Gunning Fog Index

Updated on April 19, 2024
Article byKhalid Ahmed
Reviewed byDheeraj Vaidya, CFA, FRM

Gunning Fog Index Meaning        

The Fog Index, also known as the Gunning Fog Index, is a readability assessment tool designed to measure the complexity of written text. It assigns a grade between 0 and 20 to indicate the level of comprehension required to understand the text.

Gunning Fog Index

You are free to use this image on your website, templates, etc, Please provide us with an attribution linkHow to Provide Attribution?Article Link to be Hyperlinked
For eg:
Source: Gunning Fog Index (

This tool enables writers to evaluate the readability of their writing and make adjustments for better reader understanding. It helps ensure that even complex concepts can be accessible to a wider audience. Texts are classified into different genres based on their accessibility scores. Skillful writers strike a balance between simplicity and complexity, creating content that is both engaging and informative.

Key Takeaways

  • The Fog Index, commonly referred to as the Gunning Fog Index, evaluates the readability of text.
  • It assigns a grade from 0 to 20 to indicate the complexity of written content, helping determine the required comprehension level.
  • To calculate it, divide the total word count by the number of sentences, calculate the percentage of long words by multiplying the count of long words by 100, and then sum these components.
  • While providing convenience, the index disregards essential factors like the reader’s background, education, metaphors, and text length when assessing readability.

Gunning Fog Index Explained 

The Gunning Fog Index is a readability metric that gauges the difficulty of written text by assigning it a specific grade. Created by Robert Gunning in 1952, the index aims to determine the ease of comprehension for readers. It addresses readability and comprehension challenges faced by readers when engaging with written material.

Furthermore, the index provides insight into the educational level required to understand and absorb a given text upon first reading. It quantifies the level of education a reader needs to comprehend an English text, considering factors such as the use of long words and sentence length. The index employs a formula to calculate a reader’s grade level score based on their comprehension ability.

Lower scores indicate easier comprehension, while higher scores indicate greater complexity. Scores range from 0 to 20, reflecting the level of education necessary to grasp the text’s content. For example, a score of 6 means that someone with a sixth-grade education can understand the text. Conversely, scores exceeding 17 suggest that a reader should have at least a graduate-level education to comprehend the text.

Financial Modeling & Valuation Courses Bundle (25+ Hours Video Series)

–>> If you want to learn Financial Modeling & Valuation professionally , then do check this ​Financial Modeling & Valuation Course Bundle​ (25+ hours of video tutorials with step by step McDonald’s Financial Model). Unlock the art of financial modeling and valuation with a comprehensive course covering McDonald’s forecast methodologies, advanced valuation techniques, and financial statements.


Robert Gunning proposed the Fog Index In 1952. The formula and calculation is straightforward.

Step 1: Compute Average Sentence Length

Average Sentence Length = Total Words / Sentences

Step 2: Count Words of Three or More Syllables

Long Words per 100 Words = Long Words / (Total Words / 100) 

(Count the number of words of three or more syllables per each 100 words in the sample. Words that become three syllables with the addition of “ed” or “es” are not counted, nor are words that are capitalized or easy compound words)

Step 3: Calculate the Fog Index

Fog Index = (Average Sentence Length + Long Words per 100 Words) * 0.4


Let us use a few examples:

Example #1

 Let’s say you have a company’s annual report with the following statistics:

  • Total Words: 800
  • Sentences: 15
  • Long Words: 40

Compute Average Sentence Length 

  • Average Sentence Length = Total Words / Sentences 
  • = 800 / 15 ≈ 53.33 

Count Words of Three or More Syllables 

  • Long Words per 100 Words = Long Words / (Total Words / 100) 
  • = 40 / (800 / 100) ≈ 5 

Calculate the Fog Index 

  • Fog Index = (Average Sentence Length + Long Words per 100 Words) * 0.4 
  • = (53.33 + 5) * 0.4 ≈ 23.73 

In this example, the calculated Fog Index is approximately 23.73. This score indicates a higher level of complexity and may be challenging for many readers.

Example #2

Imagine someone is reading financial news articles about different companies’ stock performance. Some articles are easy to understand, while others use intricate language and long sentences. They can determine which articles are more reader-friendly using the Gunning Fog Index. If an article has a low index score, say 7, it suggests it’s written in a clear and accessible manner. If it has a higher score, like 13, it may be more challenging for the average reader to comprehend the information about the stocks and the market fully.

Advantages And Disadvantages 

The advantages & disadvantages are as follows:


  • One of the simplest methods for understanding and calculating readability.
  • Provides quick metrics for measuring text readability.
  • Effectively gauges the general readability of various types of texts.
  • Facilitates aligning the audience’s comprehension level with the complexity of the text.
  • Enables easy comparison of readability among different texts.
  • Even individuals with limited linguistic knowledge can utilize it.
  • Widely acknowledged and employed in the publishing and education sectors.


  • Overlooks important factors such as the reader’s background, education, use of metaphors, and text length when determining readability.
  • Assessment doesn’t account for complex sentence structures and language usage.
  • Built-in bias is unsuitable for assessing language in artistic, technical, or scientific contexts.
  • Readability isn’t solely determined by sentence or word length, introducing subjectivity.
  • The reader’s familiarity with the topic and educational background significantly influence results.
  • Emphasis on word length takes precedence over conveying content meaning.
  • Fails to consider a writer’s intention in using complex language for persuasion or artistic expression.

Frequently Asked Questions (FAQs)

1. What is the importance of the Gunning Fog Index?

The Gunning Fog Index holds significance as a tool to assess text readability. By assigning a grade to indicate the level of comprehension required, it aids writers in tailoring their content for wider audiences. This index contributes to striking a balance between complexity and accessibility, making information more understandable and engaging for readers across various domains.

2. What is a good Gunning Fog Index?

A good Gunning Fog Index score depends on the intended audience and purpose of the text. Generally, a score between 7 and 10 is suitable for a broad audience, implying a reading level of around 7th to 10th grade. However, the optimal score may vary. Adjusting the index helps ensure the content aligns with the reader’s comprehension level, promoting effective communication.

3. What is Gunning Fog vs. Flesch-Kincaid?

Both Gunning Fog and Flesch-Kincaid are readability tests but assess text complexity differently. Gunning Fog measures complexity by considering sentence length and words with three or more syllables. Flesch-Kincaid focuses on syllable count per word and average sentence length. While both aim to enhance readability, Gunning Fog emphasizes longer words’ impact on comprehension, whereas Flesch-Kincaid stresses overall sentence complexity.

This has been a guide to the Gunning Fog Index and its meaning. Here, we explain its formula, examples, advantages, & disadvantages. You can learn more about it from the following articles –

Reader Interactions

Leave a Reply

Your email address will not be published. Required fields are marked *