Gunning Fog Index

Understanding the Gunning Fog Index

The Gunning Fog Index, developed by Robert Gunning in 1952, estimates the years of formal education needed to understand a text on first reading. This formula is particularly popular in business and technical writing because it emphasizes the impact of complex words on readability.

The Formula

The Gunning Fog Index is calculated using:

0.4 × (ASL + PHW)

Where:

• ASL = Average Sentence Length (total words ÷ total sentences)
• PHW = Percentage of Hard Words (words with 3+ syllables ÷ total words × 100)

What Are "Hard Words"?

In the Gunning Fog Index, "hard words" are defined as words with three or more syllables. However, the formula excludes:

• Proper nouns (names of people, places, organizations)
• Familiar jargon and compound words
• Verbs made into three syllables by adding -ed or -es

Score Interpretation

Gunning Fog Index scores correspond to years of education:

• 6: 6th grade level - Easy to read for most audiences
• 7-8: 7th-8th grade level - Ideal for general web content
• 9-12: High school level - Standard for most publications
• 13-16: College level - Academic and professional content
• 17+: Graduate level - Highly specialized content

Historical Development

Robert Gunning, a newspaper and textbook publisher, created this index after observing that many business documents were unnecessarily complex. He found that executives often complained about unclear writing but continued to produce equally complex documents themselves. The Fog Index was designed to help writers identify and eliminate unnecessary complexity.

Key Applications

Business Communication

The Gunning Fog Index is widely used in corporate settings to ensure internal communications, reports, and customer-facing documents are appropriately readable. Most business documents should target a Fog Index of 12 or lower.

Technical Writing

Technical writers use the Fog Index to balance accuracy with accessibility, ensuring that user manuals and documentation can be understood by their intended audience.

Journalism

News organizations use the Fog Index to maintain readability standards. Most newspapers target a Fog Index between 8-12 to ensure broad accessibility.

Marketing Content

Marketers use the index to create compelling copy that doesn't alienate potential customers with unnecessary complexity.

Advantages of the Gunning Fog Index

• Complex Word Focus: Specifically addresses vocabulary difficulty
• Business Relevance: Particularly useful for professional writing
• Practical Application: Easy to apply during the writing process
• Industry Standard: Widely recognized in business and journalism
• Actionable Results: Clearly identifies areas for improvement

Limitations to Consider

• Syllable Counting: May not accurately reflect word difficulty
• Context Ignorance: Doesn't consider reader familiarity with subject matter
• Cultural Bias: Based on English language patterns
• Genre Limitations: Less accurate for creative writing or dialogue
• Technical Terms: May penalize necessary technical vocabulary

Industry Benchmarks

Publishing Standards

• Popular Magazines: 8-10 (Time, Newsweek)
• Newspapers: 8-12 (Wall Street Journal, USA Today)
• Academic Journals: 15-20 (specialized fields)
• Business Publications: 10-14 (Harvard Business Review)

Digital Content

• Website Content: 8-12 for general audiences
• Blog Posts: 8-10 for maximum engagement
• Email Marketing: 6-10 for broad appeal
• Social Media: 6-8 for quick consumption

Strategies to Improve Your Fog Index

Reduce Complex Words

Replace multi-syllable words with simpler alternatives:

• "Demonstrate" → "Show"
• "Facilitate" → "Help"
• "Utilize" → "Use"
• "Approximately" → "About"
• "Methodology" → "Method"

Shorten Sentences

Break long sentences into shorter, more digestible ones:

• Aim for 15-20 words per sentence on average
• Use periods instead of semicolons when possible
• Eliminate unnecessary subordinate clauses
• Convert passive voice to active voice

Simplify Structure

• Use bullet points for lists
• Add subheadings to break up text
• Include examples and analogies
• Remove redundant phrases

When Complex Words Are Necessary

Sometimes complex vocabulary is unavoidable or necessary for precision. In these cases:

• Define Terms: Provide clear definitions for technical terms
• Use Context: Surround complex words with simpler explanations
• Add Examples: Illustrate complex concepts with concrete examples
• Consider Audience: Ensure complexity matches reader expertise

Comparing Fog Index with Other Metrics

vs. Flesch-Kincaid

While Flesch-Kincaid considers all syllables equally, the Fog Index specifically targets complex words, making it more sensitive to vocabulary difficulty.

vs. SMOG Index

SMOG focuses on polysyllabic words throughout the entire text, while Fog Index uses a sample-based approach and considers sentence length more heavily.

Combined Analysis

Using multiple readability metrics provides a more comprehensive view of text complexity, as each formula emphasizes different aspects of readability.

Best Practices for Business Writing

Target Audience Consideration

• Internal Communications: 10-12 Fog Index
• Customer Communications: 8-10 Fog Index
• Technical Documentation: 12-14 Fog Index
• Marketing Materials: 8-10 Fog Index

Regular Assessment

Incorporate Fog Index analysis into your writing and editing process to maintain consistent readability standards across all communications.

Team Training

Train team members on readability principles and establish organization-wide standards for different types of documents.

The Gunning Fog Index remains one of the most practical readability formulas for business and professional writing, helping organizations communicate more effectively with their audiences.