How Do You Know if a Credit Card Number is Valid?

Written by Nickel - 8 Comments

 Stumble It!  Digg It!  Tip It!  del.ico.us  Facebook

As a followup to my recent post about what your credit card number means, I wanted to throw out a bit more credit card trivia and talk about how you can tell if a credit card number is valid.

As you may or may not aware, most credit card numbers are generated based on something known as the Luhn algorithm. It thus stands to reason that a credit card number is valid if (and only if) it satisfies the “Luhn check” (a.k.a., the Mod 10 check), which is a simple mathematical test that involves manipulating the credit card number, adding it up, and checking to see if it’s evenly divisible by ten.

Testing credit card numbers

Here’s how to apply the Luhn check to test whether or not a credit card number is valid:

  • Step 1a. For a card number with an even number of digits (e.g., Visa or MasterCard), double alternating digits starting with the first digit in the sequence.
  • Step 1b. For a card with an odd number of digits (e.g., American Express), double alternating digits starting with the second digit in the sequence.
  • Step 2. If the doubling resulted in a number with two digits, add them together to get a single digit number
  • Step 3. Now go back to the original credit number and replace the digits that you doubled with the new value — either the doubled value, or the doubled value with the digits added together — and add it all up.
  • Step 4. Check to see if the sum is evenly divisible by 10 (you can simply look to see whether or not it ends with a zero).

If the card number does not pass this check, then it is not a valid number. If, on the other hand, it does pass, then it may be a valid number.

Checking validity: an example

Those steps are a bit convoluted, so here’s a real world example… The following credit card image comes from the CitiCards homepage for their Platinum Select MasterCard. The number on the card is 5424 1801 2345 6789. For starters, the fact that the number starts with a “5” indicates that it’s a MasterCard (as does the little MasterCard symbol on the card).

citi mastercard

Since there are sixteen digits, we’ll start by doubling the 1st, 3rd, etc. digits and then summing as outlined above. I’ve highlighted the doubled (and in some cases summed) values in parentheses, below. I’ve also underlined the check digit.

(1+0) + 4 + (4) + 4 + (2) + 8 + (0) + 1 + (4) + 3 + (8) + 5 + (1+2) + 7 + (1+6) + 9

This totals up to 70, which is evenly divisible by 10. In other words, this is a potentially valid credit card number, though I’m sure it doesn’t correspond to a real account number. If it does, then L. Walker (the name on the card) probably isn’t too happy about his/her credit card number being spread around like this.

Published on March 1st, 2010 - 8 Comments
Filed under: Credit Cards
email this article email this article - bookmark it

About the author: Nickel is the founder and editor-in-chief of this site. He's a thirty-something family man who has been writing about personal finance since 2005, and guess what? He's on Twitter!

Related articles...

» What Do Credit Card Numbers Mean? Making Sense of Your Credit Card Number
» Establishing Credit With a New Taxpayer Identification Number
» Visa Credit Card Acceptance Guidelines
» Beware New Credit Card Fees
» MasterCard Credit Card Acceptance Guidelines
» Get Free SkyMiles from Delta
» Delta to Waive Baggage Fee for AmEx SkyMiles Credit Card Holders
» How to Report Visa and MasterCard Violations

Was this article useful? Please sign up to receive our content via e-mail:

You will receive only the daily updates, and can unsubscribe at anytime.

8 Responses to “How Do You Know if a Credit Card Number is Valid?”

  1. 1
    MikeS Says:
    March 1st, 2010 at 4:14 pm

    Yup, I’m a geek. I tested one of my cards and it’s valid.

    Very cool Nickel.

  2. 2
    SkiSpeaks Says:
    March 1st, 2010 at 7:11 pm

    Nickel,

    Like MikeS said, very cool series of articles. I did a search and there are actually tools that will run this validation automatically, like the one at:
    http://www.braemoor.co.uk/soft.....card.shtml
    (For disclosure, I have nothing to do with that site).

    In keeping with the spirit of liking finance and random trivia, I added your post to my roundup of links for the week :)

  3. 3
    Doug Says:
    March 2nd, 2010 at 1:52 am

    For some example cards that are valid you can try:

    4012888888881
    4012888888881881
    5454545454545454

    These are purely test cards that are given out by Visa and Mastercard for merchants to test their credit card frontends. For a more complicated test, the expiration on those is 12/49, their CV value is 999, the address is ‘123′ (since really only the numbers in an address matter) and the zip code is 12345.

    As a follow up to what you said yesterday, MasterCard is only 5099-5499, with 55 being Maestro. Above that its a mix of private label and Wright Express if memory serves me right.

    Discover actually processes for Diners Club International, JCB (Japan Credit Bureau), and CUPS (China Union Pay). Discover is a subset in 6011 and their new range of 6440-6599. Diners Club is 3000-3095, 3600-3699, and 3800-3899. JCB is a subset of 35, while CUPS is a subset of 62.

    That’s why AMEX has 34 and 37.

    With VISA and MC also identify what type of card you have by the BIN (Bank Identification Number), which are the first 6 digits. So merchants can identify what rate they’re going to be charged for that transaction and whether they should attempt to get the customer to enter their Debit PIN (which is cheaper for the merchant). AMEX on the other hand does not do this, which is why, for example when you shop at Walmart with an AMEX, you must always enter your zip code. Zip code information along with the purchase is known as Level 2 and must be included for certain card types. Since the merchant doesn’t know based on the BIN with AMEX, the only safe move is to always ask for it.

  4. 4
    Gregg With 3 G's Says:
    March 2nd, 2010 at 9:13 pm

    The card given in the example, 5424 1801 2345 6789, adds up to 70, not 67. Supposedly, this means it could be a valid card. If this really is the example given on the CitiBank site as an “invalid” card it raises questions about their accuracy.

  5. 5
    Nickel Says:
    March 2nd, 2010 at 9:24 pm

    Gregg: You are indeed correct, and I’m apparently lame when it comes to working a calculator. The example is laid out properly, but I added it up wrong. I’ve fixed it. Thanks!

  6. 6
    Kevin Says:
    March 3rd, 2010 at 1:57 am

    I got my wallet, grabbed two cards, and did the calcs… they were both valid!

    Thanks for the post.

  7. 7
    adetayo mathew adebayo Says:
    March 23rd, 2010 at 5:44 am

    i need a card that is valid

  8. 8
    samsong Says:
    March 23rd, 2010 at 12:48 pm

    i want to know if this credit card are valid and if valid i need the whole info about it 4271035222415929

Leave a Reply

Financial Shopping Center

Mortgage rates are at an all-time low

Earn interest on your savings

Find lower insurance rates

Featured Credit Cards

Great deals...

Readers’ choice...

Recent articles...

Most talked about...

How to save money on insurance

Signup for free Newsletter