### Credit Card Validation - Check Digits

Possible uses for this information:

- When a user has keyed in a credit card number (or scanned it) and you want to validate it before sending it our for debit authorization.
- When issuing cards, say an affinity card, you might want to add a check digit using the MOD 10 method.

## 1.Prefix, Length, and Check Digit Criteria

Here is a table outlining the major credit cards that you might want to validate.

CARD TYPE | Prefix | Length | Check digit algorithm |

MASTERCARD | 51-55 | 16 | mod 10 |

VISA | 4 | 13, 16 | mod 10 |

AMEX | 34 37 | 15 | mod 10 |

Diners Club/ Carte Blanche | 300-305 36 38 | 14 | mod 10 |

Discover | 6011 | 16 | mod 10 |

enRoute | 2014 2149 | 15 | any |

JCB | 3 | 16 | mod 10 |

JCB | 2131 1800 | 15 | mod 10 |

## 2. LUHN Formula (Mod 10) for Validation of Primary Account Number

The following steps are required to validate the primary account number:

**Step 1:** Double the value of alternate digits of the primary account number beginning with the second digit from the right (the first right--hand digit is the check digit.)

**Step 2:** Add the individual digits comprising the products obtained in Step 1 to each of the unaffected digits in the original number.

**Step 3:** The total obtained in Step 2 must be a number ending in zero (30, 40, 50, etc.) for the account number to be validated.

**For example**, to validate the primary account number *49927398716:*

**Step 1:**

` 4 9 9 2 7 3 9 8 7 1 6`

x2 x2 x2 x2 x2

--------------------------------

18 4 6 16 2

**Step 2:** 4 +(1+8)+ 9 + (4) + 7 + (6) + 9 +(1+6) + 7 + (2) + 6

**Step 3:** Sum = 70 : Card number is validated

**Note:** Card is valid because the 70/10 yields no remainder.

The great folks at IC*VERIFY* are the original source of this data, I only formatted it in HTML.

Here is an Microsoft Excel worksheet that will validate a number for you (useful for understanding the algorithm, it is in a .ZIP compressed format)

Make sure that you:

- have started with the rightmost digit (including the check digit) (figure odd and even based upon the rightmost digit being odd, regardless of the length of the Credit Card.) ALWAYS work right to left.
- the check digit counts as digit #1 (assuming that the rightmost digit is the check digit) and is not doubled
- double every second digit (starting with digit # 2 from the right)
- remember that when you double a number over 4, (6 for example) you don't add the result to your total, but rather the sum of the digits of the result (in the above example 6*2=12 so you would add 1+2 to your total (not 12).
- always include the Visa or M/C/ prefix.

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