The Fog Index Calculator helps determine how difficult a piece of writing is to understand. It evaluates the complexity of text based on sentence length and the percentage of complex words. This calculator is especially useful for educators, content creators, editors, and professionals who want to make sure their writing matches the reading level of their audience.
This tool falls under the Readability and Education Tools category. It simplifies the process of evaluating written material by automatically identifying if a text is suitable for middle school students, high school readers, college-level learners, or advanced professionals. Writers use it to revise content for clarity, accessibility, and engagement.
formula of Fog Index Calculator
Fog Index = 0.4 × (ASL + PHW)
Where:
ASL = Average Sentence Length
ASL = Total Words / Total Sentences
PHW = Percentage of Hard Words
PHW = (Number of Hard Words / Total Words) × 100
Hard Words are words with 3 or more syllables, excluding:
- Proper nouns
- Familiar compound words
- Words with common suffixes like -ing, -ed, or -es
Interpretation Guide:
- 6–8: Easy to read (Middle school level)
- 9–12: Standard readability (High school level)
- 13–16: Difficult (College level)
- 17+: Very difficult (Professional level or above)
The Fog Index gives you an approximate grade level required to understand the text. A lower score means your writing is easier to read.
Common Readability Reference Table
| Fog Index Score | Reading Level | Education Level | Ideal Usage |
|---|---|---|---|
| 6–8 | Easy to understand | Middle school students | Children’s books, web content |
| 9–12 | Fairly readable | High school students | Newspapers, general blogs |
| 13–16 | Challenging | College and academic | Reports, research summaries |
| 17+ | Very complex | Graduate-level or expert | Legal, scientific, technical writing |
This table helps users quickly understand how their content ranks and who it is suitable for.
Example of Fog Index Calculator
Let’s calculate the Fog Index for a short paragraph:
Text:
"Many professionals rely on communication tools to collaborate and share knowledge. However, unclear messages can lead to delays and misunderstandings."
Step 1: Count
Total Words = 20
Total Sentences = 2
Hard Words = 3 (communication, collaborate, misunderstandings)
Step 2: Apply formulas
ASL = 20 / 2 = 10
PHW = (3 / 20) × 100 = 15
Step 3: Fog Index = 0.4 × (10 + 15) = 0.4 × 25 = 10
Interpretation: A Fog Index of 10 means the text is appropriate for a high school reader. It is considered fairly readable and understandable to most general audiences.
Most Common FAQs
A score between 6 and 12 is typically considered good, depending on your target audience. Lower scores are best for general readers, while higher scores suit technical or professional materials.
The Fog Index is designed for English-language texts. It may not apply accurately to other languages because syllable structure and grammar rules vary.
Not necessarily. A very low score might oversimplify your content, especially if you're writing for an educated audience. The key is to match the index with the audience's reading level.