LEXILES
The Lexile Framework for Reading is an approach to reading measurement that matches students to appropriately challenging reading materials. The Lexile Framework measures both reader ability and text difficulty on the same scale, called the Lexile scale.  This system is used with 100 Book Challenge homework.
The Lexile developmental reading scale ranges from 200L for beginning readers to 1700L for advanced text.  For more information visit the Lexile information page.