Under ISO/IEC 7812, the maximum length of a payment-card number is 19 digits.
Payment-card numbers are structured identifiers used to distinguish card issuers and individual card accounts. They commonly contain an issuer identification number, an account identifier, and a final check digit. The check digit is calculated using the Luhn algorithm, which helps detect many typing and transcription errors but does not prove that a card is genuine or active.
Many familiar cards have 16 digits, which is why that length is often mistaken for a universal rule. Card-number lengths vary by payment network and product. Some American Express numbers have 15 digits, while other systems use different lengths within the ISO/IEC 7812 limit.
The maximum applies to the primary account number itself, not to every number printed or encoded on a card. Security codes, expiration dates, and chip data are separate elements. A payment-card number also should not be confused with a bank account number, which follows different conventions.