| 01 | Lexile code | For example AD or HL. Deprecated in ONIX 3 – use code 06 instead | 0 | 24 |
| 02 | Lexile number | For example 880L. Deprecated in ONIX 3 – use code 06 instead | 0 | 24 |
| 03 | Fry Readability score | Fry readability metric based on number of sentences and syllables per 100 words. Expressed as an integer from 1 to 15 in <ComplexityCode> | 14 | |
| 04 | IoE Book Band | UK Institute of Education Book Bands for Guided Reading scheme (see https://www.ucl.ac.uk/reading-recovery-europe/ilc/publications/which-book-why). <ComplexityCode> is a color, eg ‘Pink A’ or ‘Copper’ | 23 | |
| 05 | Fountas & Pinnell Text Level Gradient | <ComplexityCode> is a code from ‘A’ to Z+’. See http://www.fountasandpinnellleveledbooks.com/aboutLeveledTexts.aspx | 23 | 30 |
| 06 | Lexile measure | The Lexile measure in <ComplexityCode> combines the Lexile number (for example 620L or 880L) and optionally the Lexile code (for example AD or HL). Examples might be ‘880L’, ‘AD0L’ or ‘HL600L’. See https://lexile.com/about-lexile/lexile-overview/ | 24 | |
| 07 | ATOS for Books | Advantage-TASA Open Standard book readability score, used for example within the Renaissance Learning Accelerated Reader scheme. <ComplexityCode> is the ‘Book Level’, a real number between 0 and 17. See http://www.renaissance.com/products/accelerated-reader/atos-analyzer | 25 | 47 |
| 08 | Flesch-Kincaid Grade Level | Flesch-Kincaid Grade Level Formula, a standard readability measure based on the weighted number of syllables per word and words per sentence. <ComplexityCode> is a real number typically between about -1 and 20 | 26 | |
| 09 | Guided Reading Level | Use this code for books levelled by the publisher or a third party using the Fountas and Pinnell Guided Reading methodology | 30 | |
| 10 | Reading Recovery Level | Used for books aimed at K-2 literacy intervention. <ComplexityCode> is an integer between 1 and 20 | 30 | |
| 11 | LIX | Swedish ‘läsbarhetsindex’ readability index used in Scandinavia. Only for use in ONIX 3.0 or later | 55 | |
| 12 | Lexile Audio measure | Lexile Audio measure from MetaMetrics’ Framework for Listening. The code in <ComplexityCode> indicates the difficulty of comprehension of audio material (for example 600L or 1030L). Only for use in ONIX 3.0 or later. See https://lexile.global/the-lexile-framework-for-listening/ | 56 | |