signal for vocabulary-heavy difficulty. It is not trying to judge style or expertise. It is asking a narrower question: are there enough long words in this text that many readers may need a higher reading level?

Use SMOG when your content needs to be clear, safe, and easy to act on. Then compare it with the Flesch-Kincaid calculator so you can see whether both sentence structure and vocabulary are moving in the right direction.