Flesch-Kincaid and Readability Scores: What They Mean for Your Writing

Readability scores condense the complexity of your writing into a single number. They're useful, widely used, and frequently misunderstood. Here's how the major formulas work, what the numbers actually mean, and — importantly — when you should stop caring about them.

A Brief History of Readability Research

The formal study of readability began in the 1920s, when educators started asking why some textbooks were comprehensible to students and others weren't. Early researchers — including Mabel Vogel and Carleton Washburne in the 1920s and Rudolf Flesch in the 1940s — developed the insight that two text characteristics predict comprehension difficulty more than anything else: word length (as a proxy for vocabulary difficulty) and sentence length (as a proxy for syntactic complexity).

From this insight grew a family of readability formulas that calculate complexity scores from these two measurable variables. The formulas differ in exactly how they weight these factors and what units they output, but the underlying logic is essentially the same across all of them.

The Flesch Reading Ease Score

Developed by Rudolf Flesch in 1948 and later refined with J. Peter Kincaid for the U.S. Navy, the Flesch Reading Ease score remains the most widely used readability formula. It produces a score from 0 to 100:

Higher scores mean easier reading. Here's how to interpret the scale:

For most web content intended for a general audience, a score between 60 and 70 is considered the plain English target. The U.S. government's plain language guidelines recommend writing at the 8th-grade reading level for public-facing documents.

Flesch-Kincaid Grade Level

The Flesch-Kincaid Grade Level formula, developed in 1975 for the U.S. Navy to assess military technical manuals, converts the same inputs into a U.S. school grade level:

A score of 8.0 means the text is readable by an average 8th-grade student (approximately 13–14 years old). Most major style guides — including the federal plain language guidelines and the Associated Press Style Guide — recommend targeting a grade level of 8 or below for general audiences. Consumer-facing content often targets grade 6.

An important note: these grade levels are not a measure of audience intelligence. They're a measure of reading difficulty. Writing at a 6th-grade level means using clear, direct language — which is more cognitively efficient for everyone, including expert readers, not just struggling ones.

Gunning Fog Index

Robert Gunning developed the Fog Index in 1952. It emphasizes the use of "complex words" — words with three or more syllables — as a measure of complexity:

The result is a grade level. A score of 12 corresponds to high school senior level; scores above 17 are considered extremely difficult. The Wall Street Journal targets a Fog Index around 11. Reader's Digest averages about 8.

The Fog Index is particularly useful for business and technical writing, where the temptation to use jargon-heavy multisyllabic words is strong. If your Fog score is creeping above 12 in a document intended for a broad audience, you probably need to simplify your vocabulary.

SMOG Formula

SMOG (Simple Measure of Gobbledygook) was developed by G. Harry McLaughlin in 1969 and is widely used in health communications. It counts polysyllabic words (three or more syllables) in a sample of 30 sentences:

SMOG is considered more accurate than Flesch-Kincaid for health literacy applications, because health comprehension is especially sensitive to vocabulary complexity. Many health communication guidelines recommend targeting a SMOG grade of 6 or lower for patient-facing documents.

Dale-Chall Readability Formula

The Dale-Chall formula takes a different approach: instead of using syllable count as a proxy for word difficulty, it references a list of 3,000 common words that 80% of 4th graders can reliably read. Words not on this list are counted as "difficult words." The formula then combines the percentage of difficult words with average sentence length.

Dale-Chall is considered more accurate than syllable-based formulas for evaluating educational materials, because syllable count is an imperfect proxy for familiarity. "Butterfly" has three syllables but is easy; "zeal" has one syllable but might be unfamiliar to young readers.

What Actually Drives These Scores

Since almost all readability formulas depend on sentence length and word length, improving your score means:

• Shorter sentences: Aim for an average of 15–20 words per sentence for general audiences. Break long, complex sentences into two shorter ones.
• Shorter words: Prefer "use" over "utilize," "show" over "demonstrate," "help" over "facilitate." Single-syllable Anglo-Saxon words are almost always clearer than their multi-syllable Latinate equivalents.
• Active voice: Active sentences tend to be shorter and more direct than passive constructions.
• Avoiding unnecessary hedging: Phrases like "it should be noted that" and "in order to" add syllables and sentence length without adding meaning.

The Limitations: What Readability Scores Cannot Measure

Readability formulas are useful tools with real limitations. They cannot measure:

• Clarity of argument: A text can have simple words and short sentences but still be logically incoherent or misleading.
• Logical structure: Good writing moves the reader through ideas in a sequence that makes sense. Readability scores are blind to whether the organization of ideas is clear or chaotic.
• Appropriate vocabulary for the audience: A medical journal article with a high grade level isn't poorly written — it's appropriately written for an expert audience. Context always matters more than the score.
• Engagement: Writing can score perfectly on the Flesch scale and be deeply boring. Readability and engaging writing are not the same thing.

Use readability scores as a diagnostic, not a goal. If your score is significantly higher than your target audience would need, that's a signal to simplify. But optimizing your writing toward a score — shortening every sentence mechanically, replacing every long word — will produce writing that is technically readable but hollow. The goal is always clear communication, and a readability score is just one data point toward that goal.