The SMOG Formula: A Complete Guide

Written communication suffers when people have difficulty understanding what they’re reading. You might want to assess the readability of your text and make adjustments where necessary. The SMOG formula is a readability testing tool designed to grade a text based on the number of sentences and syllables per word.

This article discusses the SMOG formula and how to use the tool to test the readability of your text.

What is the SMOG Formula?

A good readability formula measures the level of readability of a text by assessing the sentences or words.

The SMOG (Simplified Measure of Gobbledygoop) formula was created by McLaughlin. It is a tool that determines what written content is the most readable with an objective assessment.

SMOG readability formula estimates the years of education an individual needs to comprehend a text. It measures the level of text readability by considering the number of sentences and syllables per word.

SMOG Readability Formula: How it Works

The SMOG readability formula defines a numeric value, known as the SMOG grade, which estimates the readability of a given text. When using the SMOG formula to calculate the reading grade level of your text, consider the following steps:

1: Begin with the entire written text to be assessed.

2: Count off ten sentences near the beginning, 10 in the middle, and ten near the end to make 30 sentences.

3: Circle all the polysyllabic words from the sample of the 30 sentences. Include repetitions of the same word, and total the number of words circled.

4: Find the nearest perfect square of the total number of polysyllabic words and then take its square root.

5: Add a constant of 3 to the figure obtained in step 4 above. The value obtained is the SMOG Grade, i.e., the reading grade level a person must have attained to comprehend the text being assessed fully. SMOG grade = 3 + Square root of Polysyllable Count.

Rules governing the McLaughlin’s SMOG Readability Formula

McLaughlin’s SMOG readability formula is premised upon the following.

1. A sentence is a string of words punctuated with a period (.), an exclamation mark (!), or a question mark (?).

2. Consider long sentences with a semi-colon as two separate sentences.

3. Consider hyphenated words as a single word.

4. Written numbers (spelled out or in numeric form) should also be considered. If written in numeric form, they should be pronounced to determine the number of syllables they have.

5. Proper nouns that are polysyllabic should also be counted.

6. Read abbreviations as though non-abbreviated to determine if they are polysyllabic.

7. To test a written text with fewer than 30 sentences, follow the steps below:

i. Take note of the number of polysyllabic words in the text.

ii. Take note of the number of sentences in the text.

iii. Find the average number of polysyllabic words per sentence by dividing the figure obtained in (i) by that obtained in (ii), i.e.,

Average = Total Number of Polysyllabic Words

Total Number of Sentences.

iv. Multiply the average gotten in (iii) by the number of sentences fewer than 30.

v. Add the figure gotten in (iv) to the total number of polysyllabic words.

vi. Calculate the square root and add the constant of 3.

An easy way to calculate the SMOG reading grade level of a text is by using the SMOG conversion table. Count the number of polysyllabic words in a chain of 30 sentences and look up the approximate grade level on the conversion table.

SMOG Conversion Table

Total Polysyllabic Word Counts Approximate Grade Level
















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To Wrap Up

The SMOG readability formula is used to gauge a written text’s comprehensiveness. It measures the reading grade level a person must have attained if they want to comprehend a written text fully. If the readability is greater than 10, we expect the content to be more challenging to understand.

Pam is an expert grammarian with years of experience teaching English, writing and ESL Grammar courses at the university level. She is enamored with all things language and fascinated with how we use words to shape our world.

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