Determining text complexity level is now easier than ever.

The ATOS readability formula is a research-proven tool to guide students to appropriate-level books. ATOS takes into account the most important predictors of text complexity—average sentence length, average word length, and word difficulty level. If you’re confused by the content on this page, this blog gives more explanation into book levels and why they can sometimes seem inconsistent.
Graphic showing the qualitative, quantitative and reader & task relationships to ATOS and Accelerated Reader


Qualitative factors include levels of meaning, structure, language conventionality, clarity, and knowledge demands. Interest level (i.e. content and maturity level of text) and genre are provided in AR BookFinder.


Quantitative dimensions include word length or frequency and sentence length that can only be measured efficiently by computer software.

Reader and task

Reader and task considerations include students’ motivation, knowledge, and background interests along with the purpose and complexity of task assigned, and are best made by the teacher.

Guide students to books at the appropriate level.

Star Reading determines students’ reading level range or Zone of Proximal Development (ZPD) and automatically imports results into AR. Students are encouraged to read mostly within their ZPD to achieve the greatest growth. AR provides ATOS readability and interest level information on over 39,000 book titles.

Determine ATOS Level for text

This works best for text: sentences, stories, magazine and newspaper articles, test items, and other classroom materials.

ATOS for text

Determine ATOS Level for books

To get the most accurate ATOS analysis, use this option to submit the contents of a book.

ATOS for entire books

ATOS for estimated word count


Learn more about Accelerated Reader

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