The Gunning Fog Index is a readability test designed to estimate the years of formal education needed to understand a text on the first reading. It uses the average sentence length and the percentage of complex words (words with three or more syllables) in a passage. The formula helps writers assess the clarity of their writing, making it easier for readers to comprehend. A higher Gunning Fog Index indicates more complex writing, while a lower score suggests simpler text. This tool is useful for educators, authors, and content creators aiming to tailor their material for specific audiences, ensuring effective communication.