Readability Scores Explained
Flesch-Kincaid, Gunning Fog, SMOG, and Coleman-Liau — how readability formulas work and when to use each.
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Readability Scores Explained Readability scores estimate how difficult a text is to read by measuring sentence length, word length, and syllable count. They don't measure whether content is good — only whether it's linguistically accessible to a given audience. Understanding the four main formulas helps you choose the right one and interpret results without over-indexing on a single number. --- Why Readability Scores Exist The formulas were developed in the mid-20th century for practical, measurable purposes: Rudolph Flesch developed his Reading Ease formula in 1948 to help writers simplify language for general readers Robert Gunning created the Fog Index in 1952 to help business writers assess whether their prose was too dense G. Harry McLaughlin developed SMOG in 1969 specifically for…
Frequently Asked Questions
What is a readability score?
A readability score is a numeric estimate of the education level or age needed to understand a piece of text. It's calculated from measurable surface features — sentence length, word length, and syllable count — rather than meaning or content quality.
What is the Flesch Reading Ease formula?
Flesch Reading Ease = 206.835 − (1.015 × average sentence length) − (84.6 × average syllables per word). Scores range from 0 (very difficult) to 100 (very easy). Most web content aims for 60–70, which corresponds to roughly a 7th–8th grade reading level.
What is Gunning Fog index?
Gunning Fog = 0.4 × (average words per sentence + percentage of complex words). 'Complex' words have 3+ syllables. The result approximates the US grade level needed to read the text. Fog scores above 12 are considered difficult for general audiences; most successful business writing targets 8–10.
What readability level should web content aim for?
Web content targeting a general audience should aim for Flesch-Kincaid Grade Level 6–8 (roughly middle school). The US government's plain language guidelines recommend grade 8 as the maximum for public-facing content. Technical documentation for specialists can target grade 12+.
How do I improve readability?
Shorten sentences (aim for 15–20 words average), use common words instead of multi-syllable alternatives, break up long paragraphs, and use active voice. The most impactful change is usually splitting run-on sentences — sentence length has the largest weight in most readability formulas.
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