Check Digits

The last digit of the reference number is always a check digit. The check digit provides a means of validating the reference number to prevent erroneous data from being entered.

The check digit is calculated in this manner:

1. Each digit in the basic reference number (for example, the invoice number) that is to be verified is multiplied by the weights 7, 3, and 1 from right to left.

   That is, the right-most digit is multiplied by 7, the next digit to the left is multiplied by 3, the next digit to the left is multiplied by 1, the next digit to the left is multiplied by 7, and so on until the first digit has been multiplied.

2. The results of the multiplications are added together.

3. The sum is rounded up to the nearest 10, and then the sum is subtracted from the rounded number.

The resulting difference is the check digit, which becomes the last digit of the reference number. If the resulting difference is 10, the check digit is 0.