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I’ve written about how the numbers on a credit card actually contain a lot more information than most folks think. For example, you can find out the card category (Visa, MasterCard, etc.), issuing bank, country, and even type of card (World MasterCard, Visa Signature, business, etc.) just from the 1st 6 digits!

But did you know you can use the **last** digit of almost any credit card number to figure out if the card is a **fake**? All it takes is a little bit of math. I’ll show you how!

### How Do You Do It?

The last digit of any credit card number is called the **check** **digit **or **checksum**. Its only purpose is to validate the credit card number.

Credit card issuers use a formula called the Luhn Algorithm to create the check digit. And you can use the same formula to figure out if a credit card number is phony!

Let’s use the generic credit card number from the Barclaycard Arrival Plus website as an example:

Here’s how to do it (**math alert!**). 😉

#### 1. Drop the Last Digit (the Check Digit)

First, drop the last digit from the card number. So 5412 7500 1234 5678 becomes:

5412 7500 1234 567

#### 2. Reverse the Digits

In this case, reversing the digits gives us:

765 4321 0057 2145

#### 3. Multiply Odd Digits by 2

Starting with the 1st digit, multiply every other digit by 2. So now we have:

(14)6(10)4(6)2(2)0(0)5(14)2(2)4(10)

#### 4. Subtract 9 From Any Numbers Over 9

Then, subtract 9 from any double digit number that results. For example, the 1st number is now 14, and 14 – 9 = 5 So now you get:

(5)6(1)4(6)2(2)0(0)5(5)2(2)4(1)

#### 5. Add All the Numbers Together

Add all the numbers. So in this case:

5 + 6 + 1 + 4 + 6 + 2 + 2 + 0 + 0 + 5 + 5 + 2 + 2 + 4 + 1 =

45

#### 6. Add the Check Digit

Remember the check digit we dropped at the beginning? In this case, it was **8**. Add it back to the sum:

45 + 8 =

53

#### 7. Is the Number Divisible by 10?

If the final number is divisible by 10, then it **could** be a valid credit card number.

If it’s **NOT** divisible by 10, it’s definitely **NOT** a real credit card number. In this case, **53** is **NOT** divisible by 10, so it’s a fake credit card number!

Here’s another way of looking at it. The check digit is the number you would have to add to the sum (from running the card number through the algorithm) to make a number divisible by 10.

So in our example, the sum was **45**. The check digit would have to be **5** for this to be a valid credit card number, because 45 + 5 = 50, and 50 **is** divisible by 10!

Neat, huh?

### If You’re Not in the Mood for Math

Instead of doing the math yourself, you can also enter a credit card number into this free Online Credit Card Validator. It’ll run the calculations for you and tell you if a card number is (potentially) valid or definitely fake.

If you enter the Barclaycard Arrival Plus example, this tool tells you right away it’s invalid.

But if you change the last digit to a 5, it says it is valid.

**Note:** I would **NOT** enter your real credit card number, because it’s not an encrypted site. Even though a thief would probably still need your name, expiration date, and verification code, it’s better to be safe!

**Bottom Line**

A credit card number is not just a random sequence of digits. And the last number, the check digit, is used to validate the credit card number.

By doing a little math (or using a free online tool), you can figure out if a credit card is fake.

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Looks like there’s an unnecessary step (2) involved. Do we really need to reverse?

Perhaps if it’s a 15-digit amex?

Why would I want to do this? Is there a point?

Running out of content? Not surprising given the only content in last 3 months are all credit card pumps.

Uh, all numbers are divisible by 10. Some numbers will just have a left over :-0

With no leading zeros the reversal is an extra step.

1) You double every other digit starting with the last number before check digit and working right to left. Or reverse number if working right to left messes with your brain. As long as you do the operations on the correct digits it doesn’t matter.

2) Sum each doubled number:

example if digit was 7

Double: 7*2=14:

sum of 14=5

3) sum all digits

4) multiply total sum by 9

Last digit of result from step 4 is the check digit.

5412 7500 1234 5678

7*2=14 sum 5

5*2=10 sum 1

3*2=6

1*2=2

0

7=5

1*2=2

5=1

Add all resulting digits and digits not doubled. =45

Multiply by 9 = 405

Check digit is 5

This method of check digit catches. many of the most common errors when entering a number.

This doesn’t work for a valid Visa card I tried! Please double check the steps