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The Gunning’s Fog Index (or FOG) Readability FormulaThe English language dances with elegance and complexity. It invites us all to communicate, understand, and learn. Yet, its richness can sometimes create barriers. When words become too complex or sentences too long, the message may get lost. This is where the Gunning Fog Index steps into the dance.

The Gunning Fog Index Readability Formula, or simply called FOG Index, is attributed to American textbook publisher, Robert Gunning, who graduated from Ohio State University. Gunning observed that most high school graduates were unable to read. Much of this reading problem was a writing problem. His opinion was that newspapers and business documents were full of “fog” and unnecessary complexity.

Gunning realized the problem early on in his career and became the first to take the new readability research into the workplace. In 1944 Gunning founded a consulting firm specializing in readability. He spent the next few years testing and working with 60+ large city daily newspapers and popular magazines, helping writers and editors write to their audience. In 1952, Gunning published a book, The Technique of Clear Writing and created an easy-to-use Fog Index.

The index considers two factors. First, the length of sentences. Shorter sentences are easier to understand, and so they lower the fog. Second, it looks at the number of complex words. These are words with 3+ syllables that are uncommon English words. As the number of complex words goes up, so does the fog.

The tool then generates a score. This score tells us the grade level we need to understand a text. The higher the score, the higher the education level.

Why is this important? Because readability matters. Whether in classrooms, businesses, or newsrooms, clear communication paves the way for understanding. It removes barriers. It invites engagement. It fosters learning.

The formula is as follows:


Gunning Fog Index

$$0.4
\times (ASL + 100 \times \frac{n_{wsy>=3}}{n_{w}})$$

where \(n_{wsy>=3}\) = the number of words with 3-syllables or more. | NW = number of total words.


To measure sentence length, the index counts the number of words per sentence. It finds the average—a simple sum divided by the total number of sentences.

Next, the index considers complex words. But what makes a word complex? For the Fog Index, it’s all about syllables. Words with three or more syllables count as complex. Yet, there are exceptions. Proper names, simple compound words (like “sunflower” or “bookkeeper”), and words made complex by suffixes –ed or –es (like “created” or “trespasses”) do not count as complex.

To find the percentage of complex words, the index divides the number of complex words by the total number of words. Then it multiplies by 100 to get a percentage.

Finally, the Fog Index combines these two factors. It adds the average sentence length to the percentage of complex words. Then it multiplies the result by 0.4. The outcome is a number that represents the grade level needed to understand the text.

A score of 9 means a 9th grader could understand the text. A score of 12 requires a high school student’s reading level. A score above 17 indicates a university graduate level of comprehension.

Gunning Fog Index Results

Applications of the Gunning Fog Index

Applications of the Gunning Fog Index

This tool, simple yet powerful, finds its purpose in various fields.

  • In education, the Fog Index helps tailor learning materials to students. It ensures texts suit their comprehension levels. Teachers, therefore, find it an invaluable ally. They can choose books and design lessons that are just challenging enough.
  • Journalists, too, find the Fog Index useful. It guides them to write articles that their readers can grasp. It nudges them to simplify sentences and choose simpler words. The result? News that reaches and informs a wider audience.
  • Businesses also benefit from the Fog Index. It helps them craft clear, concise messages. Whether in marketing materials or internal communications, clarity reigns supreme. The Fog Index ensures their messages reach their intended audience with minimal misinterpretation.
  • Healthcare professionals use the Fog Index to develop patient information materials. It is crucial that health-related documents are easily understood by patients, regardless of their literacy levels. The Fog Index aids healthcare providers in creating clear and concise patient communication, thus ensuring that health instructions and medical information are comprehensible to all.
  • Legal documents, notorious for their complexity, often hinder comprehension. Lawyers and legal professionals use the Fog Index to assess and improve the readability of these documents. By simplifying language and reducing sentence length, they can make legal jargon more accessible to clients and the public.
  • Government agencies use the Fog Index to make their communications more transparent and understandable. These agencies need to communicate a wide range of information to the public, from new policies to safety guidelines. By using the Fog Index, they can ensure their messages are readable and readily understandable, fostering a well-informed citizenry.
  • In the world of technology, software developers and user experience (UX) designers use the Fog Index to create user-friendly interfaces and documentation. Clarity in instructions and guidelines is essential for an optimal user experience. By adhering to the principles of the Fog Index, tech professionals can provide users with easy-to-understand instructions, enhancing user satisfaction.
  • Lastly, writers and editors in all fields make use of the Gunning Fog Index. Whether they’re crafting a novel, editing an article, or producing content for a website, they turn to the Fog Index as a way to gauge and adjust the readability of their work. It serves as a valuable tool for ensuring their writing reaches and resonates with their intended audience.

Limitations of the Gunning Fog Index

Despite its wide-ranging applications, the Fog Index has limitations.

Limitations of the Gunning Fog Index

  1. The Fog Index does not consider the nuances of language and content context. All words of three syllables or more are scored complex, even when they are commonplace. Words like “unfortunately” or “beautiful” are common in everyday language, yet the Fog Index marks them as complex due to their syllable count. This leads to higher text complexity.
  2. The Fog Index’s reliance on sentence length as a measure an also mislead. It views shorter sentences as inherently simpler. Yet, a short sentence filled with dense jargon may prove much harder to understand than a longer sentence written in plain English.
  3. The Fog Index does not account for the structure of a sentence. A sentence with multiple clauses and parentheses might be more difficult to understand than a longer but straightforward sentence. The Index’s sentence length over structure may underestimate complexity in some cases.
  4. It does not consider reader familiarity. A text might use simple language and short sentences, but if it discusses a specialized or unfamiliar topic, readers may struggle to comprehend it.
  5. It fails to consider cultural and regional differences. A word considered complex in one culture might be commonplace in another. Likewise, a short sentence might be more difficult to understand if it uses region-specific idioms or references.

These limitations remind us that the Fog Index is not a standalone solution. It works best in tandem with good judgment and knowledge of the target audience. It serves as a guide, not an absolute rule.

You can score your text using the Gunning Fog Index, along with other popular formulas, with our Average Reading Level Consensus Calc.