"After typing in a query on Google.com, look at the column of tools on the left side of the search results and you will see "All Results."
At the bottom of that column, click on "More Search Tools." Then you will see a list of tools and in the middle under Dictionary, you will see "Reading Level."
After you click on Reading Level you will see at the top of the page, three reading level choices: basic, intermediate and advanced with the percentages right next to the levels. The percentages indicate how many results are in each reading level, and you can simply click on the level to display the results that you want." (Tip from HuffPost Education)
The complexity of the vocabulary and the sentence structure determines the reading level. Naturally, the more complicated the idea or the topic is, the more complicated the reading level. For example, a search for "cartoons" found only 5% of the material was written at the advanced level. On the other hand, the search illustrated below for "time space continuum" found only 3% of the articles were written at the basic level.