Now more than ever, clarity and readability in writing are essential. One of the most widely used methods for assessing the difficulty of written material is the Gunning Fog Index. This index produces a number that represents the years of schooling a person would need to read and understand your work.
In other words, the Gunning Fog Index is a measure of text readability. It calculates readability based on the average number of words per sentence and the number of complex words, determining how many years of formal education an average reader would need to comprehend the text.
In today’s article, we will explore what the Fog Index is, how to calculate it, and what your score means.
What Is the Fog Index?
The Gunning Fog Index is a readability test used to assess the complexity of English writing. It was developed by Robert Gunning in 1952 to estimate the years of formal education a reader would need to understand a text on the first reading. A higher score indicates more difficult text, often requiring higher educational attainment, while a lower score suggests the text is easier to read and comprehend.
In practice, the Gunning Fog Index is commonly used to determine the grade level of school textbooks. However, among English teachers (both language and literature), it is sometimes seen as a limiting factor because it discourages the introduction of new vocabulary. As a result, students may be exposed only to materials that match or fall below their current reading level, reducing opportunities to learn advanced words and concepts.
On the other hand, the index is valuable for tailoring presentations, job training materials, and business communications. However, its application in education is often misinterpreted and misused. Gunning did not create the index to oversimplify writing but rather to make it clearer and more readable—”less foggy.” He developed it after reviewing countless student papers filled with unnecessarily long words and excessive padding.
Today, the index is widely used in business writing courses that focus on effective communication and plain English, ensuring that messages are conveyed quickly and clearly. While technical documents may require longer words, their readability can be improved by using shorter sentences and well-structured paragraphs to enhance flow.
The index is also extensively applied in government documents and public awareness materials, helping ensure that information is accessible to a broad audience.
Mathematically, the Gunning Fog Index is an equation that quantifies the level of education needed to comprehend a sample of English text. The “ideal” score is considered to be 8 or 9. Long sentences and complex words increase the score, while shorter sentences and simpler words lower it.
Most Americans read at approximately an eighth-grade level. In many American workplaces, an unofficial policy encourages editing materials down to a 7th- or 8th-grade reading level. This practice helps limit jargon, promotes a direct writing style, and minimizes the risk of misunderstandings. If you want to lower your Fog Index score, consider breaking up long, complex sentences into shorter, more digestible ones.
The Gunning Fog Index is a widely used metric and is built into many writing applications. By using it, writers can refine their style and adapt content to their target audience, achieving a balanced mix of word choice, sentence length, and structure.
The Gunning Fog Index is calculated using the following formula:
Grade Level = 0.4 (ASL + PHW)
Where:
ASL (Average Sentence Length) is calculated by dividing the total number of words in the text by the total number of sentences.
PHW (Percentage of Hard Words) is determined by dividing the number of complex words (those with three or more syllables, excluding proper nouns, familiar jargon, and compound words) by the total number of words.
How to Calculate the Fog Index
Here’s how to calculate the Average Sentence Length (ASL) and the Percentage of Hard Words (PHW) for the Gunning Fog Index:
1. Average Sentence Length (ASL)
To calculate ASL, follow these steps:
- Count the Total Number of Words: Go through your text and count all the words.
- Count the Total Number of Sentences: Identify and count all the sentences in the text.
- Calculate ASL: Divide the total number of words by the total number of sentences using the formula:
ASL=Total Words/Total Sentences
2. Percentage of Hard Words (PHW)
To calculate PHW, use the following steps:
- Count the Total Number of Complex Words: Identify and count the words with three or more syllables. Exclude proper nouns, familiar jargon, and compound words.
- Count the Total Number of Words: This is the same total number of words used in the ASL calculation.
- Calculate PHW: Divide the number of complex words by the total number of words, then multiply by 100 to get the percentage:
PHW = (Total Complex Words/Total Words)×100
Once you have calculated both ASL and PHW, you can plug these values into the Gunning Fog Index formula to determine the readability grade level of your text. This helps in assessing how easily your content can be understood by readers.
Example Calculation
Sentence: “The server experienced a critical failure due to a hardware malfunction.”
Step 1: Calculate ASL (Average Sentence Length)
1. Count the Total Number of Words:
- There are 10 words in the sentence.
2. Count the Total Number of Sentences:
- There is 1 sentence.
3. Calculate ASL:
ASL = Total Words/Total Sentences = 10/1 = 10
Step 2: Calculate PHW (Percentage of Hard Words)
1. Identify Complex Words:
– In this sentence, the complex words are “experienced” (4 syllables) and “malfunction” (3 syllables).
– So, Total Complex Words = 2.
2. Count the Total Number of Words:
– Total words = 10.
