This is a nifty online text editor that also calculates readability and estimates grade level and estimated reading time, too. Use it to check your writing from time to time. You'll need a sample of at least 100 words. More is better.
Here's a short video tutorial. (Text transcript is available here.)
Grade 6 does not always come across as juvenile.
Grade 14 doesn't always mean you're intelligent.
Grade 9 or lower is our target.
Don't get hung up on the grade level in the Hemingway app or any other readability measurement (Fleischmann, Gunning, etc.) These readability indexes were developed long before we had computer screens, tablets, smart phones, and Kindle. Also, when these readability "grades" were created, people with disabilities were not included in most classrooms or professions. Accessibility was not even on the radar.
And consider this: Generation Z students don't read the same way people did back when these indexes were created, either.
"...many learned to read and follow directions by playing computer games."
—Beth L. Hewett in Reading to Learn and Writing to Teach. St. Martin's Press, 2015.
(For some interesting literacy stats see http://www.impact-information.com/impactinfo/literacy.htm).
(Just FYI, all the pages in this section fall between 5th and 7th grade according to the Hemingway App).