Gera Tools

ICD-10 Chapter Reference

Browse all 22 ICD-10-CM chapters with code range and description

Reference table of all 22 ICD-10-CM chapters with their alphanumeric code ranges and disease categories, plus a code-to-chapter lookup. Filter by keyword or range for fast medical coding lookups.

ICD-10-CM chapter reference

ICD-10-CM (the United States Clinical Modification of the WHO’s ICD-10) organises every diagnosis code into 22 chapters. Each chapter covers a body system or category of conditions and owns a contiguous block of alphanumeric code ranges, such as A00-B99 for infectious diseases or I00-I99 for the circulatory system. This tool lists all 22 chapters and lets you type a code to see which chapter it falls in.

How it works

An ICD-10-CM code begins with a letter (the first character) followed by two digits, then an optional decimal and further characters. The first three characters — the “category” — fall inside a chapter’s range. To resolve a code, the lookup parses the leading letter and two digits, then walks the chapter list comparing that prefix against each range’s lower and upper bounds. For example I21 (acute myocardial infarction) sorts between I00 and I99, so it belongs to Chapter 9, Diseases of the circulatory system.

Note that a few ranges end on a letter-suffixed boundary like O00-O9A; the comparison treats those coarsely, so always confirm sub-block membership against the published code set.

Tips and notes

  • Letters are not all used left-to-right: chapter order broadly follows A-Z but some letters span two chapters (H00-H59 eyes vs H60-H95 ears) and some appear out of sequence (U00-U85 is the last chapter, “special purposes”, and contains U07.1 for COVID-19).
  • ICD-10-CM differs from WHO ICD-10 and from ICD-10-PCS (inpatient procedures). This reference is the diagnosis (CM) chapter scheme.
  • The code set is updated annually (effective 1 October in the US); verify ranges against the current fiscal-year files before billing.