Dale-Chall Readability Formula

Edgar Dale and Jeanne Chall developed this formula in 1948 to measure reading difficulty by checking whether words are familiar to a typical 4th-grade reader, rather than counting syllables or characters.

At a Glance

Output	Raw score (4–10+) that maps to grade bands
Best for	Educational materials, patient health content, general adult audiences
Method	r.dale_chall()
What it counts	Unfamiliar words (not on the 3,000-word list), words per sentence
Minimum text	100 words (default)

When to Use This Metric

• You are writing for a general adult audience and vocabulary familiarity is the primary concern. Dale-Chall has more validation research for educational and patient materials than any other formula in this library.
• You are checking patient education materials, health pamphlets, or consumer health information. Multiple systematic reviews name Dale-Chall as the highest-validity formula for these use cases.
• You are assessing government documents or public information materials intended for general adult comprehension.
• You need to check text aimed at grade 4 and above. For younger readers (grades 1–3), use Spache instead.

How It Works

Dale-Chall checks each word in the text against a list of approximately 3,000 words that 80% of American 4th-grade students recognized during data collection. Words not on the list are counted as “difficult.” The formula then combines the percentage of difficult words with average sentence length.

raw_score = 0.1579 × (difficult_words / total_words × 100) + 0.0496 × (total_words / sentences)

If more than 5% of words are difficult, the formula adds a fixed adjustment:

adjusted_score = raw_score + 3.6365

The score can appear to jump discontinuously

When the percentage of difficult words crosses the 5% threshold, the formula adds 3.6365 to the raw score. This is built into the formula, not an error. A text with 4.9% difficult words will score noticeably lower than a nearly identical text with 5.1% difficult words. The adjustment reflects the non-linear reality that frequent unfamiliar words disrupt reading fluency in ways a linear formula cannot capture.

The word list

The library implements the “New Dale-Chall” — the 1948 formula coefficients combined with the revised 1995 word list (Chall & Dale, Readability Revisited). The 1995 revision expanded the list from about 769 words to approximately 3,000.

Words on the list in their base form are also familiar in these inflected forms: regular plurals (-s, -es), past tense (-ed), progressive (-ing), comparative (-er), and superlative (-est). Derivational suffixes that create new words are not covered. For example, complete is on the list, but completion is counted as difficult.

Proper nouns (names of people and places) are treated as familiar regardless of whether they appear on the list.

Score Interpretation

Dale-Chall scores do not map to individual grade levels. They map to grade bands. The formula cannot distinguish between grade 1 and grade 4 text; both fall in the same band.

Raw Score	Grade Band	Reading Level
4.9 and below	Grades 1–4	Elementary school
5.0–5.9	Grades 5–6	Upper elementary
6.0–6.9	Grades 7–8	Middle school
7.0–7.9	Grades 9–10	Early high school
8.0–8.9	Grades 11–12	Late high school
9.0–9.9	College	Undergraduate
10.0 and above	College graduate	Graduate/professional

A score of 7.5, for example, places a text at approximately grades 9–10. The grade_levels field returns this band as a list: ["9", "10"].

Return Values

r.dale_chall() returns a DaleChallResult object:

Field	Type	Description
score	float	Raw Dale-Chall score, typically 4–10
grade_levels	list[str]	Grade band for this score, e.g. ["9", "10"] or ["college"]
grade_level	str	First item in grade_levels (the lower bound of the band)

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

• Does not work well for technical or software documentation. Words like “cookie,” “stream,” “execute,” “crash,” and “enter” appear on the familiar-word list in their everyday English senses. A page about browser cookies or command-line syntax will score as easier than it actually is for readers without that domain knowledge.

The polysemy problem

The Dale-Chall word list contains words, not word senses. “Cookie” (the baked good) and “cookie” (the browser storage mechanism) receive the same score. A text heavy with technical terminology that happens to share names with everyday words will score misleadingly easy. This is a concrete limitation for software documentation, user interfaces, and technical writing aimed at non-specialist audiences.

• Not appropriate for grades 1–3. The formula’s lowest score band covers grades 1–4 as a single undifferentiated range. It cannot distinguish between a text appropriate for grade 1 and one appropriate for grade 4. Use Spache for primary-grade text.

• The word list reflects 1984 American English. The 1995 revision was calibrated on word familiarity data from approximately 1984. Words common in contemporary life but absent from that era — “app,” “email,” “wifi,” “streaming” — are not on the list and will be counted as difficult. Agricultural vocabulary from mid-20th-century American life, such as “haystack” and “bushel,” appears on the list even though many contemporary readers may not recognize these words.

• Derivational suffixes count as difficult. A text using abstract nouns ending in -tion, -ment, or -ation will score harder than a text using the same root words in base form, because the suffixes create “new words” not covered by the list. Complete is familiar; completion is difficult.

Porter stemming in this library

The library uses Porter stemming to match words against the Dale-Chall list. This is an engineering compromise, not a validated approach. Porter stemming aggressively reduces words to root forms, which can cause some derivational forms to match list entries when the original Dale-Chall rules would mark them as difficult. For example, completion might stem to complet, which matches complete on the list. Scores may be slightly lower than a strict Dale-Chall implementation would produce.

Example

from readscore import Readability

text = """
Before you take this medicine, tell your doctor about all other medicines you take,
including vitamins and supplements. Some medicines can interact with each other and
cause problems. Your doctor needs this information to keep you safe. Store this
medicine at room temperature, away from heat and direct light. Keep it out of
reach of children. Do not use it after the expiration date on the package. If you
miss a dose, take it as soon as you remember. If it is almost time for your next
dose, skip the missed dose. Do not take two doses at the same time to make up for
a missed one. Contact your doctor or pharmacist if you have questions.
"""

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

print(f"Score: {result.score:.2f}")         # Score: ~5.12
print(f"Grades: {result.grade_levels}")     # ['5', '6']
print(f"Primary grade: {result.grade_level}")  # '5'

See Also

• Spache Readability Formula — same word-list approach, designed for grades 1–3
• Choosing a Metric — decision guide for all nine metrics, including when to choose Dale-Chall over syllable-counting formulas