3. Calculate PHW:
PHW = (Total Complex Words/Total Words) x 100 = (2/10) x 100 = 20%
Now, plug ASL and PHW into the Gunning Fog Index formula:
Grade Level = 0.4 x (ASL + PHW) = 0.4 x (10 + 20) = 0.4 x 30 = 12
The Gunning Fog Index for the sentence “The server experienced a critical failure due to a hardware malfunction” is 12, indicating that the text is suitable for a 12th-grade reading level. This example highlights how technical language can affect readability in the IT field.
What Does My Score Mean?
The Gunning Fog Index score can be interpreted as follows:
0-6: Easily understandable by an average 6th grader.
7-8: Understandable by an average 8th grader.
9-10: Understandable by a high school student.
11-12: Understandable by a college student.
13-16: Understandable by someone with a graduate-level education.
17+: Very complex; may require specialized knowledge or advanced education.
In the example above, a score of 12 suggests that the text is suitable for a college-level audience.
When Is the Gunning Fog Index Most Useful?
The Gunning Fog Index is particularly useful in various contexts, including:
- Academic Writing: Students and researchers can use the Fog Index to ensure their writing is accessible to the intended audience.
- Business Communication: Professionals can evaluate reports, proposals, and emails to ensure clarity and effectiveness in communication, especially when addressing clients or stakeholders.
- Content Creation: Writers and marketers can assess blog posts, articles, and advertisements to ensure they resonate with their target audience. In communications, writers aim for Grade 7 reading level or lower, though technical and legal content may reach a Grade 10 level.
- Educational Materials: Educators can use the Fog Index to tailor reading materials to the appropriate comprehension level for their students.
Apps to Calculate the Fog Index
Here are a few apps and tools that calculate the Gunning Fog Index, a readability measure that estimates the years of formal education needed to understand a text. These tools are especially helpful for writers, editors, and anyone looking to make their texts clear and easy to read.
Hemingway Editor (Web and Desktop)
- Platforms: Web; Desktop, Mac, Windows.
- Features: identifies complex sentences and common mistakes, helping simplify your writing. It provides a readability score, including the Gunning Fog Index, so you can understand the level of schooling required to read your text.
Grammarly (Web, Desktop, and Mobile)
- Platforms: Web, Desktop (Mac, Windows), Mobile (iOS, Android).
- Features: Grammarly is an all-in-one writing assistant that checks grammar, punctuation, and spelling. It also provides a readability score— including the Gunning Fog Index— to help ensure your writing is clear and concise in both tone and structure.
Readable (Web)
- Platforms: Web.
- Features: Readable is a handy, free readability analyzer that provides scores from multiple tests, including the Gunning Fog Index. It also offers suggestions for improving the readability of your text.
Write & Improve (Web)
- Platforms: Web.
- Features: Write & Improve is a free, user-friendly tool that calculates the Gunning Fog Index and offers suggestions to improve your text’s readability. It also checks for common writing issues like passive voice, sentence length, and more.
ProWritingAid (Web, Desktop, and Mobile)
- Platforms: Web, Desktop (Mac, Windows), Mobile (iOS, Android).
- Features: ProWritingAid is a comprehensive writing tool that provides grammar and style checks, readability assessments, and much more. It also offers a readability score, including the Gunning Fog Index, Flesch Kinkaid grade among other metrics to help refine and improve your writing.
The apps listed above can help make your text more accessible and comprehensible. However, keep in mind that while these tools are useful, it’s always beneficial to consult with a human editor or proofreader for a more thorough review.
Conclusion
In conclusion, the Gunning Fog Index is a valuable tool for anyone looking to enhance the clarity and readability of their writing. By understanding how to calculate the index and interpret the results, you can improve your communication skills and ensure your message is easily understood by your audience. Whether you’re a student, professional, or content creator, incorporating the Fog Index into your writing process can lead to more effective and impactful communication.
Good luck with your technical writing!
ClickHelp Team
Author, host and deliver documentation across platforms and devices
FAQ
What is the Gunning Fog Index?
The Gunning Fog Index is a readability test that measures the complexity of English writing. It estimates the number of years of formal education required for a reader to understand a piece of text on the first reading.
How can I improve my Fog Index score?
To lower your Fog Index score and improve readability:
Use shorter sentences.
Replace complex words with simpler alternatives.
Avoid jargon and unnecessary padding.
Break up long sentences into shorter, more digestible ones.
Does the Fog Index discourage advanced vocabulary?
While the Fog Index encourages clarity, some educators argue that it may limit vocabulary expansion. The key is to strike a balance—using clear, concise language without oversimplifying content unnecessarily.
Can the Fog Index be used for technical or legal writing?
Yes, but technical and legal documents often contain unavoidable jargon. Writers can improve readability by structuring content effectively, using shorter sentences, and defining complex terms where necessary.
Why is the ideal Fog Index score around 8 or 9?
Most Americans read at approximately an eighth-grade level. For effective communication, especially in business and public documents, a score of 7-9 ensures broad accessibility without oversimplification.
