Reading levels explained: Flesch, Gunning Fog, SMOG, Coleman-Liau

Readability formulas come up in three places: in educational publishing, where they help match books to grade levels; in technical writing, where companies set readability targets for documentation; and in plain-language initiatives in government and healthcare, where lower-grade-level writing broadens access. The four formulas you will meet most often (Flesch Reading Ease, Flesch-Kincaid Grade Level, Gunning Fog Index, SMOG Index, and Coleman-Liau Index) all output a single number that translates roughly to a US school grade. They get to that number through different routes, which is why a single piece of writing can score differently across them.

What readability scores actually measure

All four formulas approximate reading difficulty by counting two kinds of thing: word complexity and sentence length. Word complexity is usually estimated either by syllable count (Flesch, Gunning Fog, SMOG) or by character count (Coleman-Liau). Sentence length is measured directly as words per sentence. The formulas combine these into a single score.

What the formulas do not measure is also worth knowing. They do not measure vocabulary familiarity (a piece of writing full of common four-syllable words like "regulation" and "department" can score harder than one with unfamiliar two-syllable words). They do not measure logical complexity (a convoluted argument in short sentences may score easy but read hard). They do not measure cohesion, paragraph structure, or audience expertise. A low readability score is a useful signal that prose might be too dense, not a verdict on whether the writing is good. The reverse holds too: a high readability score does not mean a piece is well written.

Flesch Reading Ease and Flesch-Kincaid Grade Level

Rudolf Flesch developed his original Reading Ease formula in 1948 and published it in the second edition of The Art of Readable Writing (1949). The formula outputs a score from roughly 0 to 100 where higher is easier. The calculation is:

206.835 - 1.015 (words / sentences) - 84.6 (syllables / words)

A score of 90 to 100 is described as very easy, suitable for fifth-grade readers (about age 10 to 11). A score of 60 to 70 is plain English, the range that most newspapers and Reader's Digest aim for. A score of 30 to 50 is in academic territory; below 30 is difficult and typical of scholarly writing or legal text.

The Flesch-Kincaid Grade Level was developed in 1975 by Peter Kincaid and colleagues for the US Navy. It uses the same inputs but converts the score to a US grade level number directly:

0.39 (words / sentences) + 11.8 (syllables / words) - 15.59

A grade level of 8 means the text suits an eighth grader (age 13 to 14). A grade level of 12 corresponds to a US high school senior. A grade level of 16 corresponds to a US college senior. The Microsoft Word readability statistics built into Word for decades use the Flesch-Kincaid Grade Level as their default reporting metric, which is why this is the formula most writers encounter first.

Both Flesch formulas penalise long sentences and polysyllabic words. They are sensitive to short sentences and reward direct prose. The formulas were calibrated on English and break down on other languages without adaptation.

Gunning Fog Index

Robert Gunning developed his Fog Index in 1952 as a tool for business writing, published in The Technique of Clear Writing (1952, second edition 1968). The formula is:

0.4 ((words / sentences) + 100 (complex words / words))

Gunning defined "complex words" as words of three or more syllables, excluding proper nouns, jargon, and compound words made from two simple words. This exclusion makes Gunning Fog slightly more forgiving of certain vocabulary than Flesch-Kincaid: a paragraph full of named places or standard compound terms will not score as difficult as a paragraph full of abstract three-syllable concept words.

The output is also a grade level number, but Gunning intended his index specifically as a measure of suitability for casual business and journalism readers. A Fog Index of 7 to 8 is ideal for general business writing. A score above 12 is typically considered too dense for general audiences. TheWall Street Journal and The Times typically run between 10 and 12 on the Gunning scale; this is the target band for non-specialist news writing.

SMOG Index

G. Harry McLaughlin developed the SMOG Index in 1969 to provide more accurate readability scores for healthcare and adult literacy materials. He published the formula in the Journal of Reading in 1969. SMOG stands for "Simple Measure of Gobbledygook." The formula is:

1.0430 sqrt(complex_words (30 / sentences)) + 3.1291

SMOG counts polysyllabic words (three or more syllables) in a sample of 30 sentences. McLaughlin's design goal was a formula that correlated well with "100 percent comprehension," meaning a grade level where readers fully understand the text rather than just grasping the gist. SMOG is the formula most widely used in US healthcare for patient education materials, because the Federal Centers for Disease Control and Prevention recommend it for health communications.

