Readability Checker and
Grade Level Calculator.

The oldest and most widely recognized readability score, in use since 1948. It measures two things: how long your sentences run and how many syllables your words carry. The result lands on a 0 to 100 scale where higher means easier. A score above 90 reads like a children's book. 60 to 70 is plain English, the range most content for a general audience should hit. 30 to 50 is college level reading, and anything below 30 is dense academic or legal prose.

Use it as the first benchmark for blog posts, marketing copy, and emails. If the score sits below 50 and your readers are not academics, the text is working against you. The formula is so established that some US states require minimum Reading Ease scores for insurance policies. When someone says "readability score" without naming a formula, this is usually the one they mean.

The same two inputs as Reading Ease, converted into a US school grade. A result of 8.0 means an average eighth grader can follow the text on first read. That single number is why this formula became the standard: "Grade 8" communicates instantly, while "Reading Ease 65" needs a lookup table. The conversion was commissioned by the US Navy in 1975 to grade technical manuals, and grade level targets are still written into US government plain language guidance today.

It is also the score most SEO and writing tools report, including Yoast and Semrush, so when a client hands you a readability target, it is almost certainly this one. How to read the number: Grade 7 to 9 is the standard target for blogs, product pages, and general marketing copy. Grade 6 or below suits broad consumer audiences. Grade 10 to 12 fits professional content where readers bring domain knowledge, and results above 13 should be a deliberate choice, not an accident of long sentences.

One caveat: the formula measures structural complexity, not clarity. A Grade 8 text with muddled logic still loses the reader. Treat the grade level calculator above as a ceiling check, not as proof of good writing.

Developed by Robert Gunning in 1952 to make business writing and journalism readable. Like Flesch, it starts from sentence length, but instead of counting syllables everywhere it tracks the share of complex words, defined as words with three or more syllables. That one choice changes what the formula punishes: jargon and technical vocabulary raise the Fog Index faster than anything else.

Reading the result: Fog 8 is nearly universal readability, Fog 12 is the ceiling Gunning recommended for business documents, and academic papers routinely score 16 and above. Choose the Fog Index when vocabulary is the thing you are unsure about. If your Flesch scores look fine but Fog is high, the problem is not sentence length. It is the density of long words, and the fix is swapping terms, not splitting sentences.

The first of the two character based formulas in the set. Instead of syllables it counts letters per 100 words and sentences per 100 words, then converts the result into a US grade level. The reason is precision: syllable counting in software is a heuristic that guesses at pronunciation, while character counting is exact. Coleman-Liau produces the identical result in every implementation, which is why it was designed in 1975 with machine scoring in mind.

Choose it when you need scores that stay consistent across different tools, or when your text carries unusual vocabulary such as brand names and product terms that break syllable heuristics. If Coleman-Liau disagrees sharply with the Flesch scores on the same text, that gap itself is information about how atypical your vocabulary is.

The Automated Readability Index, usually shortened to ARI, was built in 1967 for the US military to score technical manuals in real time. The hardware of the era explains the design: electric typewriters could count keystrokes but not syllables, so the formula uses characters per word and words per sentence, and outputs a US grade level.

Because it is character based like Coleman-Liau, the two usually land within a grade of each other. When both of them diverge from the syllable formulas, your text has unusually long or unusually short words relative to their syllable count, which is common in technical writing full of product names and acronyms. Use ARI for documentation, UI copy, and any automated quality check where you want a fast, deterministic score.

SMOG stands for Simple Measure of Gobbledygook, a name its author chose in 1969 with full intent. The formula counts polysyllabic words across a sample of 30 sentences and converts the count into a grade level. It was created for health care materials and remains the recommended standard for patient information in medical publishing.

SMOG typically runs one to two grades higher than Flesch-Kincaid on the same text, and that is by design: it targets full comprehension rather than partial understanding, because misreading a dosage instruction has different consequences than skimming a blog post. It needs at least 30 sentences for a reliable result, so the checker flags shorter texts. Choose SMOG for medical, legal, and safety content, or any writing where the cost of being misunderstood is high.