How to validate a Credit Card Number?
Most credit card number can be validated using the Luhn algorithm, which is more or a less a glorified Modulo 10 formula!
The Luhn Formula:
- Drop the last digit from the number. The last digit is what we want to check against
- Reverse the numbers
- Multiply the digits in odd positions (1, 3, 5, etc.) by 2 and subtract 9 to all any result higher than 9
- Add all the numbers together
- The check digit (the last number of the card) is the amount that you would need to add to get a multiple of 10 (Modulo 10)
Luhn Example:
| Step | Total | |||||||||||||||
| Original Number: | 4 | 5 | 5 | 6 | 7 | 3 | 7 | 5 | 8 | 6 | 8 | 9 | 9 | 8 | 5 | 5 |
| Drop the last digit: | 4 | 5 | 5 | 6 | 7 | 3 | 7 | 5 | 8 | 6 | 8 | 9 | 9 | 8 | 5 | |
| Reverse the digits: | 5 | 8 | 9 | 9 | 8 | 6 | 8 | 5 | 7 | 3 | 7 | 6 | 5 | 5 | 4 | |
| Multiple odd digits by 2: | 10 | 8 | 18 | 9 | 16 | 6 | 16 | 5 | 14 | 3 | 14 | 6 | 10 | 5 | 8 | |
| Subtract 9 to numbers over 9: | 1 | 8 | 9 | 9 | 7 | 6 | 7 | 5 | 5 | 3 | 5 | 6 | 1 | 5 | 8 | |
| Add all numbers: | 1 | 8 | 9 | 9 | 7 | 6 | 7 | 5 | 5 | 3 | 5 | 6 | 1 | 5 | 8 | 85 |
| Mod 10: | 85 modulo 10 = 5 (last digit of card) | |||||||||||||||
List of credit card number formats:
Credit Card Issuer Starts With ( IIN Range ) Length ( Number of digits )
American Express 34, 37 15
Diners Club 300, 301, 302, 303, 304,
305 14
305 14
Carte Blanche
Diners Club - 36 14
International
Diners Club - 54 16
USA & Canada
Discover 6011, 622126 to 622925,644,
645, 646, 647, 648, 649, 65 16
InstaPayment 637, 638, 639 16
JCB 3528 to 3589 16
Laser 6304, 6706, 6771, 6709 16-19
Maestro 5018, 5020, 5038, 5893,
6304, 6759, 6761, 6762,
6763 16-19
MasterCard 51, 52, 53, 54, 55 16-19
Visa 4 13-16
Visa Electron 4026, 417500, 4508,
4844, 4913, 4917 16
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