The Fry Readability Formula (or Fry Readability Graph) is a readabilitymetric for English texts, developed by Edward Fry. It has been suggested that Readability test be merged into this article or section. ... See: International System of Units, colloquially called the Metric System, and also metrication. ... The English language is a West Germanic language that originates in England. ...
It uses the average number of sentences and syllables per hundred words, with which the difficulty is read off a graph.
I also looked at the readability levels for the passages of the Neale Reading Analysis (Neale, 1989) and found that these were within about one year of the comprehension norms.
Studies of the readability of secondary textbooks (particularly Science) have also found that some are too hard for many pupils (Chiang-Soong and Yager, 1993).
Readability is not just the result of word and sentence length and you can find out more from an excellent on-line article written by Keith Johnson.