Converting and Understanding ATOS, Lexile, and ZPD Ranges

Breanna | October 8, 2021 |

Understanding ATOS, ZPD ranges, and Lexile levels can feel like reading Latin if it’s your first time diving in. Sure, you know about reading programs, including accelerated reader, but do you know the scientific building blocks of how and why the program works? It’s a bit more methodical than looking at a book’s blurb and saying, “Yup, that looks like a fourth grader could read it.” Yep, if they aren’t labelled you usually have to look up the appropriate reading level.

However complex, these scores are an integral part of accelerated reader programs and serve the purpose of telling you where your child is at in terms of their reading development. 

Girl reading in a library
ATOS, LEXILE and ZPD ranges help young readers choose the correct difficulty of book

What are ATOS levels?

Using the ATOS readability formula, the ATOS levels represent the difficulty of the text. An example of an ATOS level would be 4.5 which equates to a text that could be read by a fourth grader in their fifth month of school. 

ATOS factors = average sentence length + average word length + vocabulary grade level + the number of words in a book 

What are ZPD Ranges?

If you’re looking at a range of ATOS levels, it may also be expressed as a ZPD range, or a zone of proximal development. Books within this range will challenge a reader while encouraging growth and not causing frustration. Here’s a post on how you convert ZPD ranges to an approximation of reading age.

What are Lexile Levels?

These levels measure the complexity of the text, whether it’s a book, short story, article, etc. This ranges from 200L for early readers to 1600L for experienced/advanced readers. 

Lexile factors = sentence length + word frequency 

How Convert ATOS to Lexile to ZPD Range

ZPD Level ATOS Level Lexile Level Grade Equivalent (USA)
5.0 – 5.9781-910fifth 
6.0 – 6.9921-1019sixth

Breanna is a former therapist turned writer. She earned a master’s in psychology in 2015 but ultimately decided she would like to be more present for her family. Currently flexing her writing skills as a stay-at-home mother of two toddlers (with another baby on the way), she enjoys writing in the parenting and home/DIY niches. She also writes fiction and has been published in a handful of literary magazines and a fiction anthology. In her spare time, Breanna enjoys blogging, painting, running, and drinking an absurd amount of coffee.


Understanding STAR reading levels and scores

How to Find Books Based on ATOS, IL, and Lexile Levels


1 thought on “Converting and Understanding ATOS, Lexile, and ZPD Ranges”

  1. Thank you for the simple direct explanation. Been teaching many years and are having to relearn the difference between using AR (ZPD) and Lexiles (Reading A to Z program). “teaching old dog forgotten tricks”!!


Leave a Comment