I have found some great resources to find the reading level of a book by grade level, by Lexile number, by DRA level, and by guided reading level. These are the four ways most children's reading levels are measured.
First you must know which leveling system you are using. Here's a chart that compares different leveling systems.
Use these resources to level the books you own.
Renaissance Learning has a huge number of children's books in their database. The company sells AR tests. The search is meant for interested buyers, but anyone can use the search. Search the title of a book. If the company sells a test for the book, you will find the grade level for your book.
Kansas Book Connect has search for books options. It gives a description of books and a grade level.
The Lexile Framework for Reading is a great resource to find books if a child has a Lexile number, or you want to find a Lexile number for a child. Find a Book for You is a great resource to find books by level and interest.
At Scholastic Book Wizard you can search for a particular title or search according to reading level. There are buttons to specify a book search for grade level, Lexile, DRA, or guided reading.
If you can't find the reading level for a book at any of these sites, try The SMOG Readability Formula.
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