Flesch-Kincaid grade level.
http://www.editcentral.com/gwt/com.editcentral.EC/EC.html
This is an interactive web page for checking a sample of writing. It is modeled after the ancient Unix utilities style and diction. Enter or copy text into the first box below. The scores to the right give the readability of the text according to various formulas.
The Flesch reading ease score is based on a range of 0-100, with lower values for harder text and higher values for easier text; the other scores show the approximate (US) school grade of the text. Grades 13-16 correspond to college level; grades higher than 16 correspond to graduate school level.