Readability scorer: Flesch, Flesch-Kincaid, Gunning Fog and SMOG explained

A readability score answers one question: how hard is this text to read on first pass. It does not judge whether the writing is good, true or well organised. It counts two things, sentence length and word length, and turns them into a number. The free Cubbbix Readability Scorer gives you four of the standard numbers at once, so you can see where the difficulty sits instead of trusting a single grade.

Flesch Reading Ease, published by Rudolf Flesch in 1948, runs from 0 to 100, and higher is easier: 206.835 minus 1.015 times average sentence length, minus 84.6 times average syllables per word. A score of 60 to 70 is plain English. Flesch-Kincaid Grade, recalculated by Kincaid and colleagues in 1975 for the US Navy, turns the same two counts into a US school grade: 0.39 times average sentence length, plus 11.8 times average syllables per word, minus 15.59. Grade 8 means an eighth-grade reader can follow it.

Figure 1: Four scores from one paste, so you can see whether sentences or words are the problem.

Fog and SMOG: the long-word scores

Gunning Fog, from Robert Gunning in 1952, is 0.4 times (average sentence length plus 100 times complex words divided by all words), where complex means 3 or more syllables. It estimates years of formal education needed to read the text on first reading. SMOG, from Harry McLaughlin in 1969, is 1.043 times the square root of (complex words times 30 divided by sentences), plus 3.1291. It was designed to be counted by hand across 30 sentences, so on a short passage it is a rough guide, and the tool says so on the page.

Read the four together. If Reading Ease says easy but Fog says hard, your sentences are short but your words are long. If the grade says 12 but your readers are general readers, shorten sentences first: cutting a 30-word sentence in two moves every score more than swapping one long word does. Aim for grade 8 or below for general web writing, and grade 6 or below for public information that everyone must understand.

Figure 2: The tool lists your longest sentences, because one long sentence is usually the fastest fix.

What the scores cannot see

These formulas were built for English school and training texts. They do not work for other languages, and scores on non-English text are meaningless. They cannot see whether your argument holds, whether your facts are right, or whether your tone fits the reader. A text can score grade 6 and still be wrong. Use the score as a check on clarity after you have written something true, not as a target to game by chopping every sentence to five words.

Figure 3: Shorten sentences first. It moves every score more than swapping single words.

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