Check digit

A check digit is the simplest form of checksum. The check digit of a multi-digit number is calculated after a certain calculation rule from the remaining digits. By calculating and comparing the Verification code input errors can be detected.

If the check digit can take ten values ​​, an accidental coincidence occurs with invalid numbers with 10% probability. When using two check digits this error probability is only at 1%.

Check digits can be used both for manual keyboard input as well as automatic data acquisition, such as OCR or barcode readers. In a data acquisition device can be rejected at a different check digit input.

Example: ID code of Deutsche Post

The calculation of the check digit takes place when the identity code of Deutsche Post in modulo-11 method

Code: 4 7 3 1 2 4 8 2 Weighting: 8 6 4 2 3 5 9 7 Products: 32 42 12 2 6 20 72 14 Total: 200 Modulo 11: 200/11 = 18 remainder 2 Verification: difference 11 = 11 - 2 = 9 exceptions: If difference = 10 to 11: check digit becomes 0 If difference = 11 to 11: check digit is 5 Identity code of Deutsche Post / DHL Calculation and testing:

  • From left to right of the bodies 8, 6, 4, 2, 3, 5, 9 and 7 are weighted. In contrast to a simple summation of the code numbers so and permutations of two digits can be detected ..
  • The individual code and weighting numbers are multiplied and then added the products.
  • The sum is computed modulo 11, that is, it is only the remainder of the division accounted for by 11 (so only one digit ). The rest of the formation may also take place directly after the product formation.
  • The check digit is derived from the difference of the rest to 11 ( see exceptions above).

Other examples

  • IBAN
  • EAN codes in consumer goods
  • ISBN for the book trade
  • ISSN for periodicals
  • ID card numbers
  • Institution Plate ( IK ) for healthcare providers
  • ISIN for the identification of securities
  • German Insurance Number
  • Euro bank notes with serial number check digit
  • Series scheme of Deutsche Bahn AG
  • Pharmacy Central Number
  • Tax identification number
  • Luhn algorithm
International Securities Identification Number
577600
de