When assigning text reading to students it is helpful to appropriately target the grammatical complexity and vocabulary level for the audience you are teaching. I teach conceptual physics to a very diverse student body. In the same class with sophomore English language learners I will have senior IB diploma candidates. It is helpful to be able to offer them text that covers the same topic with different levels of language complexity. In some cases I write the text myself. More frequently I will pull something from wikipedia or the online texts available to me. Regardless of the source, it is helpful to compare the texts based on some common lexical measurement tools.
Google has a reading level tool that can be applied to a page of search results. Google’s “Reading Level” is limited. It will bin your search results into Basic, Intermediate, or Advanced categories. As yet I have been unable to figure out how to get the tool to do anything but evaluate the entire content of the sites that pop up in their search results. This can be helpful if you use entire websites, say for a research activity, in a lesson, but only if you have spent some time determining what these three categories mean for your students. To find the tool look to the left margin of a google search page: click show search tools: then click Reading level.
A better resource that I discovered recently is the Readability Calculator at Online-Utility.org. This was impressive. The tool allows you to either enter a URL or directly copy and paste text into a text box. Their algorithm is more or less spelled out in the analysis, and the results are estimated for several different metrics including the Flesh-Kincaid. This is a must bookmark for any teacher.