Get your credit card out of your wallet for a minute. See the 16-digit number embossed on your card? Have you ever wondered what those numbers stand for or who chooses a set of numbers for every card out there?

The credit card numbers you find on every credit card are actually meticulously structured. Unlike many people think, credit card numbers are not randomly generated.

The chosen numbers and specific order, define the brand and type of credit card, and more.

In this post, we’ll walk through the systems and reasons behind a credit card number and how credit card numbers are calculated. 

Just don’t worry, you don’t have to know any of this by heart! As long as you have your credit card number on hand, you don’t have to memorize the structure of it.

The first digit

Take any credit card. The first digit of a credit card number is known as the Major Identity Identifier (MII). 

The MII identifies what type of credit card it is. Here is a list and the reference number for first digits.

First digit

Amex cards always start with a 3

Visa cards always start with a 4

Mastercards always start with a 5

Discover cards always start with a 6

Digits 1- 6 of a credit card number

The next 5 digits represent the issuing bank and card type (aka BIN).

Here are some examples: 

414720 means it is a Chase Visa Signature card

414709 means it is a Capital One Visa Signature card

542418 means it is a Citi Mastercard

420767 means it is a Chase Debit card

440066 means it is a Bank of America Visa signature card

You can test many more credit card BIN numbers here.

Digits 7-15 of a credit card number

The digits following the first 6 digits, excluding the last digit of the number, are to personally identify the cardholder and the account number of the card issuer.

The last digit

While all digits of the credit card number up until now are determined by the credit card issuer, the last digit in the number is mathematically calculated by the Luhn algorithm. The last digit is referred to as the Check digit, or the Luhn Check, and is based on all other digits in the credit card number.

The check digit is to ensure the accuracy of the credit card number. If you verbally give over your credit card number to someone, the credit card processor can detect from the check digit if any numbers were dictated wrong. 

The Luhn algorithm

The check digit ultimately validates the credit card number.

Here is how the check digit is calculated by the Luhn algorithm.

1. Let’s create a test credit card number: 4320956975142439. For now, let’s leave the check digit (9) on the side. We’re going to trace how it came to be.

2. Beginning from the first digit (4 in our case), we’re extracting every second number. Here’s what we get. 42967123

3. Multiply each digit by 2

4×2=8

2×2=4

9×2=18

6×2=12

7×2=14

1×2=2

2×2=4

3×2=6

4. If any product of the multiplication is a double-digit, add up the two digits to swap the result for a single-digit sum.

9×2=18 → 1+8=9

6×2=12 → 1+2=3

7×2=14 → 1+4=5

Here’s what we get when we put the results in a string: 8, 4, 9, 3, 5, 2, 4, 6

5. Now we’ll add up the above digits and the digits we left at step 1. 

 3+0+5+9+5+4+4  +  8+4+9+3+5+2+4+6 = 71

6. Now, what number must be added to our sum (71) for us to have a number that is divisible by ten? The closest number divisible by ten, working upward, in our case is 80. To get to 80, we must add the number 9 to our sum of 71. 

That makes 9 our check digit!

The length of a credit card number

The standard length of a credit card number is 16 digits, but there are many issuers who have credit card numbers that are either longer or shorter than that.

Here’s a reference list.

Number of digits

Amex cards always have 15 digits

Visa cards always have 13, 16 or 19 digits

Mastercards always have 16 digits

Discover cards always have 16 digits

The format always stays the same and the check digit will always be the last digit in the number.

Conclusion

There’s the reasoning behind credit card numbers and your proof that the numbers are not a random selection. Why does all this make a difference to you? It does for a few reasons but we will leave it for another post. 

Let me guess that you’re reaching for your credit cards now to test the check digit 🙂