What Is a Readability Checker?
A readability checker scores how easy or difficult a piece of text is to read, using mathematical formulas that combine sentence length, word length, and syllable counts. These formulas were originally developed for education and government use — to verify that textbooks, legal notices, and public health materials matched the reading level of their intended audience — and have since become standard tools in SEO, UX writing, and content editing.
This tool runs your text through five independently developed formulas simultaneously: Flesch Reading Ease, Flesch-Kincaid Grade Level, Gunning Fog Index, SMOG Index, and Coleman-Liau Index. Because each formula weighs its inputs differently, comparing all five at once gives a far more reliable read than trusting any single score.
The Five Formulas Explained
Flesch Reading Ease produces a score from 0 to 100, where higher means easier to read. It's the most widely cited readability metric and the basis for Microsoft Word's built-in readability statistics.
Flesch-Kincaid Grade Level uses the same underlying inputs as Reading Ease but rescales the output to a US school grade level, making it more intuitive for educators: a score of 8 means an eighth-grader should understand the text without difficulty.
Gunning Fog Index estimates the years of formal education needed to understand a text on first reading, with heavy emphasis on the percentage of "complex" words (three or more syllables). It is widely used in journalism and government plain-language standards.
SMOG Index (Simple Measure of Gobbledygook) was developed specifically for healthcare materials and is considered one of the most reliable formulas for predicting whether a patient or general reader will fully comprehend a passage, not just decode it.
Coleman-Liau Index is unusual in that it uses letter counts rather than syllable counts, making it more reliable for text with unusual proper nouns or technical terms where syllable-counting heuristics tend to break down.