The F–K formula was first used by the Army for assessing the difficulty of technical manuals in 1978 and soon after became a United States Military Standard.

Navy research directed by Kincaid delved into high-tech education (for example, the electronic authoring and delivery of technical information), usefulness of the Flesch–Kincaid readability formula, computer aids for editing tests, illustrated formats to teach procedures, and the Computer Readability Editing System (CRES). 'The Flesch–Kincaid' (F–K) reading grade level was developed under contract to the U.S. Peter Kincaid developed the Grade Level evaluation for the United States Navy. Rudolf Flesch devised the Reading Ease evaluation somewhat later, he and J. The results of the two tests correlate approximately inversely: a text with a comparatively high score on the Reading Ease test should have a lower score on the Grade-Level test. Although they use the same core measures (word length and sentence length), they have different weighting factors. There are two tests: the Flesch Reading-Ease, and the Flesch–Kincaid Grade Level. The Flesch–Kincaid readability tests are readability tests designed to indicate how difficult a passage in English is to understand.

Graphs of Flesch-Kincaid reading ease (red) and grade level (gray) scores against average syllables per word and average words per sentence

Dr seuss computer paper