Readability scores are one of those metrics that most writers know exist but very few use correctly. The two most common — Flesch-Kincaid Reading Ease and Gunning Fog Index — measure fundamentally different things and are useful in different contexts. Here's what they actually mean and when each one matters.
Flesch-Kincaid Reading Ease (the 0–100 scale)
The formula:
206.835 − (1.015 × avg words per sentence) − (84.6 × avg syllables per word)
The higher the score, the easier the text. The thresholds:
| Score | Level | Example |
|---|---|---|
| 90–100 | Very Easy | Children's books, simple instructions |
| 70–80 | Easy | Plain-language guidelines, customer emails |
| 60–70 | Standard | News articles, company blogs |
| 50–60 | Fairly Difficult | Trade publications |
| 30–50 | Difficult | Academic papers |
| 0–30 | Very Difficult | Legal documents, scientific journals |
Most online content aimed at a general audience should score 60–70. Developer documentation often scores 50–65. Marketing copy for consumer products should score 70+.
Flesch-Kincaid Grade Level (the school grade scale)
The grade level formula:
0.39 × avg words per sentence + 11.8 × avg syllables per word − 15.59
A score of 8 means the text is readable by an 8th grader. A score of 14 roughly corresponds to college level. The same text that scores 65 on Reading Ease might score at grade 8.
Important caveat: grade level is not a quality judgement. "Write at a 6th grade level" does not mean your content should be simple — it means your sentences should be short and your word choices direct. Ernest Hemingway's "The Old Man and the Sea" scores at a grade 4 level. Academic prose often scores at grade 16+ — which is why people find it exhausting to read.
Gunning Fog Index
The formula:
0.4 × (avg words per sentence + % of complex words)
Where "complex words" = words with 3 or more syllables (excluding proper nouns, compound words, and common verb forms).
Gunning Fog is more sensitive to jargon than Flesch-Kincaid, because it specifically targets polysyllabic words. The target for most content is a Fog Score under 12.
| Fog Score | What it means |
|---|---|
| < 8 | Easy — accessible to most readers |
| 8–12 | Ideal for general audiences |
| 12–14 | Difficult — academic/technical |
| > 14 | Very hard — technical specialists only |
When scores disagree
If Flesch Reading Ease is low (hard) but Gunning Fog is also high — you have both long sentences AND lots of complex words. Both need attention.
If Reading Ease is okay but Fog is high — your sentences are short but you're using too much technical jargon. Consider simplifying terminology or adding explanations.
If Reading Ease is low but Fog is acceptable — your words are simple but your sentences are too long. Break them up.
Real-world benchmarks
Here's how some known publications score on Flesch Reading Ease:
- The New York Times: 60–65
- Wikipedia (average): 50–55
- Harvard Business Review: 45–55
- Scientific journals: 25–35
- Legal contracts: 20–30
- UK government guidance (target): 65+
For developer documentation specifically, aim for 50–65. You'll use technical terms that lower the score, but keeping sentences short compensates.
What actually improves readability
The formulas respond to two primary variables: sentence length and word complexity. In practice:
Improve sentence length:
- Break sentences over 25 words into two
- Use bullet points for lists of three or more items
- Put one idea per sentence
Reduce word complexity:
- Replace "utilise" with "use"
- Replace "implement" with "build" or "add"
- Replace "functionality" with "features"
- Define jargon the first time you use it
What doesn't help: choosing short words that still have multiple syllables. "Operationalise" and "identification" are both polysyllabic even though they may feel more "accessible" than technical terms.
Using readability scores in practice
The scores are best used as diagnostics, not targets. The workflow:
- Write naturally, aiming for clarity
- Check scores at the end (free readability checker)
- If Fog Score > 14: look for sentences with 3+ polysyllabic words and simplify
- If Grade Level > 12 for general-audience content: find your longest sentences and split them
Don't write to a score. A short, punchy paragraph can score badly if it uses technical terms. A long, flowing description can score well if it uses common words. The score is a signal, not a goal.
Special cases
Technical documentation: aim for Fog < 14, Grade Level 10–12. You need technical terms; keep sentences short to compensate.
Marketing copy: aim for Reading Ease 70+, Grade Level < 8. Short sentences, active voice, concrete nouns.
Legal/compliance text: scores don't matter as much as precision — but if you can rewrite without sacrificing accuracy, do it.
Headlines: readability scores don't apply well to very short text. Test the body, not isolated headings.
Readability scores are most useful when you're stuck on a piece and can't identify why it feels heavy. Run it through a checker, and the Fog Score usually tells you immediately: too many polysyllabic words per sentence. Fix that, and the clarity improves even if the score looks the same.
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