If you have a particular article, passage, or short story you want to use with students, this free website is a good go-to for figuring out if it's in the ballpark for the level of students you are teaching (using Reading Anchor Standard 10). The website offers a variety of readability formulas for you to use and gives you guidance about which one to choose. Copy/paste text into a text box and click a button. You also have the option of running 7 formulas at one time (it's the first option provided).
NOTE: Use this in conjunction with a qualitative analysis of the text and an assessment of the readers/task.