The Raygor estimate graph is a readability metric for English text. It was developed by Alton L. Raygor, who published it in 1977.[1]
The US grade level is calculated by the average number of sentences and letters per hundred words. These averages are plotted onto a specific graph where the intersection of the average number of sentences and the average number of letters/word determines the reading level of the content. Note that this graph is very similar to the Fry readability formula's graph.[2]
This graph is primarily used in secondary education to help classify teaching materials and books into their appropriate reading groups.
The formula
Extract a 100-word passage from the selection.[2] If the material is long, take a subsample from the beginning, middle, and end.
Count the number of sentences in each passage. Count a half sentence as 0.5.
Count the number of words in each passage containing six or more letters.