SMOG Readability Formula: Your Tool for Clearer Health Communication

As health communicators, we want as many people as possible to understand our writing. When making or editing materials, we can use the Simplified Measure of Gobbledygook (SMOG) to achieve this goal. The SMOG gives you an estimate of how many years of education someone would need to understand your writing. Use it as a quick source of info to help your work reach more people.

What is the SMOG?

The SMOG is a readability formula developed in the 1960s. It uses the number of polysyllabic words (words with more than 2 syllables) to provide an estimate of a document’s reading grade level.

When should I use the SMOG?

You can use the SMOG in many situations. Use it to highlight readability issues as you draft materials. Use it to compare materials you might use for your audience. Use it to assess how clearly your organization communicates. For general audiences, a common gol is an 8^th grade reading level or lower.

Why should I use the SMOG?

Compared to other readability formulas, the SMOG is quick and simple. It doesn’t require counting every word or every syllable in a document. It is an easy source of quantitative data to help you communicate more clearly.

How do I use the SMOG for long texts (at least 30 sentences)?

Follow the steps below to use the SMOG on a document at least 30 sentences long:

Step 1: Choose 30 sentences.

Pick a passage of 10 sentences at the start of the document, 10 in the middle, and 10 at the end. Here are some tips:

• Copy and paste the sentences into a new computer document or underline them in a print document.
• Periods, question marks, and exclamation points mark the ends of sentences in formal documents.
• A line break might mark the ends of sentences in less formal documents.

Step 2: Count every word with more than 2 syllables.

In your 30-sentence sample, count every polysyllabic word. Here are some tips:

• Highlight the words so it’s easier to keep track of them.
• Hyphenated words count as a single word.
• Include numbers in this count. Treat them as if they were written out in text.
• Include proper nouns in this count.
• Include abbreviations as if they were spelled out completely. You might make exceptions for abbreviations that your audience treats as words, e.g. COVID or TV.

Step 3: Use the SMOG formula.

For a simplified version of the SMOG, take the square root of polysyllabic word count from Step 2. Then add 3 to the square root to get the estimated reading grade level.

For example, let’s say we counted 26 polysyllabic words Step 2. The square root of 26 is about 5.1, and then we add 3 to get the estimated reading grade level. This total is 8.1, which means we might still want to edit our text to meet the common 8^th grade benchmark.

For a more precise version of the SMOG, use our SMOG calculator spreadsheet.

How do I use the SMOG for shorter texts (less than 30 sentences)?

The SMOG formula is the same for documents less than 30 sentences long. The only difference is that you will act as if you had enough sentences to reach 30. Follow these steps:

Step 1: Count sentences

Count the number of sentences in your text.

Step 2: Count every word with more than 2 syllables.

Count every polysyllabic word. Use the same tips as outlined above.

Step 3: Find the average number of polysyllabic words per sentence.

Divide the number of polysyllabic words from Step 2 by the number of sentences from Step 2. This will give you the average number of polysyllabic words per sentence.

For example, let’s say we counted 20 sentences in Step 1. And let’s say we counted 80 words with more than 2 syllables in Step 2. We would divide 80 by 20 to calculate an average of 4 polysyllabic words per sentence.

Step 4: Calculate a hypothetical polysyllabic word count, as if there were 30 sentences in your passage

Our SMOG formula requires a passage of 30 sentences. If we have less than 30 sentences, we must calculate a hypothetical polysyllabic word count. To do so:

• Subtract 30 minus the sentence count from Step 1.
• Multiply this number by the average polysyllabic words per sentence from Step 3.
• Add this number to the actual polysyllabic word count from Step 2 to get a hypothetical total polysyllabic word count, as if we had 30 sentences in our passage.

We will use this hypothetical polysyllabic word count in the SMOG formula in our next step.

As an example, let’s say again that we counted 20 sentences in Step 1 and 80 polysyllabic words in Step 2, giving us an average of 4 polysyllabic words per sentence in Step 3.  To apply the SMOG, we need to act as if we had 30 sentences. To do so:

• We would subtract 30 minus 20, giving us 10 additional sentences’ polysyllabic words to account for.
• We would then multiply 10 times 4, the average number of polysyllabic words from Step 3. This gives us a total of 40 additional polysyllabic words we would have expected from 10 additional sentences..
• We then add 40 plus 80, the polysyllabic word count from Step 2. This gives us a total of 120 polysyllabic words we would have expected in a longer passage.

Step 5: Use the SMOG formula.

For a simplified version of the SMOG, take the square root of the hypothetical polysyllabic word count from Step 4. Then add 3 to the square root to get the estimated reading grade level.

For example, let’s say we calculated a hypothetical polysyllabic word count of 120 in Step 4. The square root of 120 is about 11, and then we add 3 to get the estimated reading grade level. This total is 14, which means we might still want to edit our text to meet the common 8^th grade benchmark.

For a more precise version of the SMOG, use our SMOG calculator spreadsheet.

What are some limits of the SMOG?

The SMOG was developed for print documents. When you apply it to digital media, you have to make decisions about what counts as a word. This requires audience research to answer questions about whether and how people read elements like URLs and hashtags.

Readability formulas don’t tell you whether your audience actually understands the content. You still need community feedback and audience research. Also, keep in mind: readability is just one aspect of clear communication. See our guides on graphic design, graphs, and inclusivity for more tips.