SMOG tends to produce higher grade-level scores than Flesch-Kincaid for the same passage, sometimes by two or three grades. This is by design: the higher score reflects a more conservative threshold for what counts as "readable at this grade level." If a passage scores SMOG grade 9, then a ninth grader should be able to understand it fully, not partially.

Coleman-Liau Index

Meri Coleman and T. L. Liau developed their formula in 1975, published in the Journal of Applied Psychology. The distinctive feature of Coleman-Liau is that it does not count syllables at all. It uses character count per word instead:

0.0588 L - 0.296 S - 15.8

Where L is the average number of letters per 100 words and S is the average number of sentences per 100 words. The choice of character count over syllable count was practical: when Coleman and Liau were working, syllable counting was harder to automate than character counting. For modern text-processing tools, both are easy, but Coleman-Liau retains the advantage of working consistently regardless of how the syllable counter handles edge cases like contractions and hyphenated words.

Coleman-Liau scores tend to fall between Flesch-Kincaid and SMOG for the same passage. It is less sensitive to long words than SMOG and less sensitive to long sentences than Flesch-Kincaid. Where the other formulas disagree, Coleman-Liau usually sits in the middle, which makes it a useful tiebreaker when you want a single number to summarise three different scores.

When to use which

For general writing advice, Flesch-Kincaid Grade Level is the most widely understood and the most commonly reported. If you need to communicate a readability target to a non-specialist, Flesch-Kincaid is the formula most people recognise.

For business and journalism writing aimed at a general adult audience, Gunning Fog is the historically appropriate measure. The target band of 7 to 12 on the Fog scale corresponds well to what mainstream newspapers actually publish.

For healthcare information, patient education materials, or any communication where comprehension is essential rather than nice to have, SMOG is the established choice. The US National Library of Medicine, the CDC, and the UK National Health Service style guides all reference SMOG.

For automated text analysis at scale, Coleman-Liau is robust because it does not depend on syllable counting heuristics. It also tends to give stable scores on highly technical text (with many proper nouns and acronyms) where syllable-based formulas can drift.

Target levels by genre

Hemingway's prose famously scores around Flesch-Kincaid grade 4 to 5, which is the readability of a young reader's chapter book. This is not an accident of subject matter; Hemingway worked at short sentences and simple words deliberately.

Modern fiction targeting adult general readers (think contemporary literary novels) typically sits in the grade 7 to 9 range. Genre fiction, especially thrillers and romance, often scores lower (grade 6 to 8) because shorter sentences carry more of the pace.

Newspaper journalism aimed at a broad readership usually sits between grade 8 and grade 11, depending on publication. Tabloid newspapers cluster around grade 7 to 9; mid-market broadsheets around grade 9 to 11; the quality broadsheets (the Financial Times, the Economist) often run grade 12 or higher.

Academic writing for journals routinely scores grade 14 to 18, which corresponds to undergraduate or postgraduate reading levels. This is appropriate for the audience but is one of the reasons academic writing is often inaccessible to general readers, and why plain-language summaries of academic work have value.

Government plain-language guidance often targets grade 8 (the UK Government Digital Service style guide) or grade 6 to 8 (the US Plainlanguage.gov guidance). The lower targets reflect the goal of communicating with the widest possible adult population, including those with lower formal literacy.

Marketing copy for general consumer audiences typically targets grade 6 to 8. Anything more complex slows the reader and reduces conversion. The actual scores of long-running brand websites tend to confirm this: Apple's product copy hovers around grade 7, while business-to-business technology companies often write at grade 11 or 12 because their audience expects more density.

Practical tips for lowering a reading level

Most prose lowers a grade or two with two changes: split long sentences, and replace one or two polysyllabic words per paragraph with shorter alternatives. A 35-word sentence reliably scores higher than two 17-word sentences carrying the same meaning. "Utilise" can almost always become "use"; "demonstrate" can become "show"; "additional" can become "more".

Resist the temptation to mechanically strip every long word. Some polysyllabic words are exactly right for their meaning, and the score chasing produces prose that reads as childish rather than clear. The goal is the kind of prose Flesch praised in 1949: "the prose people want to read, written in the words they actually use."

What to do next

Score your own writing across all four formulas with the Phrasit reading level analyzer, check the prose for sentence-length and word-choice issues with the grammar checker, and compare before-and-after edits with the text comparator to see exactly which sentences moved the score.