Check Digit Calculation - Note: The Last number of a CC Number is its check digit
- Start from the last digit and move to the first digit.
- Every other number is doubled.
- If it is a two-digit number, add the digits together.
- Keep a running sum.
- At the end, get the rightmost digit of the sum.
- Subtract that number by 10
- The rightmost digit of that number is the check digit
Example Calculation:
Number: 00123456789
Digit: 9 * 2 = 18 = (1 + 8) = 9
Sum: 9
Digit: 8 * 1 = 8 = (0 + 8) = 8
Sum: 17
Digit: 7 * 2 = 14 = (1 + 4) = 5
Sum: 22
Digit: 6 * 1 = 6 = (0 + 6) = 6
Sum: 28
Digit: 5 * 2 = 10 = (1 + 0) = 1
Sum: 29
Digit: 4 * 1 = 4 = (0 + 4) = 4
Sum: 33
Digit: 3 * 2 = 6 = (0 + 6) = 6
Sum: 39
Digit: 2 * 1 = 2 = (0 + 2) = 2
Sum: 41
Digit: 1 * 2 = 2 = (0 + 2) = 2
Sum: 43
Digit: 0 * 1 = 0 = (0 + 0) = 0
Sum: 43
Digit: 0 * 2 = 0 = (0 + 0) = 0
Sum: 43
Digit: * 1 = 0 = (0 + 0) = 0
Sum: 43
Sum: 9
Digit: 8 * 1 = 8 = (0 + 8) = 8
Sum: 17
Digit: 7 * 2 = 14 = (1 + 4) = 5
Sum: 22
Digit: 6 * 1 = 6 = (0 + 6) = 6
Sum: 28
Digit: 5 * 2 = 10 = (1 + 0) = 1
Sum: 29
Digit: 4 * 1 = 4 = (0 + 4) = 4
Sum: 33
Digit: 3 * 2 = 6 = (0 + 6) = 6
Sum: 39
Digit: 2 * 1 = 2 = (0 + 2) = 2
Sum: 41
Digit: 1 * 2 = 2 = (0 + 2) = 2
Sum: 43
Digit: 0 * 1 = 0 = (0 + 0) = 0
Sum: 43
Digit: 0 * 2 = 0 = (0 + 0) = 0
Sum: 43
Digit: * 1 = 0 = (0 + 0) = 0
Sum: 43
Rightmost Digit of Sum: 3
Check Digit: 10 - (3) = 7
Check Digit: 10 - (3) = 7
Javascript Code to Calculate Check Digit:
function dualMod10(str,checkDigit){
str=str+''; //Convert to string
var weight=2; /* weight to apply to digit being checked */
var sum=0; /* sum of weights */
var digit; /* digit being checked */
/* compute the sum */
for (var cnt = str.length; cnt>0; cnt--){
digit = weight * (str.charAt(cnt-1));
/* add tens digit to ones digit to the sum */
sum = sum + Math.floor(digit / 10) + (digit % 10);
weight=weight==2?1:2;
}
/* subtract ones digit of the sum from 10 return ones digit of result */
var mod = (10-sum%10)%10;
return (mod==(checkDigit*1) );
}
PHP Code to Calculate Check Digit:
function dualMod10($str,$checkDigit){
$weight=2; /* weight to apply to digit being checked */
$sum=0; /* sum of weights */
/* compute the $sum */
for ($cnt = strlen($str); $cnt>0; $cnt--){
$digit = $weight * ($str{($cnt-1)});
/* add tens digit and ones digit to the sum */
$sum = $sum + floor($digit / 10) + ($digit % 10);
$weight=$weight==2?1:2;
}
/* subtract ones digit of the sum from 10 return ones digit of that result */
$mod = (10-$sum%10)%10;
return ($mod==($checkDigit) );
}
