The Fog index, developed by Robert Gunning, is a well known and simple formula for measuring readability. The index indicates the number of years of formal education a reader of average intelligence would need to read the text once and understand that piece of writing with its word sentence workload.
This score rates text on a 100 point scale. The higher the score, the easier it is to understand the text. A score of 60 to 70 is considered to be optimal.
This score rates text on U.S. grade school level. So a score of 8.0 means that the document can be understood by an eighth grader. A score of 7.0 to 8.0 is considered to be optimal.
out of curiosity, i ran it on a paper that (wicked smart) geeklet recently sent me:
Fog : 21.1004
Flesch : 15.9839
Flesch-Kincaid : 17.9149