Letters of the alphabet are treated like numbers in either
modulus 10 or modulus 11 check digit calculations. This is done
by assigning a digit to each letter of the alphabet as follows:
A B C D E F G H I J K L M N 0 P Q R S T U V W X Y Z ( =space)
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 0 |
A value to be checked may be preceded by a plus or minus sign;
however, the sign is ignored in the check digit calculations. To
illustrate, perform a modulus 10 check digit calculation on a value
that contains letters as well as numbers:
Assume a basic value | 3 4 G 1 2 H |
Change the letters to digits | 3 4 7 1 2 8 |
Multiply alternate digits by 2 | 8 2 16 |
Digits not multiplied | 3 7 2 |
Sum with digits that were not multiplied | 3+8+7+2+2+1+6 = 29 |
Subtract sum from next higher number
ending with zero | 30-29 = 1 |
Check digit = | 1 |
Self-checking number | 34G12H1 |
