Flesch-Kincaid Readability

What is readability?

Readability measures how easy or difficult a piece of text is to read. It does not judge the quality or accuracy of writing — only how accessible the language is to a given audience. Educators, publishers, and policy-makers use readability formulas to match texts to readers at an appropriate level.

The Flesch-Kincaid tests

There are two related formulas, both developed from research by Rudolf Flesch and later refined by J. Peter Kincaid for the United States Navy.

Flesch Reading Ease

This score rates text on a 100-point scale. Higher scores mean the text is easier to read.

Formula:

Reading Ease = 206.835
             − 1.015 × (total words ÷ total sentences)
             − 84.6  × (total syllables ÷ total words)

Score	Grade Level	Audience
90 – 100	5th Grade	Very easy. Understood by an average 11-year-old.
80 – 89	6th Grade	Easy. Conversational English.
70 – 79	7th Grade	Fairly easy.
60 – 69	8th–9th Grade	Plain English. Standard for most documents.
50 – 59	10th–12th Grade	Fairly difficult.
30 – 49	College	Difficult. Academic or technical writing.
0 – 29	College Graduate	Very difficult. Specialised professional material.

Flesch-Kincaid Grade Level

This converts readability into a US school grade level, making it directly interpretable: a score of 8.0 means the text should be understandable by an 8th-grader (age ~13–14).

Formula:

Grade Level = 0.39 × (total words ÷ total sentences)
            + 11.8 × (total syllables ÷ total words)
            − 15.59

A brief history

In 1948, Rudolf Flesch published his Reading Ease formula to help writers produce clearer prose. In the 1970s, J. Peter Kincaid, working for the US Navy, adapted the formula into the Grade Level metric so that training manuals could be matched to the reading ability of enlisted sailors. The Flesch-Kincaid Grade Level became a US Department of Defense standard (MIL-STD-1472) and is now built into word processors such as Microsoft Word.

Examples

Simple text — "The cat sat on the mat."

Reading Ease: ~116 (off the scale — extremely simple)
Grade Level: ~−1 (below 1st grade)

Mid-range text — "The committee convened to discuss the proposed amendments to the existing regulatory framework."

Reading Ease: ~30
Grade Level: ~15 (college graduate)

Limitations

Syllable counting is approximate. English syllable rules are irregular; automated counters use dictionary look-ups and heuristics that are accurate but not perfect.
English only. The coefficients were calibrated for English. Other languages have different syllable structures and would need separate formulas (e.g., the Amstad formula for German).
Length matters. Very short texts (fewer than ~30 words) produce unreliable scores because small changes in word or sentence count swing the result dramatically.
Readability ≠ comprehensibility. A text can score as "easy" while discussing a topic that requires deep background knowledge. The formula measures surface features of language, not conceptual difficulty.
Not a writing quality metric. A low grade level does not mean the writing is good, and a high grade level does not mean it is bad. Technical writing aimed at specialists is expected to score high.

Further reading

Flesch, R. (1948). A new readability yardstick. Journal of Applied Psychology, 32(3), 221–233.
Kincaid, J.P., et al. (1975). Derivation of new readability formulas for Navy enlisted personnel. Research Branch Report 8-75, Naval Technical Training Command.
DuBay, W.H. (2004). The Principles of Readability. Impact Information.