If you have ever wondered whether your writing is too complex, or whether your documentation is actually readable, the Flesch-Kincaid readability formula has the answer. Here is what it measures, how to interpret the score, and what to do about it.
What is the Flesch-Kincaid formula?
Rudolf Flesch developed the original readability formula in 1948 while working for the Associated Press. J. Peter Kincaid adapted it for the US Navy in 1975 to evaluate training manuals. The result was two related metrics:
Flesch Reading Ease score:
``
RE = 206.835 - (1.015 × ASL) - (84.6 × ASW)
`
Where:
- ASL = average sentence length (words per sentence)
- ASW = average syllables per word
The score runs from 0 to 100. Higher = easier.
Flesch-Kincaid Grade Level:
<code>
<p>GL = (0.39 × ASL) + (11.8 × ASW) - 15.59</p>
</code>
Same inputs, different weights. The output maps directly to US school grades — Grade 8 means an average 8th grader can understand it.
How to read the scores
| Reading Ease | Grade Level | What it means |
|---|---|---|
| 90–100 | 5th grade | Very easy. Comics, simple instructions. |
| 70–80 | 6–7th grade | Easy. Plain English. Many newspapers. |
| 60–70 | 8–9th grade | Standard. Most web content targets here. |
| 50–60 | 10–12th grade | Fairly difficult. Academic writing, corporate comms. |
| 30–50 | College | Difficult. Academic journals. |
| 0–30 | Professional | Very difficult. Legal documents, medical literature. |
For most web content — documentation, blog posts, marketing copy — a reading ease of 60–70 and grade level of 8–10 is the target.
Famous publications as benchmarks:
- The Sun (tabloid): Reading Ease ~65, Grade 8
- The New York Times: ~60, Grade 10
- Harvard Law Review: ~32, Grade 16+
- IKEA instructions: ~80+, Grade 6
What drives complexity
Looking at the formula, only two things matter: sentence length and word length (in syllables).
Sentence length is the biggest lever. A 30-word sentence contributes far more to complexity than a 15-word sentence with a few long words. When you split a long sentence in two, the score improves noticeably.
Word length (syllables) is the second factor. The formula doesn't care about word frequency or meaning — it counts syllables. "Approximately" (5 syllables) scores worse than "about" (1 syllable) regardless of context.
This means:
- Short sentences + short words = high readability
- Long sentences + long words = low readability
The formula can be gamed (write nothing but "The dog ran." sentences) but it correlates well with comprehension in practice.
What the formula doesn't measure
Flesch-Kincaid is a formula, not a human. It doesn't evaluate:
- Clarity — whether the idea itself is clearly expressed
- Logic — whether the argument follows
- Vocabulary — whether you're using the right word, just the shorter one
- Tone — formality, warmth, authority
- Structure — paragraph organisation, headings, visual hierarchy
A text can score Grade 6 and still be incomprehensible if the ideas are jumbled. A Grade 12 text can be perfectly clear if the complexity serves the subject.
Use readability scores as a signal, not a target. If your grade level is consistently above 14 for content aimed at general readers, that's a useful diagnostic.
Other readability metrics
Most readability tools return multiple scores. Here's what each adds:
Gunning Fog Index: Similar to FK Grade Level but focuses on words with 3+ syllables ("complex words"). More sensitive to technical jargon. Formula: 0.4 × (ASL + percentage of complex words)`. Newspaper writing targets a Fog of 8–12.
SMOG Index: "Simple Measure of Gobbledygook" — counts polysyllabic words per 30 sentences. Used widely in health literacy assessments. More conservative than FK in estimating grade level.
Coleman-Liau Index: Uses characters per word rather than syllables — more consistent for programmatic text analysis where syllable counting is imprecise.
Automated Readability Index (ARI): Uses characters per word and words per sentence. Calibrated for children's reading materials. Often returns higher grade levels than FK for the same text.
In practice, the FK Reading Ease and FK Grade Level are the two most referenced metrics. Running multiple scores and averaging them gives a more robust estimate than any single formula.
Checking your writing's readability
You can check the reading level of any text using an online readability checker — paste your text and get all six scores at once. No signup, no character limit, runs in the browser.
Practical improvements
When your readability score is too low:
1. Split long sentences. Find every sentence over 25 words and break it in two. This is the single highest-impact change.
2. Replace multi-syllable words with shorter equivalents:
- "approximately" → "about"
- "utilise" → "use"
- "endeavour" → "try"
- "commence" → "start"
- "demonstrate" → "show"
3. Cut passive voice. "The report was written by the team" → "The team wrote the report." Active voice is shorter and more direct.
4. Break up paragraphs. For web content, aim for 2–4 sentences per paragraph. Short paragraphs increase perceived readability even when individual sentences are complex.
5. Remove redundant qualifiers. "Very unique", "absolutely essential", "completely finished" add syllables without adding meaning.
6. Use subheadings. Not captured by the formula but dramatically improves reader navigation and comprehension, especially for skimmers.
The right target
Different audiences warrant different targets:
- Technical documentation for developers: Grade 10–12 is acceptable and expected — your audience reads this stuff for a living.
- Blog posts and marketing copy: Grade 8–10, Reading Ease 60–70.
- Healthcare content for patients: Grade 6 or lower, Reading Ease 70+.
- Legal content that must be understood by the general public: Grade 8 target (many jurisdictions now require this).
The US government's plain language guidelines recommend Grade 8 for all public-facing documents. The NHS aims for Grade 7.
Readability formulas are over 70 years old and still widely used because they capture something real — the cognitive load of parsing long sentences and complex vocabulary. Use them as a calibration tool, not a constraint, and your writing will almost always be clearer for it.
Originally published at https://snappytools.app/readability-checker/