Check digits are used to validate long numbers that have a lot of digits in them.

A check digit is an extra digit placed at the end of long number that can be used to check if the number has been input correctly.

Check digits are often used to check numbers that have been input using direct data entry devices such as bar code scanners or light pens.

The value of a check digit is worked out by performing a calculation using the individual digits that make up a number. This calculation gives the value of the check digit which is then added as an extra digit to the end of the number.

Calculating check digits using the modulus-11 method

• Each digit is assigned a weight starting at 2 with the right hand digit;
• Each digit is multiplied by its weight;
• The results of these calculations are added together to give a total;
• The total is divided by 11;
• The remainder is subtracted from 11 to give the check digit. The two exceptions are: If the remainder is 0 and the result is 11 the check digit is 0, not 11. If the remainder is 1 and the result is 10 the check digit is X, not 10.