Donald Trump speaks at a fourth-grade level. While this isn't news to anyone who's compared his speeches to Barack Obama's elocution or almost anyone else, it is news that there's an analysis suggesting this. Bill Frischling of Factbase published a piece that looked at the past 15 presidents (dating back to Herbert Hoover) and ran their oration abilities through the Flesch-Kincaid grade level scale.
Trump was dead last by any significant margin. His 4.6 rating was 1.3 levels behind the next lowest-ranking president Harry Truman, the largest margin between presidents. For the record, Obama was No. 3 with a 9.7, while Hoover topped the list with 11.3. Factbase also ran a number of other metrics that weren't flattering toward Trump. The president ranked the lowest when it came to the stats "Vocabulary Word Complexity" (averaging 1.33 syllables) and "Vocabulary Word Diversity" (2,605 unique words).
As the post's title "'Stable Genius' – Let’s Go to the Data" suggests, the study is directly pegged to Trump's Twitter rant where he called himself a "stable genius" in response to Michael Wolff’s recently published book Fire and Fury: Inside the Trump White House, which repeatedly called his mental health into question. Wolff also told The Daily Mail that some White House staffers were worried that he may be suffering from dementia, learning disabilities, and ADHD.
Frischling noted in the analysis post that vocabulary isn't necessary a representation of intelligence. Like a myriad of other things, it doesn't really affirm Trump's "stable genius" either. See the study in full here.