The Flesch–Kincaid readability tests are designed to indicate how difficult a passage in English is to understand. There are two tests, the Flesch–Kincaid Reading Ease, and the Flesch–Kincaid Grade Level Formula.
We use the Flesch–Kincaid Grade Level Formula to present scores as per US grade level. We believe that this makes it easier for teachers, parents, librarians, and others to judge the readability level of each item of content.
You can read more about the Flesch–Kincaid readability tests here.