Books get a level
The ATOS formula weighs average sentence length, word length, word difficulty, and the book's overall length. The result is expressed as grade.month—so 2.8 means late-2nd-grade difficulty. It measures the text, not its themes.
Books by ATOS® level
ATOS levels, decoded—grade by grade.
ATOS is the readability scale behind Accelerated Reader, and it reads like a grade: a 3.5 book suits a typical 3rd grader mid-year. If your child has an AR level or a ZPD range from STAR testing, start with the matching band below.
- Scale
- 0.0–13.0+
- Score comes from
- STAR Reading test
- Used by
- AR classrooms
Pick a range
ATOS ranges by grade
Because ATOS mirrors grade numbers, each band below simply spans its grade. We translate your pick to the matching Lexile range—the level data behind Bookroo's filters—so expect a close match rather than an exact one.
How ATOS works
A formula with a school year built in
Renaissance Learning designed ATOS to make text difficulty legible to anyone who understands grade levels.
Readers get a ZPD
In Accelerated Reader schools, the STAR Reading assessment gives each child a Zone of Proximal Development—a personal range like 3.2–4.6. Books inside the zone are meant to stretch a reader just enough to keep growing.
Level isn't points
AR also assigns points, but those track length and quiz value, not difficulty—chasing points rewards long books, not better-fit ones. And like every readability score, ATOS says nothing about content maturity, so keep age in the mix.
Translate between systems
How ATOS compares
One child can come home with an ATOS zone, a Lexile score, and a guided-reading letter in the same semester. Here's the decoder ring.
| Grade level | Lexile | ATOS | Fountas & Pinnell | Browse books |
|---|---|---|---|---|
| Kindergarten | BR40L–230L | 0.0–1.0 | A–C | Browse → |
| 1st grade | 190L–530L | 1.0–1.9 | C–I | Browse → |
| 2nd grade | 420L–650L | 2.0–2.9 | I–M | Browse → |
| 3rd grade | 520L–820L | 3.0–3.9 | M–P | Browse → |
| 4th grade | 740L–940L | 4.0–4.9 | P–S | Browse → |
| 5th grade | 830L–1010L | 5.0–5.9 | S–V | Browse → |
| 6th grade | 925L–1070L | 6.0–6.9 | V–Y | Browse → |
| 7th grade | 970L–1120L | 7.0–7.9 | Y–Z | Browse → |
| 8th grade | 1010L–1185L | 8.0–8.9 | Z–Z+ | Browse → |
| 9th–12th grade | 1050L–1385L | 9.0+ | Z+ | Browse → |
Ranges follow published grade-level charts for each system and overlap on purpose—readers rarely sit inside one tidy box. Bookroo's level filters run on Lexile measures, so ATOS and Fountas & Pinnell ranges are translated to their closest Lexile equivalents when you browse.
Good to know
Common ATOS questions
- How do I read an ATOS book level?
- As grade and month. A book at 4.5 has the text difficulty a typical 4th grader handles in the fifth month of the school year. That makes ATOS the most intuitive scale if you think in grade levels.
- What is a ZPD range?
- The STAR Reading test reports a Zone of Proximal Development—a personal band like 2.6–3.7. Books inside it should be challenging without being frustrating. If you have a ZPD, browse with its bounds; the grade ranges here are the general-purpose version of the same idea.
- Are ATOS levels the same as AR points?
- No. The ATOS level describes how hard a book is to read; Accelerated Reader points describe how long it is (and how much quiz credit it earns). A short, difficult book can be worth fewer points than a long, easy one.
- How does ATOS compare to Lexile?
- Both are computed readability measures and correlate closely—ATOS on a grade-decimal scale, Lexile on a 0–1600+ scale. The chart on this page lines them up. Bookroo's filters run on Lexile, so ATOS ranges are translated to their approximate Lexile equivalents when you browse.
Speak your school's language
Prefer a different system?
Pre-K–12thGrade levelNo score needed—browse by the grade your child is in right now.Explore Grade levels →BR–1385LLexileThe scale behind most school assessments—and Bookroo's deepest level data.Explore Lexile levels →A–Z+Fountas & PinnellThe A-to-Z guided-reading gradient many classrooms level with.Explore F&P levels →