The most widely used readability checkers are based on the Flesch–Kincaid system. This system uses a formula that combines the average word length with the average sentence length to show reading difficulty. It produces readability scores that are aligned with American school grades 3–12 (see School grades for more information). The scale was later expanded to include grades 13–16 for college students and a 17th grade to identify specialised technical texts.

The Flesch–Kincaid system is inaccurate at upper grade levels because of the great variety among readers with higher levels of education, reflecting their individual interests and aptitudes. It also does not take account of different language or cultural backgrounds.