# How ICD-10-CM Codes Are Structured URL: https://icdcodefinder.com/learn/icd-10-cm-code-structure Updated: 2026-10-30 > An ICD-10-CM code has 3 to 7 characters. The first character is a letter, the first three characters form the category, and a decimal follows the third character. Characters 4 to 6 add detail such as etiology, site or severity, and a 7th character, where required, adds information such as the episode of care. Only a code at the full length required is valid for reporting. ## Key takeaways - ICD-10-CM codes are 3 to 7 characters long, with a decimal after the third character. - The first three characters are the category; further characters add specificity. - A "billable" code is simply a valid code at the full number of characters required; headers are not valid for reporting. - The Tabular List is organized into chapters by body system or type of condition. ICD-10-CM (International Classification of Diseases, Tenth Revision, Clinical Modification) is the US classification for diagnoses. It is based on the World Health Organization's ICD-10 and maintained by the National Center for Health Statistics (NCHS), with CMS responsible for the procedure system ICD-10-PCS. Every code follows the same structure. ## The parts of a code | Position | What it represents | |---|---| | 1 | A letter. Together with characters 2–3 it places the code in a chapter. | | 2–3 | Numbers (and occasionally letters) completing the **category**. | | Decimal | Always after the third character. | | 4–6 | **Subcategory** detail, such as etiology, anatomic site, severity or laterality. | | 7 | An **extension** character, required only in certain categories (for example, the episode of care for injuries). | Example: S01.01XA - S01 is the category (open wound of head). - S01.01 adds the type and site (laceration without foreign body of scalp). - X is a placeholder that keeps the 7th character in position 7. - A is the 7th character for the initial encounter. ## Categories, subcategories and headers The three-character **category** is the starting point. Many categories are subdivided into four-, five- and six-character **subcategories**, each adding detail. A code is only valid for reporting when it is coded to the full number of characters required. A category or subcategory that has more specific codes below it acts as a **header**: it organizes the list and carries instructional notes for the codes beneath it, but it cannot be reported on its own. "Billable" is the common shorthand for a valid, complete code. ## Laterality and placeholders Many codes include **laterality**: separate codes for right, left and bilateral, with an unspecified option. Some categories also use the letter X as a **placeholder** inside the code, either to allow for future expansion or to keep a required 7th character in the 7th position. When the X is part of the code, the code is not valid without it. ## Chapters The Tabular List is divided into chapters, mostly by body system (for example, diseases of the circulatory system) and partly by type of condition (for example, neoplasms, or injury and poisoning). Chapters are divided into **blocks** of related categories. The last chapter, codes for special purposes, uses the letter U, which is reserved for codes assigned on an emergency or provisional basis, such as U07.1 for COVID-19. On ICD Code Finder, you can browse the full hierarchy from chapter to block to category to the most specific code, and every code page shows where it sits in that hierarchy. ## The Alphabetic Index and the Tabular List ICD-10-CM has two main parts: - The **Alphabetic Index**, an alphabetical list of terms and their corresponding codes, including the Table of Neoplasms, the Table of Drugs and Chemicals and the External Cause of Injuries Index. - The **Tabular List**, the chronological list of codes with all instructional notes. Codes are found through the Index and always verified in the Tabular List before reporting. ## Annual updates The code set is updated every fiscal year, effective October 1, and can also have mid-year updates. The fiscal year in effect on the date of service (the discharge date for inpatient stays) determines which codes are valid. ## Frequently asked questions **How many characters does an ICD-10-CM code have?** Between 3 and 7. A three-character category is valid on its own only when it has no further subdivisions; otherwise the code must be reported to the full number of characters required. **What is a billable ICD-10-CM code?** An informal term for a code that is valid for reporting because it is complete, meaning it has the full number of characters required. Codes that have more specific codes beneath them are category or subcategory headers and are not valid on a claim. **Is ICD-10-CM the same as ICD-10?** No. ICD-10 is the World Health Organization classification. ICD-10-CM is the US clinical modification maintained by the National Center for Health Statistics (NCHS) for diagnosis coding, with much more detail. Procedures in US inpatient settings use a separate system, ICD-10-PCS. --- Informational, not coding advice. This page summarizes ICD-10-CM conventions for education. Always code from the medical record using the current ICD-10-CM Tabular List, Alphabetic Index and Official Guidelines for Coding and Reporting, plus payer policy. Disclaimer: ICD Code Finder is an informational tool and is not a substitute for professional coding judgment, payer policy, or the official ICD-10-CM classification and guidelines. Not affiliated with, endorsed by, or sponsored by CMS, NCHS, or the World Health Organization. AI suggestions require review by a qualified coder.