The Fog Equation Calculator helps writers, editors, and researchers quickly assess the readability of a written passage. It calculates the Gunning Fog Index, which estimates the number of years of formal education someone needs to understand the text on the first reading.
This calculator is part of the Readability and Linguistics category. It is often used in fields like publishing, content marketing, education, and legal or business communication. By simplifying sentences and reducing complex words, users can enhance clarity and improve engagement.
Formula of Fog Equation Calculator
The formula used in the Fog Equation Calculator is:
Where:
- ASL = Average Sentence Length = Total Words ÷ Total Sentences
- PHW = Percentage of Hard Words = (Number of Hard Words ÷ Total Words) × 100
Hard Words are words with three or more syllables, excluding:
- Proper nouns (e.g., “Washington”)
- Compound words (e.g., “notebook”)
- Common suffixes (e.g., “running,” “advanced”)
This formula helps determine whether a piece of writing is suitable for general audiences, high school readers, or professionals with advanced degrees.
General Reference Table
| Type of Writing | Avg. Sentence Length | % Hard Words | Fog Index | Education Level |
|---|---|---|---|---|
| Children’s Books | 8–10 | 1–2% | 4–6 | Elementary |
| Popular Blogs | 10–12 | 3–4% | 6–8 | Middle School |
| General News Articles | 15–18 | 8–10% | 10–12 | High School |
| Business Reports | 20–25 | 12–15% | 13–16 | College |
| Academic/Legal Documents | 25+ | 15–20% | 17+ | Postgraduate |
This table offers a quick benchmark for evaluating and adjusting your writing to match the audience's comprehension level.
Example of Fog Equation Calculator
Sample Paragraph:
"The results of the experiment clearly demonstrated a significant improvement in energy efficiency among all tested materials."
- Total Words: 17
- Total Sentences: 1
- Hard Words: “experiment,” “demonstrated,” “efficiency,” “materials” → 4
Step 1: Calculate ASL
ASL = 17 / 1 = 17
Step 2: Calculate PHW
PHW = (4 / 17) × 100 = 23.5%
Step 3: Apply the Fog Formula
Fog Index = 0.4 × (17 + 23.5)
Fog Index = 0.4 × 40.5 = 16.2
This means the paragraph requires over 16 years of education—equivalent to college senior or early postgraduate level—for easy comprehension.
Most Common FAQs
A score of 8 to 10 is generally considered good for broad audiences. For technical writing, a higher score may be acceptable, but clarity is always important.
The Flesch Score is based on syllables and sentence length but provides a different scale where higher values mean easier readability. The Fog Index gives an education grade level.
Yes, especially to make dense content more readable. While technical vocabulary is expected, reducing sentence length and avoiding excessive jargon can greatly improve understanding.