Flesch Reading Ease

Rudolf Flesch developed this formula in 1948 to measure reading ease using sentence length and syllables per word. It outputs a score from 0 to 100, where higher scores mean easier text.

At a Glance

Output	0–100 ease score (higher = easier)
Best for	General English prose, adult audiences
Method	r.flesch()
What it counts	Words per sentence, syllables per word
Minimum text	100 words (default)

The score runs backwards

Flesch is the only metric in this library where a higher score means easier text. Every other metric works the opposite way: higher scores mean harder text or a higher grade level. New users get this backwards almost every time.

When to Use This Metric

• You need a widely recognized score that non-technical stakeholders can understand. Flesch is the most cited readability formula in the world, built into Microsoft Word since the early 1990s and referenced in legal requirements across multiple US states.
• You are checking whether body text is appropriate for a general adult audience. A score between 60 and 70 is standard for most adult readers.
• You must meet a legal threshold. Florida requires a score of 45 or higher on life insurance policies. The US Department of Defense uses Flesch scores for its documents and forms.
• You want a quick reading ease check without needing a precise grade level. Use Flesch-Kincaid if you need a US grade level from the same formula.

How It Works

Flesch counts two things: the average number of words per sentence, and the average number of syllables per word. Text with short sentences and simple words scores high. Text with long sentences and complex words scores low.

score = 206.835 − (1.015 × words per sentence) − (84.6 × syllables per word)

The syllable coefficient (84.6) is 83 times larger than the sentence-length coefficient (1.015). In practice, Flesch is almost entirely a measure of word complexity, not sentence length. Cutting sentence length has a small effect on the score. Using simpler words has a large one.

Score Interpretation

Score	Ease	Grade level	What this looks like
90–100	Very Easy	Grade 5	Children’s picture books
80–89	Easy	Grade 6	Most children’s books
70–79	Fairly Easy	Grade 7	Popular fiction
60–69	Standard	Grades 8–9	Most news articles
50–59	Fairly Difficult	Grades 10–12	Academic writing
30–49	Difficult	College	Legal documents
0–29	Very Confusing	College graduate	Dense academic prose

Scores outside 0–100 are possible. Dense technical text can score negative. Very simple text can score above 100. The library does not clamp these values.

Return Values

r.flesch() returns a FleschResult object:

Field	Type	Description
score	float	Raw Flesch score, typically 0–100
ease	str	One of: "very_easy", "easy", "fairly_easy", "standard", "fairly_difficult", "difficult", "very_confusing"
grade_levels	list[str]	US grade levels for this score range, e.g. ["8", "9"] or ["college"]
grade_level	str	The primary grade level (first item in grade_levels)

Minimum Requirements

The library defaults to a 100-word minimum. Passing fewer words raises a ValueError. You can lower this threshold with Readability(text, min_words=50), but scores from short texts are less reliable.

Limitations

• Flesch is primarily a vocabulary measure, not a sentence measure. The formula weights syllable count 83× more than sentence length. Simplifying sentence structure while keeping complex vocabulary will barely move the score.
• High scores do not mean clear or appropriate writing. Repetitive text, missing context, or condescending tone can all score high. The formula measures linguistic complexity, not whether readers actually understand the content.
• Prose only. The formula assumes connected sentences. Tables, bullet lists, code blocks, and forms produce unreliable scores because sentence boundaries are ambiguous.
• English only. The formula’s constants and syllable-counting rules are calibrated for English. Scores on other languages are not meaningful.
• Syllable counting is approximate. The library uses an algorithmic counter. Technical terms, proper nouns, and words with irregular pronunciation may be miscounted.

Example

from readscore import Readability

text = """
Plain language means writing that your audience can understand the first time they
read it. Short sentences help. Common words help more. You do not need to simplify
your ideas — just the way you express them. Jargon and long sentences slow people
down and make them feel excluded. Simple writing is not dumbed-down writing. It is
respectful writing.
"""

r = Readability(text)
result = r.flesch()

print(f"Score: {result.score:.1f}")        # Score: ~74.3
print(f"Ease: {result.ease}")              # fairly_easy
print(f"Grades: {result.grade_levels}")   # ['7']

See Also

• Flesch-Kincaid Grade Level — same inputs, outputs a US grade level directly instead of an ease score
• Gunning Fog — also uses sentence length and word complexity, but counts polysyllabic words rather than average syllables per word
• Choosing a Metric — decision guide for all nine metrics