Structure of an ICD-10-CM Code
| Letter | Number or Letter | ||||||
|---|---|---|---|---|---|---|---|
| 1st Digit | 2nd Digit | 3rd Digit | 4th Digit | 5th Digit | 6th Digit | 7th Digit | |
| C | 5 | 0 | . | 5 | 1 | 2* | |
| Category | Etiology, anatomic site, severity, laterality | Added code extensions for obstetric, injuries and external causes of injury | |||||
| Length | 3-7 digits |
|---|---|
| Character 1 | Letter only (all letters except U are used) |
| Character 2 | Number only |
| Characters 3-7 | Numbers or letter |
| Decimal | After 3rd character (not counted) |
| Placeholder | Use of “x” as a dummy placeholder |
| Letter Format | Letters are case-sensitive |
Updated: December 3, 2018
Suggested Citation
SEER Training Modules: Structure of an ICD-10-CM Code. U.S. National Institutes of Health, National Cancer Institute. Cited 26 December 2025. Available from: https://training.seer.cancer.gov.