What is the Coleman-Liau Index?
The Coleman-Liau Index is a readability test designed to estimate the U.S. grade level needed to understand a text. Unlike other readability formulas that rely on syllable counting, the Coleman-Liau Index uses character counts, making it easier to calculate with modern computers.
Formula
The Coleman-Liau Index is calculated using:
[\text{CLI} = 0.0588 \times L - 0.296 \times S - 15.8]
Where:
- CLI = Coleman-Liau Index (grade level)
- L = Average number of letters per 100 words
- S = Average number of sentences per 100 words
How to Calculate
- Calculate L: Count total letters in your text, divide by total words, multiply by 100
- Calculate S: Count total sentences, divide by total words, multiply by 100
- Apply the formula: Plug both values into the equation
- Interpret the result: The output represents a U.S. grade level
Calculation Example
Given:
- Average of 650 letters per 100 words (L = 650)
- Average of 7 sentences per 100 words (S = 7)
Calculation:
[\text{CLI} = 0.0588 \times 650 - 0.296 \times 7 - 15.8]
[\text{CLI} = 38.22 - 2.072 - 15.8 = 20.35]
This result indicates the text is appropriate for approximately 20th grade (college senior or graduate level).
Interpreting Results
| Grade Level | Education Level |
|---|---|
| 1-6 | Elementary school |
| 7-8 | Middle school |
| 9-12 | High school |
| 13-16 | College |
| 17+ | Graduate school |
Tips for Improving Readability
If your text scores higher than your target grade level:
- Use shorter sentences
- Choose simpler words when possible
- Break up long paragraphs
- Avoid unnecessary technical jargon
- Consider your audience's reading level
Applications
- Content Marketing: Ensure blog posts match audience reading levels
- Technical Writing: Verify documentation accessibility
- Education: Match materials to student grade levels
- Legal and Medical Writing: Assess patient or client communication clarity