Kontrollsiffror

Kontrollsiffror, or check digits, are digits added to numerical identifiers to allow error detection in data entry and transmission. They are generated from the preceding digits by applying a mathematical rule, and the resulting digit or digits help reveal mistakes such as a wrong digit, a transposed pair, or a missing character. Kontrollsiffror are widely used in financial numbers, product codes, library identifiers, and many national identification schemes to improve data integrity.

The most common methods are diverse. The Luhn algorithm (modulus 10) is used for many credit and debit card numbers as well as some other identifiers; it doubles every second digit from the right, subtracts 9 when the product exceeds 9, sums all digits, and the check digit is chosen so the total is a multiple of 10. Modulo 11 schemes are used in some identifiers such as ISBN-10, where digits are weighted and the result modulo 11 determines the check digit (often represented as X for the value 10). The IBAN system uses a two-digit check that ensures the entire international bank account number is valid under modulo 97. Other algorithms, such as Verhoeff, are designed to improve detection of certain error patterns.

Examples of use include credit card numbers with a Luhn check digit, ISBN-13 with a mod 10 check digit using alternating weights, and IBAN with a structural check for validity across countries. Limitations include that kontrollsiffror cannot catch all errors or fraud and some error patterns may go undetected; additional validation steps are often used in practice.