Automated Readability Index

The Automated Readability Index, also known as "ARI" or "auto", was originally created for U.S. Air Force materials and was designed for technical documents and manuals. This test calculates the U.S. grade level of a text sample based on sentence length and character count.

The ARI Formula

GL = (4.71 * (C/W)) + (0.5 * (W/S)) - 21.43

Where:

GL	U.S. grade level
W	Number of words
C	Number of characters
S	Number of sentences

Citations

Smith, E. A. and R. J. Senter (1967). 'Automated readability index', AMRL-TR, 66-22. Wright-Patterson AFB, OH: Aerospace Medical Division.