How many digits are there in a check number?

Answer 1

A check digit is typically a single digit 0-9 (or it may be a single byte with value 0-255), which is calculated from a formula, using the other digits in a string of digits. The digit is then appended, usually at the end of the string of digits before encoding for transmission over a communications network or creating a barcode.

The receiver (or barcode scanner) has the same formula, which was used to create the check digit, and performs the same operation, then checks to see if the digit that was received (or scanned) is the same as the calculated digit. If they match, then a successful transmission is assumed. Disk drives also use check digit methods to see if data read in might be erroneous, due to imperfections in the disk surface.