Readability Scores Explained: What Flesch-Kincaid Measures — and What It Misses

Readability scores measure sentence length and syllable count — not whether anything is actually clear

The Flesch Reading Ease score, Flesch-Kincaid Grade Level, Gunning Fog Index, SMOG Grade, Coleman-Liau Index — these formulas have been used to evaluate writing since the 1940s. They're in word processors, content management systems, and SEO tools. They're also built on a narrow proxy for readability that misses most of what actually makes writing easy or difficult to understand.

Understanding what readability scores measure — and what they don't — turns them from black-box verdicts into useful diagnostic tools.

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How the formulas work

Flesch Reading Ease

The most widely used formula, developed by Rudolf Flesch in 1948:

RE = 206.835 − (1.015 × ASL) − (84.6 × ASW)

Where:

• ASL = average sentence length (words per sentence)
• ASW = average syllables per word

Score interpretation:

Score	Level	Example reading material
90–100	Very easy	5th grade; comics, simple prose
70–80	Easy	6th–7th grade; popular fiction
60–70	Standard	8th–9th grade; newspapers
50–60	Fairly difficult	10th–12th grade; academic writing
30–50	Difficult	College level; professional journals
0–30	Very difficult	Graduate level; legal, technical documents

Flesch-Kincaid Grade Level

Converts the same inputs to a US school grade level:

FKGL = (0.39 × ASL) + (11.8 × ASW) − 15.59

A score of 8 means the text is appropriate for 8th-grade readers (approximately 13–14 years old).

Gunning Fog Index

GFI = 0.4 × (ASL + PHW)

Where PHW = percentage of "hard words" (words with 3+ syllables, excluding proper nouns, hyphenated words, and words where the third syllable is -ed or -es).

SMOG Grade

Designed specifically for health communication, considered more accurate for higher-difficulty texts:

SMOG = 3 + √(polysyllable count)

Where polysyllable count is the number of words with 3+ syllables in 30 sentences.

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What these scores actually measure

Every readability formula is a variation on the same two variables: sentence length and word length (measured by syllables or characters).

These variables are proxies for reading difficulty because:

• Longer sentences require holding more context in working memory
• Longer words are statistically rarer and thus less familiar to most readers

But they're imperfect proxies in well-documented ways.

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What readability scores get wrong

They don't measure vocabulary familiarity

A short technical sentence can be harder to understand than a long familiar one.

"Configure the SSL certificate's Subject Alternative Names to include your wildcard domain." — 14 words, relatively short sentence, but requires understanding of SSL, certificates, Subject Alternative Names, and wildcard domains.

"The bank will check your credit score before deciding whether to approve your application for a loan." — 18 words, longer sentence, but widely understood vocabulary.

Flesch-Kincaid grades the second sentence as harder. A general audience would find the first harder.

They're sensitive to jargon that isn't actually complex

Single-syllable technical terms score as easy ("TCP," "HTTP," "API") even when they're not. Multi-syllable common words score as hard ("comprehension," "comfortable," "conversation") even when they're not.

They penalise necessary complexity

Some content is inherently technical. A tutorial on database indexing written for experienced developers that achieves a "very easy" Flesch score might simply be oversimplified or stripping necessary precision. The score optimised for a general audience tells you nothing about whether the content serves its actual audience.

They ignore coherence and structure

Parallel structure, clear transitions, logical flow, well-placed topic sentences, and consistent point of view all affect readability significantly. None are captured by any formula.

They don't account for domain knowledge

Reading difficulty is relative to the reader's background. A haematologist reads a haematology paper at a very different difficulty level than a general practitioner reads the same paper. Formulas produce single numbers as if all readers are equivalent.

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What readability scores are useful for

Despite their limitations, readability formulas serve specific legitimate purposes:

Consistency checking: if a piece of writing has wildly varying sentence complexity across sections, it may indicate inconsistent voice or editing. Flesch-Kincaid scores by paragraph can reveal structural inconsistency.

Simplification targeting: when writing specifically for low-literacy audiences (public health communications, simplified legal notices, emergency instructions), readability formulas provide a verifiable target. Health literacy research confirms that simpler text has higher comprehension rates in general populations.

Early draft diagnosis: a first draft with an extremely high Fog Index (dense, polysyllabic sentences) may benefit from review even if the score itself isn't the end goal. The formula identifies a pattern worth looking at.

SEO tool context: many SEO tools include readability scores. In this context, they're a proxy for "would a general audience find this accessible?" — imperfect but directionally useful.

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Reading speed and the 238 words per minute myth

Most reading time estimates use a figure of around 200–250 words per minute for average adult reading speed. This number appears consistently because it comes from large studies — but it hides enormous variance.

Research-based figures:

• Silent reading rate for average adult: 238–264 wpm (multiple meta-analyses)
• Range: skilled readers read 300–700 wpm; struggling readers under 150 wpm
• Comprehension decreases at higher speeds (speed reading training that claims retention at 600+ wpm is not well-supported by research)
• The same person reads different texts at different speeds — technical material slower, familiar narrative faster

The reading time estimate in a word counter ("5 min read") uses the average speed applied to the word count. It's a rough estimate that varies by 2–3× depending on the actual reader and content type.

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How to use the Word & Character Counter on sadiqbd.com

1. Paste your text — any length of content
2. Read the counts:
   • Word count — primary metric for most writing contexts
   • Character count with and without spaces — for character-limited contexts (Twitter, SMS, meta descriptions)
   • Sentence count — useful for calculating average sentence length
   • Paragraph count
   • Estimated reading time — based on average reading speed
3. Use for calibration — compare word count against platform requirements or editorial targets

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Frequently Asked Questions

What's the ideal word count for a blog post? It depends on the topic and the competition. Informational posts covering complex topics thoroughly often rank better with 1,500–2,500 words. Simple how-to posts may rank well at 500–800 words. The right word count is "enough to cover the topic completely without padding." Word count is a side effect of thorough coverage, not a target to hit.

Why do readability scores vary so much between tools? Different tools implement the formulas slightly differently (particularly around sentence boundary detection, syllable counting methods, and handling of abbreviations). A score of 65 in one tool might be 62 in another for the same text. Use them directionally, not as precise measurements.

Should I optimise for readability scores in SEO? Readability is one factor in content quality — overly dense, jargon-heavy writing has lower engagement. But optimising specifically for Flesch-Kincaid at the expense of accuracy or depth serves neither readers nor search engines. Write for your actual audience; use readability scores as a sanity check, not a target.

Is the Word & Character Counter free? Yes — completely free, no sign-up required.

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Word count tells you how long something is. Readability scores tell you something about how it's written. Neither tells you whether it's actually good — that requires a human reader with appropriate domain knowledge.

Try the Word & Character Counter free at sadiqbd.com — count words, characters, sentences, and reading time for any text instantly.